[PATCH] namei: free new_dentry late

[PATCH] namei: free new_dentry late

[Pan Bian]

After calling dput(new_dentry), new_dentry is passed to fsnotify_move.
This may result in a use-after-free bug. This patch moves the put
operation late.

Fixes: 49d31c2f389a("dentry name snapshots")
Signed-off-by: Pan Bian <bianpan2016@163.com>
---
 fs/namei.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/namei.c b/fs/namei.c
index 0cab649..8b104d9 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -4498,7 +4498,6 @@ int vfs_rename(struct inode *old_dir, struct dentry *old_dentry,
 		unlock_two_nondirectories(source, target);
 	else if (target)
 		inode_unlock(target);
-	dput(new_dentry);
 	if (!error) {
 		fsnotify_move(old_dir, new_dir, old_name.name, is_dir,
 			      !(flags & RENAME_EXCHANGE) ? target : NULL, old_dentry);
@@ -4507,6 +4506,7 @@ int vfs_rename(struct inode *old_dir, struct dentry *old_dentry,
 				      new_is_dir, NULL, new_dentry);
 		}
 	}
+	dput(new_dentry);
 	release_dentry_name_snapshot(&old_name);
 
 	return error;
-- 
2.7.4

Re: [PATCH] namei: free new_dentry late

[Al Viro]

On Sat, Nov 24, 2018 at 05:23:24PM +0800, Pan Bian wrote:
> After calling dput(new_dentry), new_dentry is passed to fsnotify_move.
> This may result in a use-after-free bug. This patch moves the put
> operation late.
>
> Fixes: 49d31c2f389a("dentry name snapshots")

What does that commit have to do with anything?  The broken part is
                        fsnotify_move(new_dir, old_dir, old_dentry->d_name.name,
                                      new_is_dir, NULL, new_dentry);
and it predates that commit by 3 years - it came from da1ce0670c14 ("vfs: add
cross-rename")...

Re: [PATCH] namei: free new_dentry late

[PanBian]

On Sat, Nov 24, 2018 at 07:51:17PM +0000, Al Viro wrote:
> On Sat, Nov 24, 2018 at 05:23:24PM +0800, Pan Bian wrote:
> > After calling dput(new_dentry), new_dentry is passed to fsnotify_move.
> > This may result in a use-after-free bug. This patch moves the put
> > operation late.
> >
> > Fixes: 49d31c2f389a("dentry name snapshots")
> 
> What does that commit have to do with anything?  The broken part is
>                         fsnotify_move(new_dir, old_dir, old_dentry->d_name.name,
>                                       new_is_dir, NULL, new_dentry);
> and it predates that commit by 3 years - it came from da1ce0670c14 ("vfs: add
> cross-rename")...

Thank you for pointing out my mistake! I will correct it and resubmit
the patch!

Best regards,
Pan