Re: [RFC] bio: fix page leak bio_add_hw_page failure

[Ming Lei <ming.lei@redhat.com>]

On Mon, Jul 19, 2021 at 06:06:49PM +0100, Pavel Begunkov wrote:
> On 7/19/21 4:34 PM, Ming Lei wrote:
> > On Mon, Jul 19, 2021 at 11:53:00AM +0100, Pavel Begunkov wrote:
> >> __bio_iov_append_get_pages() doesn't put not appended pages on
> >> bio_add_hw_page() failure, so potentially leaking them, fix it. Also, do
> >> the same for __bio_iov_iter_get_pages(), even though it looks like it
> >> can't be triggered by userspace in this case.
> >>
> >> Fixes: 0512a75b98f8 ("block: Introduce REQ_OP_ZONE_APPEND")
> >> Cc: stable@vger.kernel.org # 5.8+
> >> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
> >> ---
> >>
> >> I haven't tested the fail path, thus RFC. Would be great if someone can
> >> do it or take over the fix.
> >>
> >>  block/bio.c | 15 +++++++++++++--
> >>  1 file changed, 13 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/block/bio.c b/block/bio.c
> >> index 1fab762e079b..d95e3456ba0c 100644
> >> --- a/block/bio.c
> >> +++ b/block/bio.c
> >> @@ -979,6 +979,14 @@ static int bio_iov_bvec_set_append(struct bio *bio, struct iov_iter *iter)
> >>  	return 0;
> >>  }
> >>  
> >> +static void bio_put_pages(struct page **pages, size_t size, size_t off)
> >> +{
> >> +	size_t i, nr = DIV_ROUND_UP(size + (off & ~PAGE_MASK), PAGE_SIZE);
> >> +
> >> +	for (i = 0; i < nr; i++)
> >> +		put_page(pages[i]);
> >> +}
> >> +
> >>  #define PAGE_PTRS_PER_BVEC     (sizeof(struct bio_vec) / sizeof(struct page *))
> >>  
> >>  /**
> >> @@ -1023,8 +1031,10 @@ static int __bio_iov_iter_get_pages(struct bio *bio, struct iov_iter *iter)
> >>  			if (same_page)
> >>  				put_page(page);
> >>  		} else {
> >> -			if (WARN_ON_ONCE(bio_full(bio, len)))
> >> -                                return -EINVAL;
> >> +			if (WARN_ON_ONCE(bio_full(bio, len))) {
> >> +				bio_put_pages(pages + i, left, offset);
> >> +				return -EINVAL;
> >> +			}
> > 
> > It is unlikely to happen:
> > 
> >         unsigned short nr_pages = bio->bi_max_vecs - bio->bi_vcnt;
> >         struct bio_vec *bv = bio->bi_io_vec + bio->bi_vcnt;
> >         struct page **pages = (struct page **)bv;
> > 
> > 		pages += entries_left * (PAGE_PTRS_PER_BVEC - 1);
> > 		size = iov_iter_get_pages(iter, pages, LONG_MAX, nr_pages, &offset);
> 
> Agree, mentioned in the commit, however ...
> 
> >>  			__bio_add_page(bio, page, len, offset);
> >>  		}
> >>  		offset = 0;
> >> @@ -1069,6 +1079,7 @@ static int __bio_iov_append_get_pages(struct bio *bio, struct iov_iter *iter)
> >>  		len = min_t(size_t, PAGE_SIZE - offset, left);
> >>  		if (bio_add_hw_page(q, bio, page, len, offset,
> >>  				max_append_sectors, &same_page) != len) {
> >> +			bio_put_pages(pages + i, left, offset);
> > 
> > Same with above.
> 
> ... bio_add_hw_page() is more complex and additionally does checks
> against queue_max_zone_append_sectors(), queue_max_segments(), and
> queue_virt_boundary() in of bvec_gap_to_prev().
> 
> It may be unlikely, but are you sure that those are just safety
> checks? It's not so obvious to me, so would be great if you could
> point out the other place where the verification is done.

OK, bio_add_hw_page() is special, and it needs the handling, but 
__bio_iov_iter_get_pages() needn't that since it is so obvious.


Thanks,
Ming