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

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Date:   Tue, 27 Nov 2018 11:05:36 +0100
From:   Jan Kara <jack@...e.cz>
To:     PanBian <bianpan2016@....com>
Cc:     Jan Kara <jack@...e.cz>, Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Miklos Szeredi <miklos@...redi.hu>
Subject: Re: [PATCH V2] namei: free new_dentry late

On Tue 27-11-18 17:57:12, PanBian wrote:
> 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.

I don't think that's the case. The dget() - dput() pair just looks
superfluous to me in vfs_rename(). Am I missing something Miklos?

								Honza

> > > 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
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR