05 Apr, 2019
1 commit
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What happens there is that we are replacing file->path.mnt of
a file we'd just opened with a clone and we need the write
count contribution to be transferred from original mount to
new one. That's it. We do *NOT* want any kind of freeze
protection for the duration of switchover.IOW, we should just use __mnt_{want,drop}_write() for that
switchover; no need to bother with mnt_{want,drop}_write()
there.Tested-by: Amir Goldstein
Reported-by: syzbot+2a73a6ea9507b7112141@syzkaller.appspotmail.com
Signed-off-by: Al Viro
31 Jan, 2019
1 commit
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Create a new helper, vfs_create_mount(), that creates a detached vfsmount
object from an fs_context that has a superblock attached to it.Almost all uses will be paired with immediately preceding vfs_get_tree();
add a helper for such combination.Switch vfs_kern_mount() to use this.
NOTE: mild behaviour change; passing NULL as 'device name' to
something like procfs will change /proc/*/mountstats - "device none"
instead on "no device". That is consistent with /proc/mounts et.al.[do'h - EXPORT_SYMBOL_GPL slipped in by mistake; removed]
[AV -- remove confused comment from vfs_create_mount()]
[AV -- removed the second argument]Reviewed-by: David Howells
Signed-off-by: Al Viro
21 Dec, 2018
1 commit
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Separate just the changing of mount flags (MS_REMOUNT|MS_BIND) from full
remount because the mount data will get parsed with the new fs_context
stuff prior to doing a remount - and this causes the syscall to fail under
some circumstances.To quote Eric's explanation:
[...] mount(..., MS_REMOUNT|MS_BIND, ...) now validates the mount options
string, which breaks systemd unit files with ProtectControlGroups=yes
(e.g. systemd-networkd.service) when systemd does the following to
change a cgroup (v1) mount to read-only:mount(NULL, "/run/systemd/unit-root/sys/fs/cgroup/systemd", NULL,
MS_RDONLY|MS_NOSUID|MS_NODEV|MS_NOEXEC|MS_REMOUNT|MS_BIND, NULL)... when the kernel has CONFIG_CGROUPS=y but no cgroup subsystems
enabled, since in that case the error "cgroup1: Need name or subsystem
set" is hit when the mount options string is empty.Probably it doesn't make sense to validate the mount options string at
all in the MS_REMOUNT|MS_BIND case, though maybe you had something else
in mind.This is also worthwhile doing because we will need to add a mount_setattr()
syscall to take over the remount-bind function.Reported-by: Eric Biggers
Signed-off-by: David Howells
Signed-off-by: Al Viro
Reviewed-by: David Howells
02 Nov, 2017
1 commit
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Many source files in the tree are missing licensing information, which
makes it harder for compliance tools to determine the correct license.By default all files without license information are under the default
license of the kernel, which is GPL version 2.Update the files which contain no license information with the 'GPL-2.0'
SPDX license identifier. The SPDX identifier is a legally binding
shorthand, which can be used instead of the full boiler plate text.This patch is based on work done by Thomas Gleixner and Kate Stewart and
Philippe Ombredanne.How this work was done:
Patches were generated and checked against linux-4.14-rc6 for a subset of
the use cases:
- file had no licensing information it it.
- file was a */uapi/* one with no licensing information in it,
- file was a */uapi/* one with existing licensing information,Further patches will be generated in subsequent months to fix up cases
where non-standard license headers were used, and references to license
had to be inferred by heuristics based on keywords.The analysis to determine which SPDX License Identifier to be applied to
a file was done in a spreadsheet of side by side results from of the
output of two independent scanners (ScanCode & Windriver) producing SPDX
tag:value files created by Philippe Ombredanne. Philippe prepared the
base worksheet, and did an initial spot review of a few 1000 files.The 4.13 kernel was the starting point of the analysis with 60,537 files
assessed. Kate Stewart did a file by file comparison of the scanner
results in the spreadsheet to determine which SPDX license identifier(s)
to be applied to the file. She confirmed any determination that was not
immediately clear with lawyers working with the Linux Foundation.Criteria used to select files for SPDX license identifier tagging was:
- Files considered eligible had to be source code files.
