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staging: android: ashmem: Fix lockdep issue during llseek

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commit cb57469c95 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>
This commit is contained in:
Joel Fernandes
2018-02-16 11:02:01 -08:00
committed by Greg Kroah-Hartman
parent 137cb4fd3f
commit 1d47c874cd
1 changed files with 7 additions and 8 deletions
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15
drivers/staging/android/ashmem.c
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@@ -343,24 +343,23 @@ 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;
}