linux-kernel - Re: [PATCH] fs: btrfs: fix possible use-after-free bug in error handling code of btrfs_get_root

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Message-ID: <20230823080702.GA2472023@google.com>
Date:   Wed, 23 Aug 2023 09:07:02 +0100
From:   Lee Jones <lee@...nel.org>
To:     dsterba@...e.cz, Jia-Ju Bai <baijiaju1990@...il.com>, clm@...com,
        josef@...icpanda.com, dsterba@...e.com,
        linux-btrfs@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fs: btrfs: fix possible use-after-free bug in error
 handling code of btrfs_get_root_ref()

On Wed, 23 Aug 2023, Lee Jones wrote:

> On Fri, 01 Apr 2022, David Sterba wrote:
> 
> > On Fri, Mar 25, 2022 at 04:04:17PM +0800, Jia-Ju Bai wrote:
> > > >> @@ -1850,9 +1850,10 @@ static struct btrfs_root *btrfs_get_root_ref(struct btrfs_fs_info *fs_info,
> > > >>   
> > > >>   	ret = btrfs_insert_fs_root(fs_info, root);
> > > >>   	if (ret) {
> > > >> -		btrfs_put_root(root);
> > > >> -		if (ret == -EEXIST)
> > > >> +		if (ret == -EEXIST) {
> > > >> +			btrfs_put_root(root);
> > > > I think this fix is correct, though it's not that clear. If you look how
> > > > the code changed, there was the unconditional put and then followed by a
> > > > free:
> > > >
> > > > 8c38938c7bb0 ("btrfs: move the root freeing stuff into btrfs_put_root")
> > > >
> > > > Here it's putting twice where one will be the final free.
> > > >
> > > > And then the whole refcounting gets updated in
> > > >
> > > > 4785e24fa5d2 ("btrfs: don't take an extra root ref at allocation time")
> > > >
> > > > which could be removing the wrong put, I'm not yet sure.
> > > 
> > > Thanks for the reply!
> > > 
> > > I think the bug should be introduced by this commit:
> > > bc44d7c4b2b1 ("btrfs: push btrfs_grab_fs_root into btrfs_get_fs_root")
> > > 
> > > This commit has a change:
> > >       ret = btrfs_insert_fs_root(fs_info, root);
> > >       if (ret) {
> > > +      btrfs_put_fs_root(root);
> > >           if (ret == -EEXIST) {
> > >               btrfs_free_fs_root(root);
> > >               goto again;
> > >           }
> > > 
> > > I could add a Fixes tag of this commit in my V2 patch.
> > > Is it okay?
> > 
> > I can add it myself, that's a minor thing. The fix is correct, I've
> > rewritten the changelog a bit, patch now added to misc-next, thanks.
> 
> Where is 'misc-next' please?

Nevermind.  I've just seen a) how old this thread is and b) the commit
subject was changed and subsequently committed as:

168a2f776b976 ("btrfs: fix root ref counts in error handling in btrfs_get_root_ref")

-- 
Lee Jones [李琼斯]