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

[Kangjie Lu]

On 3/14/19 4:15 AM, Qu Wenruo wrote:
> On 2019/3/14 äå4:03, Nikolay Borisov wrote:
> > On 14.03.19 Ð. 10:02 Ñ., 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.
> > Then I'd rather have ASSERT(cache)
> Isn't assert() a bad idea for production build without assert() support?

I also agree with that, in general, assert should not be used in

production runs. The first patch might be better.


> Thanks,
> Qu
>
> > > 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,