Patch "staging: android: ashmem: Fix lockdep issue during llseek" 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 lockdep issue during llseek

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-lockdep-issue-during-llseek.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: Joel Fernandes <[email protected]>
Date: Fri, 16 Feb 2018 11:02:01 -0800
Subject: staging: android: ashmem: Fix lockdep issue during llseek

From: Joel Fernandes <[email protected]>

commit cb57469c9573f6018cd1302953dd45d6e05aba7b upstream.

ashmem_mutex create a chain of dependencies like so:

(1)
mmap syscall ->
mmap_sem -> (acquired)
ashmem_mmap
ashmem_mutex (try to acquire)
(block)

(2)
llseek syscall ->
ashmem_llseek ->
ashmem_mutex -> (acquired)
inode_lock ->
inode->i_rwsem (try to acquire)
(block)

(3)
getdents ->
iterate_dir ->
inode_lock ->
inode->i_rwsem (acquired)
copy_to_user ->
mmap_sem (try to acquire)

There is a lock ordering created between mmap_sem and inode->i_rwsem causing a lockdep splat [2] during a syzcaller test, this patch fixes the issue by unlocking the mutex earlier. Functionally that's Ok since we don't need to protect vfs_llseek.

[1] https://patchwork.kernel.org/patch/10185031/
[2] https://lkml.org/lkml/2018/1/10/48

---
 drivers/staging/android/ashmem.c |   15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

@@ -343,24 +343,23 @@ static loff_t ashmem_llseek(struct file
 	mutex_lock(&ashmem_mutex);
 
 	if (asma->size == 0) {
-		ret = -EINVAL;
-		goto out;
+		mutex_unlock(&ashmem_mutex);
+		return -EINVAL;
 	}
 
 	if (!asma->file) {
-		ret = -EBADF;
-		goto out;
+		mutex_unlock(&ashmem_mutex);
+		return -EBADF;
 	}
 
+	mutex_unlock(&ashmem_mutex);
+
 	ret = vfs_llseek(asma->file, offset, origin);
 	if (ret < 0)
-		goto out;
+		return ret;
 
 	/** Copy f_pos from backing file, since f_ops->llseek() sets it */
 	file->f_pos = asma->file->f_pos;
-
-out:
-	mutex_unlock(&ashmem_mutex);
 	return ret;
 }
 

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

queue-4.9/staging-android-ashmem-fix-lockdep-issue-during-llseek.patch