Re: [PATCH net v4] ipv6: sr: enforce exact attribute length for SEG6_ATTR_DST

On Thu, 24 Sep 2026 09:36:10 +0000
Hui Peng [off-list ref] wrote:

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; 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>
Reviewed-by: Breno Leitao <leitao@debian.org>
Assisted-by: LLM
Signed-off-by: Hui Peng <redacted>
---
Changes in v4:
- Drop raw hex memory dump string from commit message as suggested by Breno
  Leitao.
- Add Reviewed-by tag from Breno Leitao.

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(-)

The v2 was applied to netdev/net.git (main) as commit 2d959c75c27f
("ipv6: sr: enforce exact attribute length for SEG6_ATTR_DST"):

https://git.kernel.org/netdev/net/c/2d959c75c27f90e9ec489d18ce5ee6b852ad4741

One general note for your future postings, not about this patch which is
already applied: please do not repost a patch less than 24 hours after
the previous posting. See the "Resending after review" section of
Documentation/process/maintainer-netdev.rst:

https://docs.kernel.org/process/maintainer-netdev.html#resending-after-review

Ciao,
Andrea