On 02/22/2018 01:02 PM, Joel Fernandes wrote:
(reposting in plain text, sorry for the previous HTML email, I should have not posted from the Phone)
On Thu, Feb 22, 2018 at 5:48 AM, Greg Kroah-Hartman gregkh@linuxfoundation.org wrote:
On Fri, Feb 16, 2018 at 11:02:01AM -0800, Joel Fernandes wrote:
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
Cc: Todd Kjos tkjos@google.com Cc: Arve Hjonnevag arve@android.com Cc: Greg Hackmann ghackmann@google.com Cc: Greg Kroah-Hartman gregkh@linuxfoundation.org Cc: stable@vger.kernel.org Reported-by: syzbot+8ec30bb7bf1a981a2012@syzkaller.appspotmail.com Signed-off-by: Joel Fernandes joelaf@google.com
Changes since first version: Don't relock after vfs call since its not needed. Only reason we lock is to protect races with asma->file. https://patchwork.kernel.org/patch/10185031/
I'd like some acks from others before I take this patch.
GregH, Todd, could you provide Acks?
Acked-by: Greg Hackmann ghackmann@google.com