Re: [PATCH v2 1/6] spec: Recommend cap on NBD_REPLY_TYPE_BLOCK_STATUS length

[Eric Blake]

On Fri, Dec 16, 2022 at 10:32:01PM +0300, Vladimir Sementsov-Ogievskiy wrote:
> On 11/15/22 01:46, Eric Blake wrote:
> > The spec was silent on how many extents a server could reply with.
> > However, both qemu and nbdkit (the two server implementations known to
> > have implemented the NBD_CMD_BLOCK_STATUS extension) implement a hard
> > cap, and will truncate the amount of extents in a reply to avoid
> > sending a client a reply so large that the client would treat it as a
> > denial of service attack.  Clients currently have no way during
> > negotiation to request such a limit of the server, so it is easier to
> > just document this as a restriction on viable server implementations
> > than to add yet another round of handshaking.  Also, mentioning
> > amplification effects is worthwhile.
> > 
> > When qemu first implemented NBD_CMD_BLOCK_STATUS for the
> > base:allocation context (qemu commit e7b1948d51, Mar 2018), it behaved
> > as if NBD_CMD_FLAG_REQ_ONE were always passed by the client, and never
> > responded with more than one extent.  Later, when adding its
> > qemu:dirty-bitmap:XYZ context extension (qemu commit 3d068aff16, Jun
> > 2018), it added a cap to 128k extents (1M+4 bytes), and that cap was
> > applied to base:allocation once qemu started sending multiple extents
> > for that context as well (qemu commit fb7afc797e, Jul 2018).  Qemu
> > extents are never smaller than 512 bytes (other than an exception at
> > the end of a file whose size is not aligned to 512), but even so, a
> > request for just under 4G of block status could produce 8M extents,
> > resulting in a reply of 64M if it were not capped smaller.
> > 
> > When nbdkit first implemented NBD_CMD_BLOCK_STATUS (nbdkit 4ca66f70a5,
> > Mar 2019), it did not impose any restriction on the number of extents
> > in the reply chunk.  But because it allows extents as small as one
> > byte, it is easy to write a server that can amplify a client's request
> > of status over 1M of the image into a reply over 8M in size, and it
> > was very easy to demonstrate that a hard cap was needed to avoid
> > crashing clients or otherwise killing the connection (a bad server
> > impacting the client negatively).  So nbdkit enforced a bound of 1M
> > extents (8M+4 bytes, nbdkit commit 6e0dc839ea, Jun 2019).  [Unrelated
> > to this patch, but worth noting for history: nbdkit's situation also
> > has to deal with the fact that it is designed for plugin server
> > implementations; and not capping the number of extents in a reply also
> > posed a problem to nbdkit as the server, where a plugin could exhaust
> > memory and kill the server, unrelated to any size constraints enforced
> > by a client.]
> > 
> > Since the limit chosen by these two implementations is different, and
> > since nbdkit has versions that were not limited, add this as a SHOULD
> > NOT instead of MUST NOT constraint on servers implementing block
> > status.  It does not matter that qemu picked a smaller limit that it
> > truncates to, since we have already documented that the server may
> > truncate for other reasons (such as it being inefficient to collect
> > that many extents in the first place).  But documenting the limit now
> > becomes even more important in the face of a future addition of 64-bit
> > requests, where a client's request is no longer bounded to 4G and
> > could thereby produce even more than 8M extents for the corner case
> > when every 512 bytes is a new extent, if it were not for this
> > recommendation.
> 
> s-o-b line missed.

I'm not sure if the NBD project has a strict policy on including one,
but I don't mind adding it.

> 
> 
> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
> 
> -- 
> Best regards,
> Vladimir
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org