[PATCH] Avoid dereferencing a possibly NULL mm

[Siddhesh Poyarekar <siddhesh.poyarekar@gmail.com>]

The NULL check for mm in exit_mm occurs after mm_release is
called. This looks wrong because mm_release dereferences mm:

...
                if (!(tsk->flags & PF_SIGNALED) &&
                    atomic_read(&mm->mm_users) > 1) {
                        /*
...

This dereference seems unsafe and hence is fixed by moving the NULL
check above mm_release.

Signed-off-by: Siddhesh Poyarekar <siddhesh.poyarekar@gmail.com>
---
 kernel/exit.c |    4 +++-
 1 files changed, 3 insertions(+), 1 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 34867cc..15fe63c 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -640,9 +640,11 @@ static void exit_mm(struct task_struct * tsk)
 	struct mm_struct *mm = tsk->mm;
 	struct core_state *core_state;
 
-	mm_release(tsk, mm);
 	if (!mm)
 		return;
+
+	mm_release(tsk, mm);
+
 	/*
 	 * Serialize with any possible pending coredump.
 	 * We must hold mmap_sem around checking core_state
-- 
1.7.7.6