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Subject[PATCH v2] kmod: Avoid deadlock by recursive kmod call.
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Tetsuo Handa wrote:
> I noticed that the system deadlocks if do_execve() request by kmod() triggered
> recursive do_execve() request.
>
> An example operation for triggering this deadlock:
>
> # : > /tmp/dummy
> # chmod 755 /tmp/dummy
> # echo /tmp/dummy > /proc/sys/kernel/hotplug
> # modprobe whatever
>
> Although this patch works for me, I'm not sure that this is a correct fix.
> Also, my analysis in the patch description may be wrong. Please check.

Well, we should not unconditionally prevent all vfork()ed threads from using
request_module() at search_binary_handler(). We should prevent only kernel
threads created by __call_usermodehelper(). Updated to filter only kmod threads.
--------------------
[PATCH v2] kmod: Avoid deadlock by recursive kmod call.

call_usermodehelper(UMH_WAIT_EXEC or UMH_WAIT_PROC) request depends on an
assumption that do_execve() will not trigger recursive
call_usermodehelper(UMH_WAIT_EXEC or UMH_WAIT_PROC) request, for
"khelper cannot start processing recursive call_usermodehelper() request until
do_execve() of original call_usermodehelper() request completes" but
"do_execve() of original call_usermodehelper() cannot be completed until
recursive call_usermodehelper() request completes".

There are several callers that break this assumption and lead to deadlock.
We need to make sure that do_execve() request by kmod thread does not trigger
recursive call_usermodehelper(UMH_WAIT_PROC or UMH_WAIT_EXEC).

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
include/linux/sched.h | 3 +++
kernel/kmod.c | 18 +++++++++++++++++-
2 files changed, 20 insertions(+), 1 deletion(-)

--- linux-3.1.7.orig/include/linux/sched.h
+++ linux-3.1.7/include/linux/sched.h
@@ -1305,6 +1305,9 @@ struct task_struct {
unsigned sched_reset_on_fork:1;
unsigned sched_contributes_to_load:1;

+ /* Prevent recursive kmod request. */
+ unsigned kmod_thread:1;
+
pid_t pid;
pid_t tgid;

--- linux-3.1.7.orig/kernel/kmod.c
+++ linux-3.1.7/kernel/kmod.c
@@ -189,6 +189,13 @@ fail:
do_exit(0);
}

+static int call_helper(void *data)
+{
+ /* Do not trigger recursive kmod call. */
+ current->kmod_thread = 1;
+ return ____call_usermodehelper(data);
+}
+
void call_usermodehelper_freeinfo(struct subprocess_info *info)
{
if (info->cleanup)
@@ -252,7 +259,7 @@ static void __call_usermodehelper(struct
pid = kernel_thread(wait_for_helper, sub_info,
CLONE_FS | CLONE_FILES | SIGCHLD);
else
- pid = kernel_thread(____call_usermodehelper, sub_info,
+ pid = kernel_thread(call_helper, sub_info,
CLONE_VFORK | SIGCHLD);

switch (wait) {
@@ -428,6 +435,15 @@ int call_usermodehelper_exec(struct subp
retval = -EBUSY;
goto out;
}
+ /*
+ * We can't call wait_for_completion() if current thread was created by
+ * call_helper(), or we will deadlock when current thread calls
+ * request_module() from search_binary_handler().
+ */
+ if (wait != UMH_NO_WAIT && current->kmod_thread) {
+ retval = -EBUSY;
+ goto out;
+ }

sub_info->complete = &done;
sub_info->wait = wait;

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