linux-kernel - [PATCH 3.16 116/136] staging: android: ashmem: fix a race condition in ASHMEM_SET

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Message-ID: <lsq.1518323471.875907406@decadent.org.uk>
Date:   Sun, 11 Feb 2018 04:31:11 +0000
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        "Viktor Slavkovic" <viktors@...gle.com>
Subject: [PATCH 3.16 116/136] staging: android: ashmem: fix a race
 condition in ASHMEM_SET_SIZE ioctl

3.16.54-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Viktor Slavkovic <viktors@...gle.com>

commit 443064cb0b1fb4569fe0a71209da7625129fb760 upstream.

A lock-unlock is missing in ASHMEM_SET_SIZE ioctl which can result in a
race condition when mmap is called. After the !asma->file check, before
setting asma->size, asma->file can be set in mmap. That would result in
having different asma->size than the mapped memory size. Combined with
ASHMEM_UNPIN ioctl and shrinker invocation, this can result in memory
corruption.

Signed-off-by: Viktor Slavkovic <viktors@...gle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 drivers/staging/android/ashmem.c | 2 ++
 1 file changed, 2 insertions(+)

--- a/drivers/staging/android/ashmem.c
+++ b/drivers/staging/android/ashmem.c
@@ -758,10 +758,12 @@ static long ashmem_ioctl(struct file *fi
 		break;
 	case ASHMEM_SET_SIZE:
 		ret = -EINVAL;
+		mutex_lock(&ashmem_mutex);
 		if (!asma->file) {
 			ret = 0;
 			asma->size = (size_t) arg;
 		}
+		mutex_unlock(&ashmem_mutex);
 		break;
 	case ASHMEM_GET_SIZE:
 		ret = asma->size;