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Date:   Wed, 22 Mar 2023 21:23:27 +0800
From:   Ming Lei <ming.lei@redhat.com>
To:     Eric Blake <eblake@redhat.com>
Cc:     josef@toxicpanda.com, linux-block@vger.kernel.org,
        nbd@other.debian.org, philipp.reisner@linbit.com,
        lars.ellenberg@linbit.com, christoph.boehmwalder@linbit.com,
        corbet@lwn.net, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, ming.lei@redhat.com
Subject: Re: [PATCH v2 2/5] block nbd: send handle in network order
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On Wed, Mar 22, 2023 at 07:29:21AM -0500, Eric Blake wrote:
> On Wed, Mar 22, 2023 at 08:47:41AM +0800, Ming Lei wrote:
> > On Tue, Mar 21, 2023 at 08:59:00AM -0500, Eric Blake wrote:
> > > On Tue, Mar 21, 2023 at 07:20:33AM +0800, Ming Lei wrote:
> > > > On Fri, Mar 17, 2023 at 03:27:46PM -0500, Eric Blake wrote:
> > > > > The NBD spec says the client handle (or cookie) is opaque on the
> > > > > server, and therefore it really doesn't matter what endianness we use;
> > > > > to date, the use of memcpy() between u64 and a char[8] has exposed
> > > > > native endianness when treating the handle as a 64-bit number.
> > > > 
> > > > No, memcpy() works fine for char[8], which doesn't break endianness.
> > > 
> > > I didn't say memcpy() breaks endianness, I said it preserves it.  By
> > > using memcpy(), you are exposing native endianness over the wire.
> > > Thus, even though a server should not be making any decisions based on
> > > the content of the handle (it is an opaque value handed back to the
> > > client unchanged), the current kernel client code DOES leak through
> > > information about whether the client is big- or little-endian;
> > 
> > How is the client cpu endianness leaked with handle defined as char[8]?
> > 
> > Suppose it is leaked, is it really one issue? Cause most of CPUs in
> > the world is little-endian.
> > 
> > > contrast to the NBD protocol saying that ALL data is
> > > network-byte-order.
> > 
> > That doesn't make sense for any data defined as char[] or byte which
> > needn't to be little or big endian.
> 
> The NBD spec defines it as a 64-bit opaque quantity - that does not
> indicate whether it is a single integer or 8 characters, but because
> it is opaque, we don't have to care.  However, if we DO treat it as an
> integer (and the kernel client code DOES do that: internally, it is
> building up a u64 integer), it is wise to consider network endianness.

That depends on if it is reasonable to convert to int.

> 
> > 
> > > 
> > > > 
> > > > > However, since NBD protocol documents that everything else is in
> > > > > network order, and tools like Wireshark will dump even the contents of
> > > > > the handle as seen over the network, it's worth using a consistent
> > > > > ordering regardless of the native endianness.
> > > > > 
> > > > > Plus, using a consistent endianness now allows an upcoming patch to
> > > > > simplify this to directly use integer assignment instead of memcpy().
> > > > 
> > > > It isn't necessary, given ->handle is actually u64, which is handled by
> > > > nbd client only.
> > > 
> > > No, re-read the whole series.  ->handle is actually char[8].  Later in
> > > the series adds ->cookie as __be64 as an alias to ->handle, precisely
> > > so that we are converting the u64 'handle' in kernel code into a
> > > big-endian value on the wire, regardless of the host type, and making
> > > it impossible for a server to inspect the wire data and learn the
> > > kernel's endianness.
> > 
> > How does server learn the client cpu endianness in this way? Is it really
> > one issue?
> 
> Not a security issue, merely a consistency one.  A server that
> inspects the handles being sent by the client, and checks whether they
> are sequential when treated as a big- or little-endian number, can
> infer whether the client is little-endian.  But there is nothing
> useful it can do with that knowledge.  Rather, the consistency factor
> is that if you have a wireshark plugin reading network traffic, and
> are trying to correlate it back to kernel traces, it is NICE if the
> wireshark plugin can display the SAME u64 number as the kernel was
> sticking into the field - and the way to do that is to have a fixed
> endianness of the u64 value over the wire.

OK, so the real motivation is only for aligning wireshark output with nbd
trace event. If yes, please add it in comment log.

BTW, the nbd trace event can be converted to any format by bcc or bpftrace
script, then you still can associate one with another.

> 
> > 
> > > 
> > > > 
> > > > > 
> > > > > Signed-off-by: Eric Blake <eblake@redhat.com>
> > > > > 
> > > > > ---
> > > > > v2: new patch
> > > > > ---
> > > > >  drivers/block/nbd.c | 10 +++++++---
> > > > >  1 file changed, 7 insertions(+), 3 deletions(-)
> > > > > 
> > > > > diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> > > > > index 592cfa8b765a..8a9487e79f1c 100644
> > > > > --- a/drivers/block/nbd.c
> > > > > +++ b/drivers/block/nbd.c
> > > > > @@ -560,6 +560,7 @@ static int nbd_send_cmd(struct nbd_device *nbd, struct nbd_cmd *cmd, int index)
> > > > >  	unsigned long size = blk_rq_bytes(req);
> > > > >  	struct bio *bio;
> > > > >  	u64 handle;
> > > > > +	__be64 tmp;
> > > > >  	u32 type;
> > > > >  	u32 nbd_cmd_flags = 0;
> > > > >  	int sent = nsock->sent, skip = 0;
> > > > > @@ -606,7 +607,8 @@ static int nbd_send_cmd(struct nbd_device *nbd, struct nbd_cmd *cmd, int index)
> > > > >  		request.len = htonl(size);
> > > > >  	}
> > > > >  	handle = nbd_cmd_handle(cmd);
> > > > > -	memcpy(request.handle, &handle, sizeof(handle));
> > > > > +	tmp = cpu_to_be64(handle);
> > > > > +	memcpy(request.handle, &tmp, sizeof(tmp));
> > > > 
> > > > This way copies handle two times, really not fun.
> > > 
> > > Indeed.  And as mentioned in the commit message, it is temporary; the
> > > second copy goes away later in the series once we can use direct
> > > integer assignment.
> > 
> > Then please merge with following patch, given it is hard to review
> > temporary change.
> 
> The underlying reason I split this patch out is that in v1 I got
> complaints that I was not taking endianness into account.  The patch
> series DOES cause an observable change (namely, a little-endian client
> now sends a value in big-endian order that it used to send in
> little-endian order) - but the change is harmless.  But if you want me
> to squash this patch back with 4/5 in v3, I'm happy to do that.
> 
> Are there any other comments on this series that I should consider
> before spending time putting out a v3?

I think 2~4 should be merged to single patch.

Thanks,
Ming