staging: android: ashmem: Fix lockdep issue during llseek

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

Acked-by: Todd Kjos <tkjos@google.com>
Cc: Arve Hjonnevag <arve@android.com>
Cc: stable@vger.kernel.org
Reported-by: syzbot+8ec30bb7bf1a981a2012@syzkaller.appspotmail.com
Signed-off-by: Joel Fernandes <joelaf@google.com>
Acked-by: Greg Hackmann <ghackmann@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 7 insertions, 8 deletions

diff --git a/drivers/staging/android/ashmem.c b/drivers/staging/android/ashmem.c
index 6dbba5aff191..d5450365769c 100644
--- a/drivers/staging/android/ashmem.c
+++ b/drivers/staging/android/ashmem.c
@@ -326,24 +326,23 @@ static loff_t ashmem_llseek(struct file *file, loff_t offset, int origin)
         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;
 }