Patch "staging: android: ashmem: Fix possible deadlock in ashmem_ioctl" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

staging: android: ashmem: Fix possible deadlock in ashmem_ioctl

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
staging-android-ashmem-fix-possible-deadlock-in-ashmem_ioctl.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Yisheng Xie <[email protected]>
Date: Wed, 28 Feb 2018 14:59:22 +0800
Subject: staging: android: ashmem: Fix possible deadlock in ashmem_ioctl

From: Yisheng Xie <[email protected]>

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

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

@@ -718,16 +718,14 @@ 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;

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/staging-android-ashmem-fix-possible-deadlock-in-ashmem_ioctl.patch