Commit 0a40db9b authored Feb 16, 2018 by Joel Fernandes Committed by Greg Kroah-Hartman Mar 18, 2018

staging: android: ashmem: Fix lockdep issue during llseek

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



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>

parent 8d5ac33d

drivers/staging/android/ashmem.c

+7 −8

Original line number	Diff line number	Diff line
		@@ -330,24 +330,23 @@ 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;
		}