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

[David Sterba]

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.