staging: android: ashmem: fix a race condition in ASHMEM_SET_SIZE ioctl

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@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 2 insertions, 0 deletions

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;