Eric Lee / linux-smarc-t335x-v3.2

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Commit 7d64224217b53b23243fd2aa428001486f2a5da5

Authored by Oleg Nesterov
Committed by Linus Torvalds
1 parent 363da4022c
Exists in master and in 4 other branches v3.2_AM335xPSP_04.06.00.11, v3.2_NEWT335xPSP_04.06.00.11, v3.2_SBC_SMARTMEN, v3.2_SMARCT335xPSP_04.06.00.11

wait_for_helper: SIGCHLD from user-space can lead to use-after-free 

1. wait_for_helper() calls allow_signal(SIGCHLD) to ensure the child
   can't autoreap itself.

   However, this means that a spurious SIGCHILD from user-space can
   set TIF_SIGPENDING and:

   	- kernel_thread() or sys_wait4() can fail due to signal_pending()

   	- worse, wait4() can fail before ____call_usermodehelper() execs
   	  or exits. In this case the caller may kfree(subprocess_info)
   	  while the child still uses this memory.

   Change the code to use SIG_DFL instead of magic "(void __user *)2"
   set by allow_signal(). This means that SIGCHLD won't be delivered,
   yet the child won't autoreap itsefl.

   The problem is minor, only root can send a signal to this kthread.

2. If sys_wait4(&ret) fails it doesn't populate "ret", in this case
   wait_for_helper() reports a random value from uninitialized var.

   With this patch sys_wait4() should never fail, but still it makes
   sense to initialize ret = -ECHILD so that the caller can notice
   the problem.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Showing 1 changed file with 5 additions and 5 deletions Side-by-side Diff

... ... @@ -175,16 +175,16 @@
175 175 struct subprocess_info *sub_info = data;
176 176 pid_t pid;
177 177  
178   - /* Install a handler: if SIGCLD isn't handled sys_wait4 won't
179   - * populate the status, but will return -ECHILD. */
180   - allow_signal(SIGCHLD);
  178 + /* If SIGCLD is ignored sys_wait4 won't populate the status. */
  179 + spin_lock_irq(&current->sighand->siglock);
  180 + current->sighand->action[SIGCHLD-1].sa.sa_handler = SIG_DFL;
  181 + spin_unlock_irq(&current->sighand->siglock);
181 182  
182 183 pid = kernel_thread(____call_usermodehelper, sub_info, SIGCHLD);
183 184 if (pid < 0) {
184 185 sub_info->retval = pid;
185 186 } else {
186   - int ret;
187   -
  187 + int ret = -ECHILD;
188 188 /*
189 189 * Normally it is bogus to call wait4() from in-kernel because
190 190 * wait4() wants to write the exit code to a userspace address.