kmod: fix race in usermodehelper code

The user mode helper code has a race in it.  call_usermodehelper_exec()
takes an allocated subprocess_info structure, which it passes to a
workqueue, and then passes it to a kernel thread which it creates, after
which it calls complete to signal to the caller of
call_usermodehelper_exec() that it can free the subprocess_info struct.

But since we use that structure in the created thread, we can't call
complete from __call_usermodehelper(), which is where we create the kernel
thread.  We need to call complete() from within the kernel thread and then
not use subprocess_info afterward in the case of UMH_WAIT_EXEC.  Tested
successfully by me.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 8 insertions, 5 deletions

diff --git a/kernel/kmod.c b/kernel/kmod.c
index 9fcb53a11f87..689d20f39305 100644
--- a/kernel/kmod.c
+++ b/kernel/kmod.c
@@ -143,6 +143,7 @@ struct subprocess_info {
 static int ____call_usermodehelper(void *data)
 {
         struct subprocess_info *sub_info = data;
+        enum umh_wait wait = sub_info->wait;
         int retval;
 
         BUG_ON(atomic_read(&sub_info->cred->usage) != 1);
@@ -184,10 +185,14 @@ static int ____call_usermodehelper(void *data)
          */
         set_user_nice(current, 0);
 
+        if (wait == UMH_WAIT_EXEC)
+                complete(sub_info->complete);
+
         retval = kernel_execve(sub_info->path, sub_info->argv, sub_info->envp);
 
         /* Exec failed? */
-        sub_info->retval = retval;
+        if (wait != UMH_WAIT_EXEC)
+                sub_info->retval = retval;
         do_exit(0);
 }
 
@@ -266,16 +271,14 @@ static void __call_usermodehelper(struct work_struct *work)
 
         switch (wait) {
         case UMH_NO_WAIT:
+        case UMH_WAIT_EXEC:
                 break;
 
         case UMH_WAIT_PROC:
                 if (pid > 0)
                         break;
                 sub_info->retval = pid;
-                /* FALLTHROUGH */
-
-        case UMH_WAIT_EXEC:
-                complete(sub_info->complete);
+                break;
         }
 }