block: fix sanity checks in blk_rq_map_user_bvec

blk_rq_map_user_bvec contains a check bytes + bv->bv_len > nr_iter which
causes unnecessary failures in NVMe passthrough I/O, reproducible as
follows:

- register a 2 page, page-aligned buffer against a ring
- use that buffer to do a 1 page io_uring NVMe passthrough read

The second (i = 1) iteration of the loop in blk_rq_map_user_bvec will
then have nr_iter == 1 page, bytes == 1 page, bv->bv_len == 1 page, so
the check bytes + bv->bv_len > nr_iter will succeed, causing the I/O to
fail. This failure is unnecessary, as when the check succeeds, it means
we've checked the entire buffer that will be used by the request - i.e.
blk_rq_map_user_bvec should complete successfully. Therefore, terminate
the loop early and return successfully when the check bytes + bv->bv_len
> nr_iter succeeds.

While we're at it, also remove the check that all segments in the bvec
are single-page. While this seems to be true for all users of the
function, it doesn't appear to be required anywhere downstream.

CC: [email protected]
Signed-off-by: Xinyu Zhang <[email protected]>
Co-developed-by: Uday Shankar <[email protected]>
Signed-off-by: Uday Shankar <[email protected]>
Fixes: 3798754 ("block: extend functionality to map bvec iterator")
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

Author: Xinyu Zhang

block/blk-map.c

@@ -600,9 +600,7 @@ static int blk_rq_map_user_bvec(struct request *rq, const struct iov_iter *iter)
 		if (nsegs >= nr_segs || bytes > UINT_MAX - bv->bv_len)
 			goto put_bio;
 		if (bytes + bv->bv_len > nr_iter)
-			goto put_bio;
-		if (bv->bv_offset + bv->bv_len > PAGE_SIZE)
-			goto put_bio;
+			break;
 
 		nsegs++;
 		bytes += bv->bv_len;