[PATCH net v3] ipv6: sr: enforce exact attribute length for SEG6_ATTR_DST

In seg6_genl_policy, SEG6_ATTR_DST is defined with .type = NLA_BINARY and
.len = sizeof(struct in6_addr). For NLA_BINARY, .len only enforces the
maximum payload length and permits shorter payloads (e.g., 0 bytes).
When seg6_genl_set_tunsrc() copies sizeof(struct in6_addr) bytes via
kmemdup(val, sizeof(*val), GFP_KERNEL), a short SEG6_ATTR_DST attribute
triggers a 16-byte out-of-bounds read past skb->tail into uninitialized
skb->head memory, which is stored in sdata->tun_src and leaked back to
userspace via SEG6_CMD_GET_TUNSRC.

Switch SEG6_ATTR_DST in seg6_genl_policy to
NLA_POLICY_EXACT_LEN(sizeof(struct in6_addr)) so that generic netlink
validation rejects any attribute whose length is not exactly
sizeof(struct in6_addr) with -ERANGE.

Tested in QEMU against Linux 7.3.0-rc3 by sending a SEG6_CMD_SET_TUNSRC
Generic Netlink message with a 0-byte SEG6_ATTR_DST attribute followed
by SEG6_CMD_GET_TUNSRC. On the unfixed kernel, SEG6_CMD_SET_TUNSRC
succeeds (err = 0) and SEG6_CMD_GET_TUNSRC leaks 16 bytes of
uninitialized kernel heap memory (tun_src =
836a61ecc4d25a1042a8d60411cfb378); with this patch applied,
SEG6_CMD_SET_TUNSRC is rejected by netlink policy validation with
-ERANGE (-34) and tun_src remains zeroed.

Fixes: 915d7e5e5930 ("ipv6: sr: add code base for control plane support of SR-IPv6")
Cc: stable@vger.kernel.org
Reviewed-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Reviewed-by: Hangbin Liu <redacted>
Assisted-by: LLM
Signed-off-by: Hui Peng <redacted>
---
Changes in v3:
- Add Reviewed-by tag from Andrea Mayer.
- Send as a fresh standalone thread (no In-Reply-To header) as requested by
  Jakub Kicinski.

Changes in v2:
- Drop the redundant nla_len(info->attrs[SEG6_ATTR_DST]) !=
  sizeof(struct in6_addr) check in seg6_genl_set_tunsrc() since
  NLA_POLICY_EXACT_LEN() in seg6_genl_policy already enforces the exact
  length, as pointed out by Hangbin Liu.

 net/ipv6/seg6.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/ipv6/seg6.c b/net/ipv6/seg6.c
index 62a7eb779202..8c2b156c227a 100644
--- a/net/ipv6/seg6.c
+++ b/net/ipv6/seg6.c

@@ -138,8 +138,8 @@ void seg6_icmp_srh(struct sk_buff *skb, struct inet6_skb_parm *opt)
 static struct genl_family seg6_genl_family;
 
 static const struct nla_policy seg6_genl_policy[SEG6_ATTR_MAX + 1] = {
-	[SEG6_ATTR_DST]				= { .type = NLA_BINARY,
-		.len = sizeof(struct in6_addr) },
+	[SEG6_ATTR_DST]				=
+		NLA_POLICY_EXACT_LEN(sizeof(struct in6_addr)),
 	[SEG6_ATTR_HMACKEYID]		= { .type = NLA_U32, },
 	[SEG6_ATTR_SECRET]			= { .type = NLA_BINARY, },