- Make and config files were included as candidates if they contained >5
lines of source
- File already had some variant of a license header in it (even if
Reviewed-by: Philippe Ombredanne
Reviewed-by: Thomas Gleixner
Signed-off-by: Greg Kroah-Hartman
01 Jul, 2017
1 commit
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This marks many critical kernel structures for randomization. These are
structures that have been targeted in the past in security exploits, or
contain functions pointers, pointers to function pointer tables, lists,
workqueues, ref-counters, credentials, permissions, or are otherwise
sensitive. This initial list was extracted from Brad Spengler/PaX Team's
code in the last public patch of grsecurity/PaX based on my understanding
of the code. Changes or omissions from the original code are mine and
don't reflect the original grsecurity/PaX code.Left out of this list is task_struct, which requires special handling
and will be covered in a subsequent patch.Signed-off-by: Kees Cook
01 Feb, 2017
1 commit
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To support unprivileged users mounting filesystems two permission
checks have to be performed: a test to see if the user allowed to
create a mount in the mount namespace, and a test to see if
the user is allowed to access the specified filesystem.The automount case is special in that mounting the original filesystem
grants permission to mount the sub-filesystems, to any user who
happens to stumble across the their mountpoint and satisfies the
ordinary filesystem permission checks.Attempting to handle the automount case by using override_creds
almost works. It preserves the idea that permission to mount
the original filesystem is permission to mount the sub-filesystem.
Unfortunately using override_creds messes up the filesystems
ordinary permission checks.Solve this by being explicit that a mount is a submount by introducing
vfs_submount, and using it where appropriate.vfs_submount uses a new mount internal mount flags MS_SUBMOUNT, to let
sget and friends know that a mount is a submount so they can take appropriate
action.sget and sget_userns are modified to not perform any permission checks
on submounts.follow_automount is modified to stop using override_creds as that
has proven problemantic.do_mount is modified to always remove the new MS_SUBMOUNT flag so
that we know userspace will never by able to specify it.autofs4 is modified to stop using current_real_cred that was put in
there to handle the previous version of submount permission checking.cifs is modified to pass the mountpoint all of the way down to vfs_submount.
debugfs is modified to pass the mountpoint all of the way down to
trace_automount by adding a new parameter. To make this change easier
a new typedef debugfs_automount_t is introduced to capture the type of
the debugfs automount function.Cc: stable@vger.kernel.org
Fixes: 069d5ac9ae0d ("autofs: Fix automounts by using current_real_cred()->uid")
Fixes: aeaa4a79ff6a ("fs: Call d_automount with the filesystems creds")
Reviewed-by: Trond Myklebust
Reviewed-by: Seth Forshee
Signed-off-by: "Eric W. Biederman"
17 Dec, 2016
1 commit
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Signed-off-by: Al Viro
06 Dec, 2016
1 commit
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Signed-off-by: Al Viro
04 Dec, 2016
1 commit
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d_mountpoint() can only be used reliably to establish if a dentry is
not mounted in any namespace. It isn't aware of the possibility there
may be multiple mounts using a given dentry that may be in a different
namespace.Add helper functions, path_is_mountpoint(), that checks if a struct path
is a mountpoint for this case.Link: http://lkml.kernel.org/r/20161011053358.27645.9729.stgit@pluto.themaw.net
Signed-off-by: Ian Kent
Cc: Al Viro
Cc: Eric W. Biederman
Cc: Omar Sandoval
Signed-off-by: Andrew Morton
Signed-off-by: Al Viro
01 Oct, 2016
1 commit
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CAI Qian pointed out that the semantics
of shared subtrees make it possible to create an exponentially
increasing number of mounts in a mount namespace.mkdir /tmp/1 /tmp/2
mount --make-rshared /
for i in $(seq 1 20) ; do mount --bind /tmp/1 /tmp/2 ; doneWill create create 2^20 or 1048576 mounts, which is a practical problem
as some people have managed to hit this by accident.As such CVE-2016-6213 was assigned.
Ian Kent described the situation for autofs users
as follows:> The number of mounts for direct mount maps is usually not very large because of
> the way they are implemented, large direct mount maps can have performance
> problems. There can be anywhere from a few (likely case a few hundred) to less
> than 10000, plus mounts that have been triggered and not yet expired.
>
> Indirect mounts have one autofs mount at the root plus the number of mounts that
> have been triggered and not yet expired.
>
> The number of autofs indirect map entries can range from a few to the common
> case of several thousand and in rare cases up to between 30000 and 50000. I've
> not heard of people with maps larger than 50000 entries.
