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

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Message-Id: <20180108184303.55563-1-ghackmann@google.com>
Date:   Mon,  8 Jan 2018 10:43:03 -0800
From:   Greg Hackmann <ghackmann@...gle.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Arve Hjønnevåg <arve@...roid.com>,
        Todd Kjos <tkjos@...roid.com>,
        Martijn Coenen <maco@...roid.com>
Cc:     devel@...verdev.osuosl.org, linux-kernel@...r.kernel.org,
        Viktor Slavkovic <viktors@...gle.com>, stable@...r.kernel.org
Subject: [PATCH] staging: android: ashmem: fix a race condition in ASHMEM_SET_SIZE ioctl

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

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>
Cc: stable@...r.kernel.org
---
 drivers/staging/android/ashmem.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/staging/android/ashmem.c b/drivers/staging/android/ashmem.c
index 0f695df14c9d..372ce9913e6d 100644
--- a/drivers/staging/android/ashmem.c
+++ b/drivers/staging/android/ashmem.c
@@ -765,10 +765,12 @@ static long ashmem_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 		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;
-- 
2.16.0.rc0.223.g4a4ac83678-goog