Re: [PATCH] btrfs: add a sanity check for btrfs root

[David Sterba <dsterba@suse.cz>]

On Fri, Oct 25, 2024 at 08:23:07PM +1030, Qu Wenruo wrote:
> 
> 
> 在 2024/10/25 15:25, Lizhi Xu 写道:
> > Syzbot report a null-ptr-deref in btrfs_search_slot.
> > It use the input logical can't find the extent root in extent_from_logical,
> > and triger the null-ptr-deref in btrfs_search_slot.
> > Add sanity check for btrfs root before using it in btrfs_search_slot.
> 
> Although I'd prefer to explain a little more about why the NULL root
> pointer can happen (caused by the rescue=all mount option), which can
> cause NULL root pointer for non-critical tree roots, like
> uuid/csum/extent or even device trees.
> 
> You don't need to bother sending an update.
> I can add such info when pushing to the maintainer's tree.
> 
> >
> > Reported-by: syzbot+3030e17bd57a73d39bd7@syzkaller.appspotmail.com
> > Closes: https://syzkaller.appspot.com/bug?extid=3030e17bd57a73d39bd7
> > Signed-off-by: Lizhi Xu <lizhi.xu@windriver.com>
> 
> Reviewed-by: Qu Wenruo <wqu@suse.com>

> > @@ -2023,6 +2023,10 @@ int btrfs_search_slot(struct btrfs_trans_handle *trans, struct btrfs_root *root,
> >   	int min_write_lock_level;
> >   	int prev_cmp;
> >
> > +	if (!root)
> > +		return -EINVAL;

The function returns errors indirectly so it's not clear which could be
ultimately returned. I did a quick search over the calls starting in
btrfs_search_slot() and it seems that EINVAL is not used so we'd know if
it ends up in some error report. The ones I found: EAGAIN, EIO, EUCLEAN,
ENOMEM.