Re: [PATCH 10/11] tap: Discard guest data on length descriptor mismatch

[David Gibson <david@gibson.dropbear.id.au>]

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On Wed, Jul 24, 2024 at 11:50:16PM +0200, Stefano Brivio wrote:
> This was reported by Matej a while ago, but we forgot to fix it. Even
> if the hypervisor is necessarily trusted by passt, as it can in any
> case terminate the guest or disrupt guest connectivity, it's a good
> idea to be robust against possible issues.
> 
> Instead of resetting the connection to the hypervisor, just discard
> the data we read with a single recv(), as we had a few cases where
> QEMU would get the length descriptor wrong, in the past.
> 
> While at it, change l2len in tap_handler_passt() to uint32_t, as the
> length descriptor is logically unsigned and 32-bit wide.
> 
> Reported-by: Matej Hrica <mhrica@redhat.com>
> Suggested-by: Matej Hrica <mhrica@redhat.com>
> Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
> ---
>  tap.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/tap.c b/tap.c
> index 44bd444..62ba6a4 100644
> --- a/tap.c
> +++ b/tap.c
> @@ -1011,15 +1011,18 @@ redo:
>  	}
>  
>  	while (n > (ssize_t)sizeof(uint32_t)) {
> -		ssize_t l2len = ntohl(*(uint32_t *)p);
> +		uint32_t l2len = ntohl(*(uint32_t *)p);
>  
>  		p += sizeof(uint32_t);
>  		n -= sizeof(uint32_t);
>  
> +		if (l2len > (ssize_t)TAP_BUF_BYTES - n)
> +			return;

Neither the condition nor the action makes much sense to me here.
We're testing if the frame can fit in the the remaining buffer space.
But we may have already read part (or all) of the frame - i.e. it's
included in 'n'.  So I don't see how that condition is useful.

Then, simply returning doesn't seem right under pretty much any
circumstances - that discards some amount of data, and leaves us in an
unsynchronized state w.r.t. the frame boundaries.

If this is just supposed to be a sanity check on the frame length,
then I think we'd be better off with a fixed limit - 64kiB is the
obvious choice.  If we hit that, we can warn() and discard data up to
the end of the too-large frame.  That at least has a chance of letting
us recover and move on to future acceptable frames.

>  		/* At most one packet might not fit in a single read, and this
>  		 * needs to be blocking.
>  		 */
> -		if (l2len > n) {
> +		if (l2len > (size_t)n) {
>  			rem = recv(c->fd_tap, p + n, l2len - n, 0);
>  			if ((n += rem) != l2len)
>  				return;

Pre-existing, but a 'return' here basically lands us in a situation we
have no meaningful chance of recovering from.  A die() would be
preferable.  Better yet would be continuing to re-recv() until we have
the whole frame, similar to what we do for write_remainder().

> @@ -1028,8 +1031,7 @@ redo:
>  		/* Complete the partial read above before discarding a malformed
>  		 * frame, otherwise the stream will be inconsistent.
>  		 */
> -		if (l2len < (ssize_t)sizeof(struct ethhdr) ||
> -		    l2len > (ssize_t)ETH_MAX_MTU)
> +		if (l2len < sizeof(struct ethhdr) || l2len > ETH_MAX_MTU)
>  			goto next;
>  		tap_add_packet(c, l2len, p);

-- 
David Gibson (he or they)	| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you, not the other way
				| around.
http://www.ozlabs.org/~dgibson

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