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Date:   Tue, 27 Nov 2018 10:25:51 +0100
From:   Jan Kara <jack@suse.cz>
To:     Pan Bian <bianpan2016@163.com>
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH V2] namei: free new_dentry late
Message-ID: <20181127092551.GC16301@quack2.suse.cz>
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On Sun 25-11-18 08:15:23, 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: da1ce0670c14("vfs: add cross-rename")
> Signed-off-by: Pan Bian <bianpan2016@163.com>

The code is actually fine AFAICT. One new_dentry reference is passed into
vfs_rename() and another one is acquired directly inside vfs_rename(). That
is the one dropped early but there's still the original reference passed in
protecting new_dentry from freeing. Am I missing something?

								Honza

> ---
> V2: correct the fixes commit information
> ---
>  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
> 
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR