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authorTetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>2012-07-30 23:42:20 +0200
committerLinus Torvalds <torvalds@linux-foundation.org>2012-07-31 02:25:20 +0200
commit0f20784d4ba3f88ca33b703b23372d8ccf6dbd42 (patch)
tree3cb33ac4afdf3679e12520f579e2e1a607f33670 /kernel/kmod.c
parentkernel/kmod.c: document call_usermodehelper_fns() a bit (diff)
downloadlinux-0f20784d4ba3f88ca33b703b23372d8ccf6dbd42.tar.xz
linux-0f20784d4ba3f88ca33b703b23372d8ccf6dbd42.zip
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>
Diffstat (limited to '')
-rw-r--r--kernel/kmod.c31
1 files changed, 29 insertions, 2 deletions
diff --git a/kernel/kmod.c b/kernel/kmod.c
index 2a8351516a0e..6f99aead66c6 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -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;