Re: [PATCH] bcache: fix NULL pointer deref in blk_add_request_payload

Lars Ellenberg
2014-02-19T13:19:01Z

On Wed, Feb 19, 2014 at 01:07:14PM +0000, Lars Ellenberg wrote:
> bch_generic_make_request_hack() tries to be smart,
> and fake a bi_max_bvecs = bi_vcnt.

I meant to include a cover letter with the Oops backtrace,
and this:

Note that this patch is relevant for 3.13 and before.

With 3.14,
this whole bch_generic_make_request_hack() function has been dropped already.

Thanks,

Lars

> If those bios have been REQ_DISCARD, and get submitted to a driver
> (md raid) that uses bio_clone, the clone will end up with bi_io_vec == NULL,
> passed down the stack, end up in sd_prep_fn and blk_add_request_payload,
> which then tries to use bio->bi_io_vec->page.
>
> Fix: try to be even smarter in bch_generic_make_request_hack(),
> and always pretend to have at least bi_max_vecs of 1,
> unless the incoming bio was already created without a single bvec.
>
> Signed-off-by: Lars Ellenberg <lars@linbit.com>
> ---
> drivers/md/bcache/io.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/md/bcache/io.c b/drivers/md/bcache/io.c
> index 9056632..a638ac6 100644
> --- a/drivers/md/bcache/io.c
> +++ b/drivers/md/bcache/io.c
> @@ -47,7 +47,7 @@ static void bch_generic_make_request_hack(struct bio *bio)
> *
> * To be taken out once immutable bvec stuff is in.
> */
> - bio->bi_max_vecs = bio->bi_vcnt;
> + bio->bi_max_vecs = bio->bi_vcnt ?: (bio->bi_io_vec ? 1 : 0);
>
> generic_make_request(bio);
> }
> --
> 1.7.10.4