Restrict unprivileged access to kernel syslog

The kernel syslog contains debugging information that is often useful during exploitation of other vulnerabilities, such as kernel heap addresses. Rather than futilely attempt to sanitize hundreds (or thousands) of printk statements and simultaneously cripple useful debugging functionality, it is far simpler to create an option that prevents unprivileged users from reading the syslog. This patch, loosely based on grsecurity's GRKERNSEC_DMESG, creates the dmesg_restrict sysctl. When set to "0", the default, no restrictions are enforced. When set to "1", only users with CAP_SYS_ADMIN can read the kernel syslog via dmesg(8) or other mechanisms. [akpm@linux-foundation.org: explain the config option in kernel.txt] Signed-off-by: Dan Rosenberg <drosenberg@vsecurity.com> Acked-by: Ingo Molnar <mingo@elte.hu> Acked-by: Eugene Teo <eugeneteo@kernel.org> Acked-by: Kees Cook <kees.cook@canonical.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Restrict unprivileged access to kernel syslog
The kernel syslog contains debugging information that is often useful during exploitation of other vulnerabilities, such as kernel heap addresses. Rather than futilely attempt to sanitize hundreds (or thousands) of printk statements and simultaneously cripple useful debugging functionality, it is far simpler to create an option that prevents unprivileged users from reading the syslog. This patch, loosely based on grsecurity's GRKERNSEC_DMESG, creates the dmesg_restrict sysctl. When set to "0", the default, no restrictions are enforced. When set to "1", only users with CAP_SYS_ADMIN can read the kernel syslog via dmesg(8) or other mechanisms. [akpm@linux-foundation.org: explain the config option in kernel.txt] Signed-off-by: Dan Rosenberg <drosenberg@vsecurity.com> Acked-by: Ingo Molnar <mingo@elte.hu> Acked-by: Eugene Teo <eugeneteo@kernel.org> Acked-by: Kees Cook <kees.cook@canonical.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Dan Rosenberg · Linus Torvalds
1 parent 203f40a5a0
Showing 6 changed files with 44 additions and 0 deletions Side-by-side Diff
Documentation/sysctl/kernel.txt
include/linux/kernel.h
kernel/printk.c
kernel/sysctl.c
security/Kconfig
security/commoncap.c
@@ -28,6 +28,7 @@
 - core_uses_pid
 - ctrl-alt-del
 - dentry-state
+- dmesg_restrict
 - domainname
 - hostname
 - hotplug
@@ -210,6 +211,19 @@
 mode, the ctrl-alt-del is intercepted by the program before it
 ever reaches the kernel tty layer, and it's up to the program
 to decide what to do with it.
+
+==============================================================
+
+dmesg_restrict:
+
+This toggle indicates whether unprivileged users are prevented from using
+dmesg(8) to view messages from the kernel's log buffer.  When
+dmesg_restrict is set to (0) there are no restrictions.  When
+dmesg_restrict is set set to (1), users must have CAP_SYS_ADMIN to use
+dmesg(8).
+
+The kernel config option CONFIG_SECURITY_DMESG_RESTRICT sets the default
+value of dmesg_restrict.
  
 ==============================================================
  
@@ -293,6 +293,7 @@
 				   unsigned int interval_msec);
  
 extern int printk_delay_msec;
+extern int dmesg_restrict;
  
 /*
  * Print a one-time message (analogous to WARN_ONCE() et al):
@@ -261,6 +261,12 @@
 }
 #endif
  
+#ifdef CONFIG_SECURITY_DMESG_RESTRICT
+int dmesg_restrict = 1;
+#else
+int dmesg_restrict;
+#endif
+
 int do_syslog(int type, char __user *buf, int len, bool from_file)
 {
 	unsigned i, j, limit, count;
@@ -704,6 +704,15 @@
 	},
 #endif
 	{
+		.procname	= "dmesg_restrict",
+		.data		= &dmesg_restrict,
+		.maxlen		= sizeof(int),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= &zero,
+		.extra2		= &one,
+	},
+	{
 		.procname	= "ngroups_max",
 		.data		= &ngroups_max,
 		.maxlen		= sizeof (int),
@@ -39,6 +39,18 @@
  
 	  If you are unsure as to whether this is required, answer N.
  
