ashmem_mutex may create a chain of dependencies like:
CPU0 CPU1
mmap syscall ioctl syscall
-> mmap_sem (acquired) -> ashmem_ioctl
-> ashmem_mmap -> ashmem_mutex (acquired)
-> ashmem_mutex (try to acquire) -> copy_from_user
-> mmap_sem (try to acquire)
There is a lock odering problem between mmap_sem and ashmem_mutex causing
a lockdep splat[1] during a syzcaller test. This patch fixes the problem
by move copy_from_user out of ashmem_mutex.
[1] https://www.spinics.net/lists/kernel/msg2733200.html
Fixes: ce8a3a9e76d0 (staging: android: ashmem: Fix a race condition in pin ioctls)
Reported-by: syzbot+d7a918a7a8e1c952bc36@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Yisheng Xie <xieyisheng1@xxxxxxxxxx>
---
drivers/staging/android/ashmem.c | 8 +++-----
1 file changed, 3 insertions(+), 5 deletions(-)
diff --git a/drivers/staging/android/ashmem.c b/drivers/staging/android/ashmem.c
index 6dbba5a..8c55706 100644
--- a/drivers/staging/android/ashmem.c
+++ b/drivers/staging/android/ashmem.c
@@ -702,16 +702,14 @@ static int ashmem_pin_unpin(struct ashmem_area *asma, unsigned long cmd,
size_t pgstart, pgend;
int ret = -EINVAL;
+ if (unlikely(copy_from_user(&pin, p, sizeof(pin))))
+ return -EFAULT;
+
mutex_lock(&ashmem_mutex);
if (unlikely(!asma->file))
goto out_unlock;
- if (unlikely(copy_from_user(&pin, p, sizeof(pin)))) {
- ret = -EFAULT;
- goto out_unlock;
- }
-
/* per custom, you can pass zero for len to mean "everything onward" */
if (!pin.len)
pin.len = PAGE_ALIGN(asma->size) - pin.offset;
--
1.7.12.4
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