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Subject[PATCH 4.15 02/84] staging: android: ashmem: Fix possible deadlock in ashmem_ioctl
Date
4.15-stable review patch.  If anyone has any objections, please let me know.

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

From: Yisheng Xie <xieyisheng1@huawei.com>

commit 740a5759bf222332fbb5eda42f89aa25ba38f9b2 upstream.

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@syzkaller.appspotmail.com
Signed-off-by: Yisheng Xie <xieyisheng1@huawei.com>
Cc: "Joel Fernandes (Google)" <joel.opensrc@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
drivers/staging/android/ashmem.c | 8 +++-----
1 file changed, 3 insertions(+), 5 deletions(-)

--- a/drivers/staging/android/ashmem.c
+++ b/drivers/staging/android/ashmem.c
@@ -709,16 +709,14 @@ static int ashmem_pin_unpin(struct ashme
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;

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