Re: [PATCH] ovl: Fix use inode directly in rcu-walk mode

[Miklos Szeredi]

On Thu, Nov 24, 2022 at 05:26:02PM +0800, Chen Zhongjin wrote:
> syzkaller reported a null-ptr-deref error:
> https://syzkaller.appspot.com/bug?id=bb281e89381b9ed55728c274447a575e69a96c35
> 
> ovl_dentry_revalidate_common() can be called in rcu-walk mode.
> As document said, "in rcu-walk mode, d_parent and d_inode should not be
> used without care". Check inode here to protect access under rcu-walk
> mode.
> 
> Fixes: bccece1ead36 ("ovl: allow remote upper")
> Reported-by: syzbot+a4055c78774bbf3498bb@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Chen Zhongjin <chenzhongjin@xxxxxxxxxx>

Hi,

Thanks for the quick analysis and patch.

I simplified the patch a bit without changing the attribution.

Thanks,
Miklos

----
From: Chen Zhongjin <chenzhongjin@xxxxxxxxxx>
Subject: ovl: fix use inode directly in rcu-walk mode

ovl_dentry_revalidate_common() can be called in rcu-walk mode.  As document
said, "in rcu-walk mode, d_parent and d_inode should not be used without
care".

Check inode here to protect access under rcu-walk mode.

Fixes: bccece1ead36 ("ovl: allow remote upper")
Reported-by: syzbot+a4055c78774bbf3498bb@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Chen Zhongjin <chenzhongjin@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # v5.7
Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
---
 fs/overlayfs/super.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

--- a/fs/overlayfs/super.c
+++ b/fs/overlayfs/super.c
@@ -139,11 +139,16 @@ static int ovl_dentry_revalidate_common(
 					unsigned int flags, bool weak)
 {
 	struct ovl_entry *oe = dentry->d_fsdata;
+	struct inode *inode = d_inode_rcu(dentry);
 	struct dentry *upper;
 	unsigned int i;
 	int ret = 1;
 
-	upper = ovl_dentry_upper(dentry);
+	/* Careful in RCU mode */
+	if (!inode)
+		return -ECHILD;
+
+	upper = ovl_i_dentry_upper(inode);
 	if (upper)
 		ret = ovl_revalidate_real(upper, flags, weak);