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kmod: avoid deadlock from recursive kmod call 

The system deadlocks (at least since 2.6.10) when
call_usermodehelper(UMH_WAIT_EXEC) request triggers
call_usermodehelper(UMH_WAIT_PROC) request.

This is because "khelper thread is waiting for the worker thread at
wait_for_completion() in do_fork() since the worker thread was created
with CLONE_VFORK flag" and "the worker thread cannot call complete()
because do_execve() is blocked at UMH_WAIT_PROC request" and "the khelper
thread cannot start processing UMH_WAIT_PROC request because the khelper
thread is waiting for the worker thread at wait_for_completion() in
do_fork()".

The easiest example to observe this deadlock is to use a corrupted
/sbin/hotplug binary (like shown below).

  # : > /tmp/dummy
  # chmod 755 /tmp/dummy
  # echo /tmp/dummy > /proc/sys/kernel/hotplug
  # modprobe whatever

call_usermodehelper("/tmp/dummy", UMH_WAIT_EXEC) is called from
kobject_uevent_env() in lib/kobject_uevent.c upon loading/unloading a
module.  do_execve("/tmp/dummy") triggers a call to
request_module("binfmt-0000") from search_binary_handler() which in turn
calls call_usermodehelper(UMH_WAIT_PROC).

In order to avoid deadlock, as a for-now and easy-to-backport solution, do
not try to call wait_for_completion() in call_usermodehelper_exec() if the
worker thread was created by khelper thread with CLONE_VFORK flag.  Future
and fundamental solution might be replacing singleton khelper thread with
some workqueue so that recursive calls up to max_active dependency loop
can be handled without deadlock.

[akpm@linux-foundation.org: add comment to kmod_thread_locker]
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Acked-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Tejun Heo <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Tetsuo Handa 2012-07-30 14:42:20 -07:00 committed by Linus Torvalds
parent 79c743dd1e
commit 0f20784d4b
1 changed files with 29 additions and 2 deletions
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31
kernel/kmod.c
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@ -45,6 +45,13 @@ extern int max_threads;
static struct workqueue_struct *khelper_wq;
/*
* kmod_thread_locker is used for deadlock avoidance. There is no explicit
* locking to protect this global - it is private to the singleton khelper
* thread and should only ever be modified by that thread.
*/
static const struct task_struct *kmod_thread_locker;
#define CAP_BSET (void *)1
#define CAP_PI (void *)2
@ -221,6 +228,13 @@ fail:
return 0;
}
static int call_helper(void *data)
{
/* Worker thread started blocking khelper thread. */
kmod_thread_locker = current;
return ____call_usermodehelper(data);
}
static void call_usermodehelper_freeinfo(struct subprocess_info *info)
{
if (info->cleanup)
@ -295,9 +309,12 @@ static void __call_usermodehelper(struct work_struct *work)
if (wait == UMH_WAIT_PROC)
pid = kernel_thread(wait_for_helper, sub_info,
CLONE_FS | CLONE_FILES | SIGCHLD);
else
pid = kernel_thread(____call_usermodehelper, sub_info,
else {
pid = kernel_thread(call_helper, sub_info,
CLONE_VFORK | SIGCHLD);
/* Worker thread stopped blocking khelper thread. */
kmod_thread_locker = NULL;
}
switch (wait) {
case UMH_NO_WAIT:
@ -548,6 +565,16 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, int wait)
retval = -EBUSY;
goto out;
}
/*
* Worker thread must not wait for khelper thread at below
* wait_for_completion() if the thread was created with CLONE_VFORK
* flag, for khelper thread is already waiting for the thread at
* wait_for_completion() in do_fork().
*/
if (wait != UMH_NO_WAIT && current == kmod_thread_locker) {
retval = -EBUSY;
goto out;
}
sub_info->complete = &done;
sub_info->wait = wait;