Re: [PATCH] btrfs: fix a NULL pointer dereference

[Su Yue]

On 2019/3/14 4:02 PM, Qu Wenruo wrote:
> On 2019/3/14 äå3:54, Nikolay Borisov wrote:
> > On 14.03.19 Ð. 9:50 Ñ., Kangjie Lu wrote:
> > > btrfs_lookup_block_group may fail and return NULL. The fix goes
> > > to out when it fails to avoid NULL pointer dereference.
> >
> > Actually no, in this case btrfs_lookup_block_group must never fail
> > because if we have an allocated eb then it must have been allocated from
> > a bg.
>
> Yep, that's the normal case.
>
> However I'm wondering if it's possible to get a bad eb which is cached.
>
> Then we could hit such situation.
>
> So I still believe being safe here still makes sense, especially who
> knows future fuzzed image will be.

Plus one.
Personally, I'd rather like the version 1.

Thanks,
Su
> Thanks,
> Qu
>
> > > Signed-off-by: Kangjie Lu <kjlu@xxxxxxx>
> > > ---
> > >   fs/btrfs/extent-tree.c | 2 ++
> > >   1 file changed, 2 insertions(+)
> > >
> > > diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> > > index 994f0cc41799..b1e7985bcb9d 100644
> > > --- a/fs/btrfs/extent-tree.c
> > > +++ b/fs/btrfs/extent-tree.c
> > > @@ -7303,6 +7303,8 @@ void btrfs_free_tree_block(struct btrfs_trans_handle *trans,
> > >   
> > >   		pin = 0;
> > >   		cache = btrfs_lookup_block_group(fs_info, buf->start);
> > > +		if (!cache)
> > > +			goto out;
> > >   
> > >   		if (btrfs_header_flag(buf, BTRFS_HEADER_FLAG_WRITTEN)) {
> > >   			pin_down_extent(fs_info, cache, buf->start,