Re: [PATCH v3] ovl: fix null pointer when filesystemdoesn'tsupportdirect IO

[Miklos Szeredi]

On Wed, Sep 22, 2021 at 04:00:47PM +0200, Miklos Szeredi wrote:

> First let's fix the oops: ovl_read_iter()/ovl_write_iter() must check
> real file's ->direct_IO if IOCB_DIRECT is set in iocb->ki_flags and
> return -EINVAL if not.

And here's that fix.  Please test.

Thanks,
Miklos

---
From: Miklos Szeredi <mszeredi@xxxxxxxxxx>
Subject: ovl: fix IOCB_DIRECT if underlying fs doesn't support direct IO

Normally the check at open time suffices, but e.g loop device does set
IOCB_DIRECT after doing its own checks (which are not sufficent for
overlayfs).

Make sure we don't call the underlying filesystem read/write method with
the IOCB_DIRECT if it's not supported.

Reported-by: Huang Jianan <huangjianan@xxxxxxxx>
Fixes: 16914e6fc7e1 ("ovl: add ovl_read_iter()")
Cc: <stable@xxxxxxxxxxxxxxx> # v4.19
Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
---
 fs/overlayfs/file.c |   15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

--- a/fs/overlayfs/file.c
+++ b/fs/overlayfs/file.c
@@ -296,6 +296,12 @@ static ssize_t ovl_read_iter(struct kioc
 	if (ret)
 		return ret;
 
+	ret = -EINVAL;
+	if (iocb->ki_flags & IOCB_DIRECT &&
+	    (!real.file->f_mapping->a_ops ||
+	     !real.file->f_mapping->a_ops->direct_IO))
+		goto out_fdput;
+
 	old_cred = ovl_override_creds(file_inode(file)->i_sb);
 	if (is_sync_kiocb(iocb)) {
 		ret = vfs_iter_read(real.file, iter, &iocb->ki_pos,
@@ -320,7 +326,7 @@ static ssize_t ovl_read_iter(struct kioc
 out:
 	revert_creds(old_cred);
 	ovl_file_accessed(file);
-
+out_fdput:
 	fdput(real);
 
 	return ret;
@@ -349,6 +355,12 @@ static ssize_t ovl_write_iter(struct kio
 	if (ret)
 		goto out_unlock;
 
+	ret = -EINVAL;
+	if (iocb->ki_flags & IOCB_DIRECT &&
+	    (!real.file->f_mapping->a_ops ||
+	     !real.file->f_mapping->a_ops->direct_IO))
+		goto out_fdput;
+
 	if (!ovl_should_sync(OVL_FS(inode->i_sb)))
 		ifl &= ~(IOCB_DSYNC | IOCB_SYNC);
 
@@ -384,6 +396,7 @@ static ssize_t ovl_write_iter(struct kio
 	}
 out:
 	revert_creds(old_cred);
+out_fdput:
 	fdput(real);
 
 out_unlock: