Messages in this thread Patch in this message | | | Subject | [PATCH] kmod: Avoid deadlock by recursive kmod call. | From | Tetsuo Handa <> | Date | Wed, 04 Jan 2012 16:43:09 +0900 |
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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. -------------------- [PATCH] kmod: Avoid deadlock by recursive kmod call.
call_usermodehelper(UMH_WAIT_EXEC) creates a CLONE_VFORK'ed thread. The vforked thread may call call_usermodehelper(UMH_WAIT_PROC) from search_binary_handler() which in turn calls wait_for_completion().
This sequence results in a deadlock because "khelper cannot start processing call_usermodehelper(UMH_WAIT_PROC) until do_execve() of vforked thread by call_usermodehelper(UMH_WAIT_EXEC) completes" but "vforked thread cannot complete do_execve() until call_usermodehelper(UMH_WAIT_PROC) completes".
We need to make sure that do_execve() request by vforked thread does not trigger call_usermodehelper(UMH_WAIT_PROC or UMH_WAIT_EXEC).
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> --- kernel/kmod.c | 9 +++++++++ 1 files changed, 9 insertions(+)
--- linux-3.1.7.orig/kernel/kmod.c +++ linux-3.1.7/kernel/kmod.c @@ -428,6 +428,15 @@ int call_usermodehelper_exec(struct subp retval = -EBUSY; goto out; } + /* + * We can't call wait_for_completion() if current thread was created + * with CLONE_VFORK flag, or we will deadlock when current thread calls + * request_module() from search_binary_handler(). + */ + if (wait != UMH_NO_WAIT && current->vfork_done) { + retval = -EBUSY; + goto out; + } sub_info->complete = &done; sub_info->wait = wait;
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