[PATCH v2] ovl: fix null pointer when filesystem doesn't support direct IO

[Huang Jianan]

At present, overlayfs provides overlayfs inode to users. Overlayfs
inode provides ovl_aops with noop_direct_IO to avoid open failure
with O_DIRECT. But some compressed filesystems, such as erofs and
squashfs, don't support direct_IO.

Users who use f_mapping->a_ops->direct_IO to check O_DIRECT support,
will read file through this way. This will cause overlayfs to access
a non-existent direct_IO function and cause panic due to null pointer:

Kernel panic - not syncing: CFI failure (target: 0x0)
CPU: 6 PID: 247 Comm: loop0
Call Trace:
 panic+0x188/0x45c
 __cfi_slowpath+0x0/0x254
 __cfi_slowpath+0x200/0x254
 generic_file_read_iter+0x14c/0x150
 vfs_iocb_iter_read+0xac/0x164
 ovl_read_iter+0x13c/0x2fc
 lo_rw_aio+0x2bc/0x458
 loop_queue_work+0x4a4/0xbc0
 kthread_worker_fn+0xf8/0x1d0
 loop_kthread_worker_fn+0x24/0x38
 kthread+0x29c/0x310
 ret_from_fork+0x10/0x30

The filesystem may only support direct_IO for some file types. For
example, erofs supports direct_IO for uncompressed files. So reset
f_mapping->a_ops to NULL when the file doesn't support direct_IO to
fix this problem.

Fixes: 5b910bd615ba ("ovl: fix GPF in swapfile_activate of file from overlayfs over xfs")
Signed-off-by: Huang Jianan <huangjianan at oppo.com>
---
Change since v1:
 - Return error to user rather than fall back to buffered io. (Chengguang Xu)

 fs/overlayfs/file.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
index d081faa55e83..38118d3b46f8 100644
--- a/fs/overlayfs/file.c
+++ b/fs/overlayfs/file.c
@@ -157,6 +157,10 @@ static int ovl_open(struct inode *inode, struct file *file)
 	if (IS_ERR(realfile))
 		return PTR_ERR(realfile);
 
+	if ((f->f_flags & O_DIRECT) && (!realfile->f_mapping->a_ops ||
+		!realfile->f_mapping->a_ops->direct_IO))
+		file->f_mapping->a_ops = NULL;
+
 	file->private_data = realfile;
 
 	return 0;
-- 
2.25.1