linux-kernel - [PATCH] Avoid dereferencing a possibly NULL mm

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Message-Id: <1338656062-25008-1-git-send-email-siddhesh.poyarekar@gmail.com>
Date:	Sat,  2 Jun 2012 22:24:22 +0530
From:	Siddhesh Poyarekar <siddhesh.poyarekar@...il.com>
To:	linux-kernel@...r.kernel.org
Cc:	Oleg Nesterov <oleg@...hat.com>, Tejun Heo <tj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Siddhesh Poyarekar <siddhesh.poyarekar@...il.com>
Subject: [PATCH] Avoid dereferencing a possibly NULL mm

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@...il.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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