+config SECURITY_DMESG_RESTRICT
+	bool "Restrict unprivileged access to the kernel syslog"
+	default n
+	help
+	  This enforces restrictions on unprivileged users reading the kernel
+	  syslog via dmesg(8).
+
+	  If this option is not selected, no restrictions will be enforced
+	  unless the dmesg_restrict sysctl is explicitly set to (1).
+
+	  If you are unsure how to answer this question, answer N.
+
 config SECURITY
 	bool "Enable different security models"
 	depends on SYSFS
@@ -895,6 +895,8 @@
 {
 	if (type != SYSLOG_ACTION_OPEN && from_file)
 		return 0;
+	if (dmesg_restrict && !capable(CAP_SYS_ADMIN))
+		return -EPERM;
 	if ((type != SYSLOG_ACTION_READ_ALL &&
 	     type != SYSLOG_ACTION_SIZE_BUFFER) && !capable(CAP_SYS_ADMIN))
 		return -EPERM;
...	...	@@ -28,6 +28,7 @@
28	28	- core_uses_pid
29	29	- ctrl-alt-del
30	30	- dentry-state
	31	+- dmesg_restrict
31	32	- domainname
32	33	- hostname
33	34	- hotplug
...	...	@@ -210,6 +211,19 @@
210	211	mode, the ctrl-alt-del is intercepted by the program before it
211	212	ever reaches the kernel tty layer, and it's up to the program
212	213	to decide what to do with it.
	214	+
	215	+==============================================================
	216	+
	217	+dmesg_restrict:
	218	+
	219	+This toggle indicates whether unprivileged users are prevented from using
	220	+dmesg(8) to view messages from the kernel's log buffer. When
	221	+dmesg_restrict is set to (0) there are no restrictions. When
	222	+dmesg_restrict is set set to (1), users must have CAP_SYS_ADMIN to use
	223	+dmesg(8).
	224	+
	225	+The kernel config option CONFIG_SECURITY_DMESG_RESTRICT sets the default
	226	+value of dmesg_restrict.
213	227
214	228	==============================================================
215	229
...	...	@@ -293,6 +293,7 @@
293	293	unsigned int interval_msec);
294	294
295	295	extern int printk_delay_msec;
	296	+extern int dmesg_restrict;
296	297
297	298	/*
298	299	* Print a one-time message (analogous to WARN_ONCE() et al):
...	...	@@ -261,6 +261,12 @@
261	261	}
262	262	#endif
263	263
	264	+#ifdef CONFIG_SECURITY_DMESG_RESTRICT
	265	+int dmesg_restrict = 1;
	266	+#else
	267	+int dmesg_restrict;
	268	+#endif
	269	+
264	270	int do_syslog(int type, char __user *buf, int len, bool from_file)
265	271	{
266	272	unsigned i, j, limit, count;
...	...	@@ -704,6 +704,15 @@
704	704	},
705	705	#endif
706	706	{
	707	+ .procname = "dmesg_restrict",
	708	+ .data = &dmesg_restrict,
	709	+ .maxlen = sizeof(int),
	710	+ .mode = 0644,
	711	+ .proc_handler = proc_dointvec_minmax,
	712	+ .extra1 = &zero,
	713	+ .extra2 = &one,
	714	+ },
	715	+ {
707	716	.procname = "ngroups_max",
708	717	.data = &ngroups_max,
709	718	.maxlen = sizeof (int),
...	...	@@ -39,6 +39,18 @@
39	39
40	40	If you are unsure as to whether this is required, answer N.
41	41
	42	+config SECURITY_DMESG_RESTRICT
	43	+ bool "Restrict unprivileged access to the kernel syslog"
	44	+ default n
	45	+ help
	46	+ This enforces restrictions on unprivileged users reading the kernel
	47	+ syslog via dmesg(8).
	48	+
	49	+ If this option is not selected, no restrictions will be enforced
	50	+ unless the dmesg_restrict sysctl is explicitly set to (1).
	51	+
	52	+ If you are unsure how to answer this question, answer N.
	53	+
42	54	config SECURITY
43	55	bool "Enable different security models"
44	56	depends on SYSFS
...	...	@@ -895,6 +895,8 @@
895	895	{
896	896	if (type != SYSLOG_ACTION_OPEN && from_file)
897	897	return 0;
	898	+ if (dmesg_restrict && !capable(CAP_SYS_ADMIN))
	899	+ return -EPERM;
898	900	if ((type != SYSLOG_ACTION_READ_ALL &&
899	901	type != SYSLOG_ACTION_SIZE_BUFFER) && !capable(CAP_SYS_ADMIN))
900	902	return -EPERM;