On Wed, Aug 02, 2023 at 08:50:21PM -0500, Eric Blake wrote:
> Previously, we had not been doing any validation of server extent
> responses, which means a client query at an offset near the end of the
> export can result in a buggy server sending a response longer than the
> export length and potentially confusing the client. The NBD spec also
> says that an extent length should be non-zero so that a successful
> block status call makes progress. It is easy enough to track that the
> server has not overflowed the export size, and that we ensure an error
> on no progress even when the buggy server claims success. Since the
> spec says a client should be prepared for a block status result to be
> truncated, the client should not care whether the truncation happened
> at the server or at libnbd after validating the server's response.
>
> In the process, this patch reorganizes some of the code so that early
> exits are obvious, leading for less indentation in the success path.
>
> Adding this sanity checking now makes it easier for future patches to
> do orthogonal support for a server's 32- or 64-bit reply, vs. a
> client's 32- or 64-bit API call. Once 64-bit replies are in play, we
> will additionally have to worry about a 64-bit reply that overflows a
> 32-bit API callback without exceeding the exportsize. Similarly,
> since nbd_get_size() already caps export size at 63 bits (based on
> off_t limitations), we have guaranteed that a 64-bit API callback will
> never see an extent length that could appear negative in a 64-bit
> signed type (at least OCaml benefits from that guarantee, since its
> only native 64-bit integer type is signed).
>
> Signed-off-by: Eric Blake <ebl...@redhat.com>
> ---
>
> v4: new patch
> ---
> generator/states-reply-chunk.c | 78 +++++++++++++++++++++++++---------
> 1 file changed, 58 insertions(+), 20 deletions(-)
>
> diff --git a/generator/states-reply-chunk.c b/generator/states-reply-chunk.c
> index 17bb5149..735f9456 100644
> --- a/generator/states-reply-chunk.c
> +++ b/generator/states-reply-chunk.c
> @@ -461,6 +461,11 @@ REPLY.CHUNK_REPLY.RECV_BS_ENTRIES:
> struct command *cmd = h->reply_cmd;
> size_t i;
> uint32_t context_id;
> + int error;
> + const char *name;
> + uint32_t orig_len, len, flags;
> + uint64_t total, cap;
> + bool stop;
>
> switch (recv_into_rbuf (h)) {
> case -1: SET_NEXT_STATE (%.DEAD); return 0;
> @@ -481,30 +486,63 @@ REPLY.CHUNK_REPLY.RECV_BS_ENTRIES:
> if (context_id == h->meta_contexts.ptr[i].context_id)
> break;
>
> - if (i < h->meta_contexts.len) {
> - int error = cmd->error;
> - const char *name = h->meta_contexts.ptr[i].name;
> -
> - /* Need to byte-swap the entries returned, but apart from that
> - * we don't validate them. Yes, our 32-bit public API foolishly
> - * tracks the number of uint32_t instead of block descriptors;
> - * see _block_desc_is_multiple_of_bs_entry above.
> - */
> - for (i = 0; i < h->bs_count * 2; ++i)
> - h->bs_entries[i] = be32toh (h->bs_entries[i]);
> -
> - /* Call the caller's extent function. */
> - if (CALL_CALLBACK (cmd->cb.fn.extent, name, cmd->offset,
> - h->bs_entries, h->bs_count * 2, &error) == -1)
> - if (cmd->error == 0)
> - cmd->error = error ? error : EPROTO;
> - }
> - else
> + SET_NEXT_STATE (%FINISH);
> + if (i == h->meta_contexts.len) {
> /* Emit a debug message, but ignore it. */
> debug (h, "server sent unexpected meta context ID %" PRIu32,
> context_id);
> + break;
> + }
>
> - SET_NEXT_STATE (%FINISH);
> + name = h->meta_contexts.ptr[i].name;
> + total = 0;
> + cap = h->exportsize - cmd->offset;
> + assert (cap <= h->exportsize);
> +
> + /* Need to byte-swap the entries returned. The NBD protocol
> + * allows truncation as long as progress is made; the client
> + * cannot tell the difference between a server's truncation or if
> + * we truncate on a length we don't like. We stop iterating on a
> + * zero-length extent (error only if it is the first extent), and
> + * on an extent beyond the exportsize (unconditional error after
> + * truncating to exportsize); but do not diagnose issues with the
> + * server's length alignments, flag values, nor compliance with
> + * the REQ_ONE command flag.
> + */
> + for (i = 0, stop = false; i < h->bs_count && !stop; ++i) {
> + orig_len = len = be32toh (h->bs_entries[i * 2]);
> + flags = be32toh (h->bs_entries[i * 2 + 1]);
> + total += len;
> + if (len == 0) {
> + stop = true;
> + if (i > 0)
> + break; /* Skip this and later extents; we already made progress */
> + /* Expose this extent as an error; we made no progress */
> + cmd->error = cmd->error ? : EPROTO;
> + }
> + else if (total > cap) {
> + /* Expose this extent as an error, after truncating to make progress
> */
> + stop = true;
> + cmd->error = cmd->error ? : EPROTO;
> + len -= total - cap;
> + }
> + h->bs_entries[i * 2] = len;
> + h->bs_entries[i * 2 + 1] = flags;
> + }
> +
> + /* Call the caller's extent function. Yes, our 32-bit public API
> + * foolishly tracks the number of uint32_t instead of block
> + * descriptors; see _block_desc_is_multiple_of_bs_entry above.
> + */
> + if (stop)
> + debug (h, "truncating server's response at unexpected extent length %"
> + PRIu32 " and total %" PRIu64 " near extent %zu",
> + orig_len, total, i);
> + error = cmd->error;
> + if (CALL_CALLBACK (cmd->cb.fn.extent, name, cmd->offset,
> + h->bs_entries, i * 2, &error) == -1)
> + if (cmd->error == 0)
> + cmd->error = error ? error : EPROTO;
> }
> return 0;
>
Acked-by: Richard W.M. Jones <rjo...@redhat.com>
... although I wonder if this might break something. I think it's
possible for an nbdkit plugin to return a zero length extent, for
example if it has a simplistic internal model of regions of the disk.
Since the client can still make progress if at least the total length
of extents returned is > 0 I'm fairly sure this would work right now.
Rich.
--
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
nbdkit - Flexible, fast NBD server with plugins
https://gitlab.com/nbdkit/nbdkit
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