linux-kernel - Re: [PATCH V2] namei: free new

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Date:   Tue, 27 Nov 2018 17:57:12 +0800
From:   PanBian <bianpan2016@....com>
To:     Jan Kara <jack@...e.cz>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH V2] namei: free new_dentry late

On Tue, Nov 27, 2018 at 10:25:51AM +0100, Jan Kara wrote:
> 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@....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?

I am not quite sure about the actual execution logic. But I guess new_dentry 
reference may be dropped outside vfs_rename in cocurrent executions.
Otherwise, there is no need to acquire & drop new_dentry reference as it
is always alive along vfs_rename.

Best regards,
Pan

> 
> 								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@...e.com>
> SUSE Labs, CR