>
> The larger the number of map entries the greater the possibility for a large
> number of active mounts so it's not hard to expect cases of a 1000 or somewhat
> more active mounts.So I am setting the default number of mounts allowed per mount
namespace at 100,000. This is more than enough for any use case I
know of, but small enough to quickly stop an exponential increase
in mounts. Which should be perfect to catch misconfigurations and
malfunctioning programs.For anyone who needs a higher limit this can be changed by writing
to the new /proc/sys/fs/mount-max sysctl.Tested-by: CAI Qian
Signed-off-by: "Eric W. Biederman"
24 Jun, 2016
1 commit
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If a process gets access to a mount from a different user
namespace, that process should not be able to take advantage of
setuid files or selinux entrypoints from that filesystem. Prevent
this by treating mounts from other mount namespaces and those not
owned by current_user_ns() or an ancestor as nosuid.This will make it safer to allow more complex filesystems to be
mounted in non-root user namespaces.This does not remove the need for MNT_LOCK_NOSUID. The setuid,
setgid, and file capability bits can no longer be abused if code in
a user namespace were to clear nosuid on an untrusted filesystem,
but this patch, by itself, is insufficient to protect the system
from abuse of files that, when execed, would increase MAC privilege.As a more concrete explanation, any task that can manipulate a
vfsmount associated with a given user namespace already has
capabilities in that namespace and all of its descendents. If they
can cause a malicious setuid, setgid, or file-caps executable to
appear in that mount, then that executable will only allow them to
elevate privileges in exactly the set of namespaces in which they
are already privileges.On the other hand, if they can cause a malicious executable to
appear with a dangerous MAC label, running it could change the
caller's security context in a way that should not have been
possible, even inside the namespace in which the task is confined.As a hardening measure, this would have made CVE-2014-5207 much
more difficult to exploit.Signed-off-by: Andy Lutomirski
Signed-off-by: Seth Forshee
Acked-by: James Morris
Acked-by: Serge Hallyn
Signed-off-by: Eric W. Biederman
18 Apr, 2015
1 commit
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Pull usernamespace mount fixes from Eric Biederman:
"Way back in October Andrey Vagin reported that umount(MNT_DETACH)
could be used to defeat MNT_LOCKED. As I worked to fix this I
discovered that combined with mount propagation and an appropriate
selection of shared subtrees a reference to a directory on an
unmounted filesystem is not necessary.That MNT_DETACH is allowed in user namespace in a form that can break
MNT_LOCKED comes from my early misunderstanding what MNT_DETACH does.To avoid breaking existing userspace the conflict between MNT_DETACH
and MNT_LOCKED is fixed by leaving mounts that are locked to their
parents in the mount hash table until the last reference goes away.While investigating this issue I also found an issue with
__detach_mounts. The code was unnecessarily and incorrectly
triggering mount propagation. Resulting in too many mounts going away
when a directory is deleted, and too many cpu cycles are burned while
doing that.Looking some more I realized that __detach_mounts by only keeping
mounts connected that were MNT_LOCKED it had the potential to still
leak information so I tweaked the code to keep everything locked
together that possibly could be.This code was almost ready last cycle but Al invented fs_pin which
slightly simplifies this code but required rewrites and retesting, and
I have not been in top form for a while so it took me a while to get
all of that done. Similiarly this pull request is late because I have
been feeling absolutely miserable all week.The issue of being able to escape a bind mount has not yet been
addressed, as the fixes are not yet mature"* 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace:
mnt: Update detach_mounts to leave mounts connected
mnt: Fix the error check in __detach_mounts
mnt: Honor MNT_LOCKED when detaching mounts
fs_pin: Allow for the possibility that m_list or s_list go unused.
mnt: Factor umount_mnt from umount_tree
mnt: Factor out unhash_mnt from detach_mnt and umount_tree
mnt: Fail collect_mounts when applied to unmounted mounts
mnt: Don't propagate unmounts to locked mounts
mnt: On an unmount propagate clearing of MNT_LOCKED
mnt: Delay removal from the mount hash.
mnt: Add MNT_UMOUNT flag
mnt: In umount_tree reuse mnt_list instead of mnt_hash
mnt: Don't propagate umounts in __detach_mounts
mnt: Improve the umount_tree flags
mnt: Use hlist_move_list in namespace_unlock
16 Apr, 2015
1 commit
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DM will switch its device lookup code to using name_to_dev_t() so it
must be exported. Also, the @name argument should be marked const.Signed-off-by: Dan Ehrenberg
Signed-off-by: Mike Snitzer
03 Apr, 2015
1 commit
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In some instances it is necessary to know if the the unmounting
process has begun on a mount. Add MNT_UMOUNT to make that reliably
testable.This fix gets used in fixing locked mounts in MNT_DETACH
Cc: stable@vger.kernel.org
Signed-off-by: "Eric W. Biederman"
24 Oct, 2014
1 commit
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Overlayfs needs a private clone of the mount, so create a function for
this and export to modules.Signed-off-by: Miklos Szeredi
12 Aug, 2014
1 commit
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Pull vfs updates from Al Viro:
"Stuff in here:- acct.c fixes and general rework of mnt_pin mechanism. That allows
to go for delayed-mntput stuff, which will permit mntput() on deep
stack without worrying about stack overflows - fs shutdown will
happen on shallow stack. IOW, we can do Eric's umount-on-rmdir
series without introducing tons of stack overflows on new mntput()
call chains it introduces.
- Bruce's d_splice_alias() patches
- more Miklos' rename() stuff.
- a couple of regression fixes (stable fodder, in the end of branch)
and a fix for API idiocy in iov_iter.c.There definitely will be another pile, maybe even two. I'd like to
get Eric's series in this time, but even if we miss it, it'll go right
in the beginning of for-next in the next cycle - the tricky part of
prereqs is in this pile"* 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/viro/vfs: (40 commits)
fix copy_tree() regression
__generic_file_write_iter(): fix handling of sync error after DIO
switch iov_iter_get_pages() to passing maximal number of pages
fs: mark __d_obtain_alias static
dcache: d_splice_alias should detect loops
exportfs: update Exporting documentation
dcache: d_find_alias needn't recheck IS_ROOT && DCACHE_DISCONNECTED
dcache: remove unused d_find_alias parameter
dcache: d_obtain_alias callers don't all want DISCONNECTED
dcache: d_splice_alias should ignore DCACHE_DISCONNECTED
dcache: d_splice_alias mustn't create directory aliases
dcache: close d_move race in d_splice_alias
dcache: move d_splice_alias
namei: trivial fix to vfs_rename_dir comment
VFS: allow ->d_manage() to declare -EISDIR in rcu_walk mode.
cifs: support RENAME_NOREPLACE
hostfs: support rename flags
shmem: support RENAME_EXCHANGE
shmem: support RENAME_NOREPLACE
btrfs: add RENAME_NOREPLACE
...
08 Aug, 2014
1 commit
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Rather than playing silly buggers with vfsmount refcounts, just have
acct_on() ask fs/namespace.c for internal clone of file->f_path.mnt
and replace it with said clone. Then attach the pin to original
vfsmount. Voila - the clone will be alive until the file gets closed,
making sure that underlying superblock remains active, etc., and
we can drop the original vfsmount, so that it's not kept busy.
If the file lives until the final mntput of the original vfsmount,
we'll notice that there's an fs_pin (one in bsd_acct_struct that
holds that file) and mnt_pin_kill() will take it out. Since
->kill() is synchronous, we won't proceed past that point until
these files are closed (and private clones of our vfsmount are
gone), so we get the same ordering warranties we used to get.mnt_pin()/mnt_unpin()/->mnt_pinned is gone now, and good riddance -
it never became usable outside of kernel/acct.c (and racy wrt
umount even there).Signed-off-by: Al Viro
01 Aug, 2014
2 commits
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While invesgiating the issue where in "mount --bind -oremount,ro ..."
would result in later "mount --bind -oremount,rw" succeeding even if
the mount started off locked I realized that there are several
additional mount flags that should be locked and are not.In particular MNT_NOSUID, MNT_NODEV, MNT_NOEXEC, and the atime
flags in addition to MNT_READONLY should all be locked. These
flags are all per superblock, can all be changed with MS_BIND,
and should not be changable if set by a more privileged user.The following additions to the current logic are added in this patch.
- nosuid may not be clearable by a less privileged user.
- nodev may not be clearable by a less privielged user.
- noexec may not be clearable by a less privileged user.
