CVE-2026-74598: ipv6: fix Route Information option length validation

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

ipv6: fix Route Information option length validation

rt6_route_rcv() validates the Route Information option (RFC 4191) length
against the prefix length, but both checks are off by one.

rinfo->length is the ND option length in units of 8 octets and it
*includes* the 8-byte option header, so an option carrying N bytes of
prefix has length == 1 + N/8.  RFC 4191 section 2.3 requires length 3
when Prefix Length is greater than 64, and 2 or 3 when it is greater
than 0.  The code accepts length >= 2 and length >= 1 respectively.

ipv6_addr_prefix() then copies prefix_len/8 bytes out of rinfo->prefix,
so a Router Advertisement with (prefix_len=128, length=2) or
(prefix_len=64, length=1) makes the kernel read up to 8 bytes past the
end of the option.  Those bytes end up in the prefix of the route that
gets installed, so they are visible to userspace:

  # RA with a Route Information option (prefix_len=128, length=2)
  # followed by a source link-layer address option, 01 01 de ad be ef ca fe
  $ ip -6 route show
  2001:db8:dead:beef:101:dead:beef:cafe via fe80::1234 dev veth0 proto ra
                     ^^^^^^^^^^^^^^^^^^ the next option, read out of bounds

When the Route Information option is the last one in the packet, those
eight bytes come from the skb tail room instead.

Reject the option lengths RFC 4191 does not allow.

The Linux kernel CVE team has assigned CVE-2026-74598 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 2.6.17 with commit 70ceb4f53929f73746be72f73707cd9f8753e2fc and fixed in 5.10.265 with commit 2f6f94eda12430fb41b24b44a71e2ea4e93561d7
	Issue introduced in 2.6.17 with commit 70ceb4f53929f73746be72f73707cd9f8753e2fc and fixed in 5.15.216 with commit 7eac87396c44a312be457ef41d4c5687883be9a2
	Issue introduced in 2.6.17 with commit 70ceb4f53929f73746be72f73707cd9f8753e2fc and fixed in 6.1.183 with commit 7309529f257ae18e72112ef9f614edfd6df229bb
	Issue introduced in 2.6.17 with commit 70ceb4f53929f73746be72f73707cd9f8753e2fc and fixed in 6.6.152 with commit 0b9e02f3bd31c888f2ccdc0ca08e546d6abe9c4d
	Issue introduced in 2.6.17 with commit 70ceb4f53929f73746be72f73707cd9f8753e2fc and fixed in 6.12.104 with commit ff3cb05289b8a4ef95fa7ea14c7d34818359edbb
	Issue introduced in 2.6.17 with commit 70ceb4f53929f73746be72f73707cd9f8753e2fc and fixed in 6.18.45 with commit 3b2231e358d26e3aec5d8040b1fb777af03c5f05
	Issue introduced in 2.6.17 with commit 70ceb4f53929f73746be72f73707cd9f8753e2fc and fixed in 7.1.9 with commit da64ed1f346ba84df574d6469fa2e422b2511719
	Issue introduced in 2.6.17 with commit 70ceb4f53929f73746be72f73707cd9f8753e2fc and fixed in 7.2 with commit d1ad8fb2ac6a1afb71dc22d9ae8efb4dda96c824

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-74598
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	net/ipv6/route.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/2f6f94eda12430fb41b24b44a71e2ea4e93561d7
	https://git.kernel.org/stable/c/7eac87396c44a312be457ef41d4c5687883be9a2
	https://git.kernel.org/stable/c/7309529f257ae18e72112ef9f614edfd6df229bb
	https://git.kernel.org/stable/c/0b9e02f3bd31c888f2ccdc0ca08e546d6abe9c4d
	https://git.kernel.org/stable/c/ff3cb05289b8a4ef95fa7ea14c7d34818359edbb
	https://git.kernel.org/stable/c/3b2231e358d26e3aec5d8040b1fb777af03c5f05
	https://git.kernel.org/stable/c/da64ed1f346ba84df574d6469fa2e422b2511719
	https://git.kernel.org/stable/c/d1ad8fb2ac6a1afb71dc22d9ae8efb4dda96c824