linux-kernel - [PATCH 10/12] proc: hold cred_guard_mutex in check_mem

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Message-Id: <1300045764-24168-11-git-send-email-wilsons@start.ca>
Date:	Sun, 13 Mar 2011 15:49:22 -0400
From:	Stephen Wilson <wilsons@...rt.ca>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Alexander Viro <viro@...iv.linux.org.uk>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Michel Lespinasse <walken@...gle.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Matt Mackall <mpm@...enic.com>,
	David Rientjes <rientjes@...gle.com>,
	Nick Piggin <npiggin@...nel.dk>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Mel Gorman <mel@....ul.ie>, Hugh Dickins <hughd@...gle.com>,
	x86@...nel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Stephen Wilson <wilsons@...rt.ca>
Subject: [PATCH 10/12] proc: hold cred_guard_mutex in check_mem_permission()

Avoid a potential race when task exec's and we get a new ->mm but check against
the old credentials in ptrace_may_access().

Holding of the mutex is implemented by factoring out the body of the code into a
helper function __check_mem_permission().  Performing this factorization now
simplifies upcoming changes and minimizes churn in the diff's.

Signed-off-by: Stephen Wilson <wilsons@...rt.ca>
---
 fs/proc/base.c |   26 ++++++++++++++++++++++----
 1 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index e52702d..f6b644f 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -191,10 +191,7 @@ static int proc_root_link(struct inode *inode, struct path *path)
 	return result;
 }
 
-/*
- * Return zero if current may access user memory in @task, -error if not.
- */
-static int check_mem_permission(struct task_struct *task)
+static int __check_mem_permission(struct task_struct *task)
 {
 	/*
 	 * A task can always look at itself, in case it chooses
@@ -222,6 +219,27 @@ static int check_mem_permission(struct task_struct *task)
 	return -EPERM;
 }
 
+/*
+ * Return zero if current may access user memory in @task, -error if not.
+ */
+static int check_mem_permission(struct task_struct *task)
+{
+	int err;
+
+	/*
+	 * Avoid racing if task exec's as we might get a new mm but validate
+	 * against old credentials.
+	 */
+	err = mutex_lock_killable(&task->signal->cred_guard_mutex);
+	if (err)
+		return err;
+
+	err = __check_mem_permission(task);
+	mutex_unlock(&task->signal->cred_guard_mutex);
+
+	return err;
+}
+
 struct mm_struct *mm_for_maps(struct task_struct *task)
 {
 	struct mm_struct *mm;
-- 
1.7.3.5

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