Re: [f2fs-dev] [PATCH 3/4] f2fs: fix NULL pointer dereference in f2fs_verity_work()

[Chao Yu]

On 2020-3-21 3:11, Eric Biggers wrote:
> On Thu, Mar 19, 2020 at 07:57:59PM +0800, Chao Yu wrote:
> > If both compression and fsverity feature is on, generic/572 will
> > report below NULL pointer dereference bug.
> >
> >  BUG: kernel NULL pointer dereference, address: 0000000000000018
> >  RIP: 0010:f2fs_verity_work+0x60/0x90 [f2fs]
> >  #PF: supervisor read access in kernel mode
> >  Workqueue: fsverity_read_queue f2fs_verity_work [f2fs]
> >  RIP: 0010:f2fs_verity_work+0x60/0x90 [f2fs]
> >  Call Trace:
> >   process_one_work+0x16c/0x3f0
> >   worker_thread+0x4c/0x440
> >   ? rescuer_thread+0x350/0x350
> >   kthread+0xf8/0x130
> >   ? kthread_unpark+0x70/0x70
> >   ret_from_fork+0x35/0x40
> >
> > There are two issue in f2fs_verity_work():
> > - it needs to traverse and verify all pages in bio.
> > - if pages in bio belong to non-compressed cluster, accessing
> > decompress IO context stored in page private will cause NULL
> > pointer dereference.
> >
> > Fix them.
> >
> > Signed-off-by: Chao Yu <yuchao0@xxxxxxxxxx>
> > ---
> >  fs/f2fs/data.c | 35 ++++++++++++++++++++++++++++++-----
> >  1 file changed, 30 insertions(+), 5 deletions(-)
> >
> > diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
> > index 5c5db09324b7..66e49fc1056e 100644
> > --- a/fs/f2fs/data.c
> > +++ b/fs/f2fs/data.c
> > @@ -187,12 +187,37 @@ static void f2fs_verify_pages(struct page **rpages, unsigned int cluster_size)
> >
> >  static void f2fs_verify_bio(struct bio *bio)
> >  {
> > -	struct page *page = bio_first_page_all(bio);
> > -	struct decompress_io_ctx *dic =
> > -			(struct decompress_io_ctx *)page_private(page);
> > +	struct bio_vec *bv;
> > +	struct bvec_iter_all iter_all;
> > +	struct decompress_io_ctx *dic, *pdic = NULL;
> > +
> > +	bio_for_each_segment_all(bv, bio, iter_all) {
> > +		struct page *page = bv->bv_page;
> > +
> > +		dic = (struct decompress_io_ctx *)page_private(page);
> > +
> > +		if (dic) {
> > +			if (dic != pdic) {
> > +				f2fs_verify_pages(dic->rpages,
> > +							dic->cluster_size);
> > +				f2fs_free_dic(dic);
> > +				pdic = dic;
> > +			}
> > +			continue;
> > +		}
> > +		pdic = dic;
> >
> > -	f2fs_verify_pages(dic->rpages, dic->cluster_size);
> > -	f2fs_free_dic(dic);
> > +		if (bio->bi_status || PageError(page)) {
> > +			ClearPageUptodate(page);
> > +			ClearPageError(page);
> > +		} else {
> > +			if (fsverity_verify_page(page))
> > +				SetPageUptodate(page);
> > +			else
> > +				SetPageError(page);
> > +		}
> > +		unlock_page(page);
> > +	}
>
> I'm a bit confused why you added SetPageError() before unlocking the page.
> The other error paths actually clear the Error flag, not set it.  I thought
> there's a reason for that?

These codes were copy&paste from f2fs_decompress_end_io(), I guess I missed to 
check that logic, so anyway, we need another patch to fix that first.

Thanks for pointing out this.

Thanks,

> - Eric
>
>
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