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Subject[PATCH] ashmem: Fix lockdep issue during llseek
Date
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>
Reported-by: syzbot+8ec30bb7bf1a981a2012@syzkaller.appspotmail.com
Signed-off-by: Joel Fernandes <joelaf@google.com>
---
drivers/staging/android/ashmem.c | 15 +++++++--------
1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/drivers/staging/android/ashmem.c b/drivers/staging/android/ashmem.c
index 7e060f32aaa8..c8b74ae53936 100644
--- a/drivers/staging/android/ashmem.c
+++ b/drivers/staging/android/ashmem.c
@@ -335,24 +335,23 @@ 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;
}

--
2.16.0.rc1.238.g530d649a79-goog
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