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Subject[PATCH] Avoid dereferencing a possibly NULL mm
Date
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


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