Re: [PATCH] fs: btrfs: fix possible use-after-free bug in error handling code of btrfs_get_root_ref()

Lee Jones
2023-08-23T08:09:50Z

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]