- atime flags may not be changeable by a less privileged user.The logic with atime is that always setting atime on access is a
global policy and backup software and auditing software could break if
atime bits are not updated (when they are configured to be updated),
and serious performance degradation could result (DOS attack) if atime
updates happen when they have been explicitly disabled. Therefore an
unprivileged user should not be able to mess with the atime bits set
by a more privileged user.The additional restrictions are implemented with the addition of
MNT_LOCK_NOSUID, MNT_LOCK_NODEV, MNT_LOCK_NOEXEC, and MNT_LOCK_ATIME
mnt flags.Taken together these changes and the fixes for MNT_LOCK_READONLY
should make it safe for an unprivileged user to create a user
namespace and to call "mount --bind -o remount,... ..." without
the danger of mount flags being changed maliciously.Cc: stable@vger.kernel.org
Acked-by: Serge E. Hallyn
Signed-off-by: "Eric W. Biederman" -
Kenton Varda discovered that by remounting a
read-only bind mount read-only in a user namespace the
MNT_LOCK_READONLY bit would be cleared, allowing an unprivileged user
to the remount a read-only mount read-write.Correct this by replacing the mask of mount flags to preserve
with a mask of mount flags that may be changed, and preserve
all others. This ensures that any future bugs with this mask and
remount will fail in an easy to detect way where new mount flags
simply won't change.Cc: stable@vger.kernel.org
Acked-by: Serge E. Hallyn
Signed-off-by: "Eric W. Biederman"
02 Apr, 2014
1 commit
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The current mainline has copies propagated to *all* nodes, then
tears down the copies we made for nodes that do not contain
counterparts of the desired mountpoint. That sets the right
propagation graph for the copies (at teardown time we move
the slaves of removed node to a surviving peer or directly
to master), but we end up paying a fairly steep price in
useless allocations. It's fairly easy to create a situation
where N calls of mount(2) create exactly N bindings, with
O(N^2) vfsmounts allocated and freed in process.Fortunately, it is possible to avoid those allocations/freeings.
The trick is to create copies in the right order and find which
one would've eventually become a master with the current algorithm.
It turns out to be possible in O(nodes getting propagation) time
and with no extra allocations at all.One part is that we need to make sure that eventual master will be
created before its slaves, so we need to walk the propagation
tree in a different order - by peer groups. And iterate through
the peers before dealing with the next group.Another thing is finding the (earlier) copy that will be a master
of one we are about to create; to do that we are (temporary) marking
the masters of mountpoints we are attaching the copies to.Either we are in a peer of the last mountpoint we'd dealt with,
or we have the following situation: we are attaching to mountpoint M,
the last copy S_0 had been attached to M_0 and there are sequences
S_0...S_n, M_0...M_n such that S_{i+1} is a master of S_{i},
S_{i} mounted on M{i} and we need to create a slave of the first S_{k}
such that M is getting propagation from M_{k}. It means that the master
of M_{k} will be among the sequence of masters of M. On the
other hand, the nearest marked node in that sequence will either
be the master of M_{k} or the master of M_{k-1} (the latter -
in the case if M_{k-1} is a slave of something M gets propagation
from, but in a wrong peer group).So we go through the sequence of masters of M until we find
a marked one (P). Let N be the one before it. Then we go through
the sequence of masters of S_0 until we find one (say, S) mounted
on a node D that has P as master and check if D is a peer of N.
If it is, S will be the master of new copy, if not - the master of S
will be.That's it for the hard part; the rest is fairly simple. Iterator
is in next_group(), handling of one prospective mountpoint is
propagate_one().It seems to survive all tests and gives a noticably better performance
than the current mainline for setups that are seriously using shared
subtrees.Cc: stable@vger.kernel.org
Signed-off-by: Al Viro
09 Nov, 2013
1 commit
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* RCU-delayed freeing of vfsmounts
* vfsmount_lock replaced with a seqlock (mount_lock)
* sequence number from mount_lock is stored in nameidata->m_seq and
used when we exit RCU mode
* new vfsmount flag - MNT_SYNC_UMOUNT. Set by umount_tree() when its
caller knows that vfsmount will have no surviving references.
* synchronize_rcu() done between unlocking namespace_sem in namespace_unlock()
and doing pending mntput().
* new helper: legitimize_mnt(mnt, seq). Checks the mount_lock sequence
number against seq, then grabs reference to mnt. Then it rechecks mount_lock
again to close the race and either returns success or drops the reference it
has acquired. The subtle point is that in case of MNT_SYNC_UMOUNT we can
simply decrement the refcount and sod off - aforementioned synchronize_rcu()
makes sure that final mntput() won't come until we leave RCU mode. We need
that, since we don't want to end up with some lazy pathwalk racing with
umount() and stealing the final mntput() from it - caller of umount() may
expect it to return only once the fs is shut down and we don't want to break
that. In other cases (i.e. with MNT_SYNC_UMOUNT absent) we have to do
full-blown mntput() in case of mount_lock sequence number mismatch happening
just as we'd grabbed the reference, but in those cases we won't be stealing
the final mntput() from anything that would care.
* mntput_no_expire() doesn't lock anything on the fast path now. Incidentally,
SMP and UP cases are handled the same way - no ifdefs there.
* normal pathname resolution does *not* do any writes to mount_lock. It does,
of course, bump the refcounts of vfsmount and dentry in the very end, but that's
it.Signed-off-by: Al Viro
25 Jul, 2013
1 commit
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When creating a less privileged mount namespace or propogating mounts
from a more privileged to a less privileged mount namespace lock the
submounts so they may not be unmounted individually in the child mount
namespace revealing what is under them.This enforces the reasonable expectation that it is not possible to
see under a mount point. Most of the time mounts are on empty
directories and revealing that does not matter, however I have seen an
occassionaly sloppy configuration where there were interesting things
concealed under a mount point that probably should not be revealed.Expirable submounts are not locked because they will eventually
unmount automatically so whatever is under them already needs
to be safe for unprivileged users to access.From a practical standpoint these restrictions do not appear to be
significant for unprivileged users of the mount namespace. Recursive
bind mounts and pivot_root continues to work, and mounts that are
created in a mount namespace may be unmounted there. All of which
means that the common idiom of keeping a directory of interesting
files and using pivot_root to throw everything else away continues to
work just fine.Acked-by: Serge Hallyn
Acked-by: Andy Lutomirski
Signed-off-by: "Eric W. Biederman"
27 Mar, 2013
1 commit
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When a read-only bind mount is copied from mount namespace in a higher
privileged user namespace to a mount namespace in a lesser privileged
user namespace, it should not be possible to remove the the read-only
restriction.Add a MNT_LOCK_READONLY mount flag to indicate that a mount must
remain read-only.CC: stable@vger.kernel.org
Acked-by: Serge Hallyn
Signed-off-by: "Eric W. Biederman"
04 Jan, 2012
15 commits
-
Signed-off-by: Al Viro
-
Signed-off-by: Al Viro
-
Signed-off-by: Al Viro
-
Signed-off-by: Al Viro
-
Signed-off-by: Al Viro
-
Signed-off-by: Al Viro
-
Signed-off-by: Al Viro
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make IS_MNT_SLAVE take struct mount * at the same time
Signed-off-by: Al Viro
-
Signed-off-by: Al Viro
-
Signed-off-by: Al Viro
-
Signed-off-by: Al Viro
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the second victim...
Signed-off-by: Al Viro
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taken out of struct vfsmount into struct mount
Signed-off-by: Al Viro
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new helper (wrapper around mnt_drop_write()) to be used in pair with
mnt_want_write_file().Signed-off-by: Al Viro
-
the only user outside of fs/namespace.c has died
Signed-off-by: Al Viro
27 Jul, 2011
1 commit
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This allows us to move duplicated code in
(atomic_inc_not_zero() for now) toSigned-off-by: Arun Sharma
Reviewed-by: Eric Dumazet
Cc: Ingo Molnar
Cc: David Miller
Cc: Eric Dumazet
Acked-by: Mike Frysinger
Signed-off-by: Andrew Morton
Signed-off-by: Linus Torvalds
17 Jan, 2011
1 commit
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Instead of splitting refcount between (per-cpu) mnt_count
and (SMP-only) mnt_longrefs, make all references contribute
to mnt_count again and keep track of how many are longterm
ones.Accounting rules for longterm count:
* 1 for each fs_struct.root.mnt
* 1 for each fs_struct.pwd.mnt
* 1 for having non-NULL ->mnt_ns
* decrement to 0 happens only under vfsmount lock exclusiveThat allows nice common case for mntput() - since we can't drop the
final reference until after mnt_longterm has reached 0 due to the rules
above, mntput() can grab vfsmount lock shared and check mnt_longterm.
If it turns out to be non-zero (which is the common case), we know
that this is not the final mntput() and can just blindly decrement
percpu mnt_count. Otherwise we grab vfsmount lock exclusive and
do usual decrement-and-check of percpu mnt_count.For fs_struct.c we have mnt_make_longterm() and mnt_make_shortterm();
namespace.c uses the latter in places where we don't already hold
vfsmount lock exclusive and opencodes a few remaining spots where
we need to manipulate mnt_longterm.Note that we mostly revert the code outside of fs/namespace.c back
to what we used to have; in particular, normal code doesn't need
to care about two kinds of references, etc. And we get to keep
the optimization Nick's variant had bought us...Signed-off-by: Al Viro