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

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

diff --git a/drivers/staging/android/ashmem.c b/drivers/staging/android/ashmem.c
index 658d640022be068651cd7546998d4f78b73e8557..8b0211d7d7a63498e43f0d21ad7cef8312467cc8 100644 (file)
--- a/drivers/staging/android/ashmem.c
+++ b/drivers/staging/android/ashmem.c
@@ -759,10 +759,12 @@ 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;