[PATCH net] selftests: net: txtimestamp: fix out-of-bounds read in raw UDP checksum

[Jamie Nguyen <jamien@nvidia.com>]

With -r (SOCK_RAW, IPPROTO_UDP) the test writes only the UDP header and
payload, and the kernel adds the IP header. get_udp_csum() still expects
the IP source and destination addresses to directly precede the UDP
header, and reads 2 * alen bytes before the start of the buffer: 8 bytes
for IPv4, 32 bytes for IPv6.

ASan reports a heap-buffer-overflow for both -4 -r and -6 -r. Without
ASan the read usually goes unnoticed, but it faults when the buffer is
the first chunk in the heap. That happens on Ubuntu 26.04, where the
kselftest runner wraps the test in uutils stdbuf, and txtimestamp.sh
fails with SIGSEGV on every run.

Even without a crash, -r packets were sent with a checksum computed
over unrelated heap memory, so their UDP checksum was invalid.

Build the pseudo-header from the destination address and checksum only
the UDP header and payload. -R and -P already write daddr as both the
source and destination address, so their checksums are unchanged. With
-r the kernel selects the source address, which equals daddr for the
loopback addresses used by txtimestamp.sh.

Fixes: b52354aa068e ("selftests: expand txtimestamp with ipv6 dgram + raw and pf_packet")
Signed-off-by: Jamie Nguyen <jamien@nvidia.com>
---
 tools/testing/selftests/net/txtimestamp.c | 23 ++++++++++++++---------
 1 file changed, 14 insertions(+), 9 deletions(-)

diff --git a/tools/testing/selftests/net/txtimestamp.c b/tools/testing/selftests/net/txtimestamp.c
index 170be19..f7bdf30 100644
--- a/tools/testing/selftests/net/txtimestamp.c
+++ b/tools/testing/selftests/net/txtimestamp.c
@@ -418,19 +418,25 @@ static uint16_t get_ip_csum(const uint16_t *start, int num_words,
 	return ~sum;
 }
 
-static uint16_t get_udp_csum(const struct udphdr *udph, int alen)
+static uint16_t get_udp_csum(const struct udphdr *udph, bool is_ipv4)
 {
-	unsigned long pseudo_sum, csum_len;
-	const void *csum_start = udph;
+	const void *addr = is_ipv4 ? (void *)&daddr.sin_addr : &daddr6.sin6_addr;
+	int alen = is_ipv4 ? sizeof(struct in_addr) : sizeof(struct in6_addr);
+	uint16_t addr16[sizeof(struct in6_addr) / sizeof(uint16_t)];
+	unsigned long pseudo_sum;
+	int i;
 
 	pseudo_sum = htons(IPPROTO_UDP);
 	pseudo_sum += udph->len;
 
-	/* checksum ip(v6) addresses + udp header + payload */
-	csum_start -= alen * 2;
-	csum_len = ntohs(udph->len) + alen * 2;
+	/* pseudo-header source and destination: both are daddr */
+	memcpy(addr16, addr, alen);
+	for (i = 0; i < alen / 2; i++)
+		pseudo_sum += 2 * addr16[i];
 
-	return get_ip_csum(csum_start, csum_len >> 1, pseudo_sum);
+	/* checksum udp header + payload */
+	return get_ip_csum((const uint16_t *)udph, ntohs(udph->len) >> 1,
+			   pseudo_sum);
 }
 
 static int fill_header_ipv4(void *p)
@@ -479,8 +485,7 @@ static void fill_header_udp(void *p, bool is_ipv4)
 	udph->len    = ntohs(sizeof(*udph) + cfg_payload_len);
 	udph->check  = 0;
 
-	udph->check  = get_udp_csum(udph, is_ipv4 ? sizeof(struct in_addr) :
-						    sizeof(struct in6_addr));
+	udph->check  = get_udp_csum(udph, is_ipv4);
 }
 
 static void do_test(int family, unsigned int report_opt)
-- 
2.43.0