[PATCH 2/4] dhcp: Make option parsing more robust, explicitly handle options 0 and 255

[Stefano Brivio <sbrivio@redhat.com>]

The initial option-scanning loop in dhcp(), so far, ignored options 0
(Pad Option, RFC 2132, Section 3.1) and 255 (End Option, RFC 2132,
Section 3.2).

As a result:

- if we ever encountered option 0 in the middle of option fields
  (never seen in practice), we would potentially terminate the loop
  too early, before scanning remaining options

- a malformed message with an option 255 followed by a length byte
  would (reliably) cause us to terminate as we would exceed the
  allocated size for the 'opts' array, which is detected as buffer
  overflow by the FORTIFY_SOURCE mechanism

The latter was reported as potential vulnerability by AISLE, but it's
not actually a vulnerability as we always terminate without carrying
on further handling, and in our security model the guest is able to
sabotage its own connectivity in any case (for example, a malformed
frame from the hypervisor would cause us to reset the connection, or
entirely flooding the flow table would cause inbound connectivity to
stop working, etc.).

The reported behaviour, however, is indeed a defect, as it affects
the functional robustness to a hypothetical issue in a DHCP client,
and that's something we definitely want to fix.

Make the option parsing loop more robust by:

- resizing 'opts' from 255 to 256 elements: there's no particular
  reason to try to save a tiny bit of memory (which shouldn't even
  be allocated in practice) instead of being defensive about it

- explicitly handle options 0 (skip one byte, continue) and 255 (stop
  processing options) in the option-scanning loop

This bug was found and an initial version of the patch was written by
the AISLE AI security scanning tool (https://aisle.com/platform).

Reported-by: AISLE
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
---
 dhcp.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/dhcp.c b/dhcp.c
index 1ff8cba..632019a 100644
--- a/dhcp.c
+++ b/dhcp.c
@@ -49,7 +49,7 @@ struct opt {
 	uint8_t c[255];
 };
 
-static struct opt opts[255];
+static struct opt opts[256];
 
 #define DHCPDISCOVER	1
 #define DHCPOFFER	2
@@ -374,6 +374,14 @@ int dhcp(const struct ctx *c, struct iov_tail *data)
 		if (!type || !olen)
 			return -1;
 
+		if (*type == 255)
+			break;
+
+		if (*type == 0) { /* Pad Option (RFC 2132, 3.1): one byte */
+			opt_len--;
+			continue;
+		}
+
 		opt_len = iov_tail_size(data);
 		if (opt_len < *olen)
 			return -1;
-- 
2.43.0