The NBD spec suggests that a server should never advertise a size inconsistent with its minimum block alignment, as that tail is effectively inaccessible to a compliant client obeying those block constraints. Since we have a habit of rounding up rather than truncating, to avoid losing the last few bytes of user input, and we cannot access the tail when the server advertises bogus block sizing, abort the connection to alert the server to fix their bug. And rejecting such servers matches what we already did for a min_block that was not a power of 2 or which was larger than max_block.
Does not impact either qemu (which always sends properly aligned sizes) or nbdkit (which does not send minimum block requirements yet); so this is mostly aimed at new NBD server implementations, and ensures that the rest of our code can assume the size is aligned. Signed-off-by: Eric Blake <ebl...@redhat.com> Message-Id: <20190329163407.26919-1-ebl...@redhat.com> --- This is the rewrite of the original 7/6 that I posted, retitled and hoisted earlier in the series (since Vladimir was right that 4/6 depends on it). I've added patches 1-3 to my NBD queue for a pull request on Monday as they now have R-b; I'd love to also have enough reviews for 3.5, 4, 5, 6, and also another thread on fixing 'qemu-img info' output, to include those as well. nbd/client.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/nbd/client.c b/nbd/client.c index de7da48246b..9f5eb79930c 100644 --- a/nbd/client.c +++ b/nbd/client.c @@ -426,6 +426,14 @@ static int nbd_opt_info_or_go(QIOChannel *ioc, uint32_t opt, nbd_send_opt_abort(ioc); return -1; } + if (info->min_block && + !QEMU_IS_ALIGNED(info->size, info->min_block)) { + error_setg(errp, "server size %" PRIu64 "is not multiple of " + "minimum block size %" PRIu32, info->size, + info->min_block); + nbd_send_opt_abort(ioc); + return -1; + } trace_nbd_receive_negotiate_size_flags(info->size, info->flags); break; -- 2.20.1