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

ipv6: fix memory leak in fib6_rule_suppress

The kernel leaks memory when a `fib` rule is present in IPv6 nftables
firewall rules and a suppress_prefix rule is present in the IPv6 routing
rules (used by certain tools such as wg-quick). In such scenarios, every
incoming packet will leak an allocation in `ip6_dst_cache` slab cache.

After some hours of `bpftrace`-ing and source code reading, I tracked
down the issue to ca7a03c41753 ("ipv6: do not free rt if
FIB_LOOKUP_NOREF is set on suppress rule").

The problem with that change is that the generic `args->flags` always have
`FIB_LOOKUP_NOREF` set[1][2] but the IPv6-specific flag
`RT6_LOOKUP_F_DST_NOREF` might not be, leading to `fib6_rule_suppress` not
decreasing the refcount when needed.

How to reproduce:
 - Add the following nftables rule to a prerouting chain:
     meta nfproto ipv6 fib saddr . mark . iif oif missing drop
   This can be done with:
     sudo nft create table inet test
     sudo nft create chain inet test test_chain '{ type filter hook prerouting priority filter + 10; policy accept; }'
     sudo nft add rule inet test test_chain meta nfproto ipv6 fib saddr . mark . iif oif missing drop
 - Run:
     sudo ip -6 rule add table main suppress_prefixlength 0
 - Watch `sudo slabtop -o | grep ip6_dst_cache` to see memory usage increase
   with every incoming ipv6 packet.

This patch exposes the protocol-specific flags to the protocol
specific `suppress` function, and check the protocol-specific `flags`
argument for RT6_LOOKUP_F_DST_NOREF instead of the generic
FIB_LOOKUP_NOREF when decreasing the refcount, like this.

[1]: https://github.com/torvalds/linux/blob/ca7a03c4175366a92cee0ccc4fec0038c3266e26/net/ipv6/fib6_rules.c#L71
[2]: https://github.com/torvalds/linux/blob/ca7a03c4175366a92cee0ccc4fec0038c3266e26/net/ipv6/fib6_rules.c#L99

References:
http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2021-47546
https://www.cve.org/CVERecord?id=CVE-2021-47546
https://git.kernel.org/stable/c/209d35ee34e25f9668c404350a1c86d914c54ffa
https://git.kernel.org/stable/c/8ef8a76a340ebdb2c2eea3f6fb0ebbed09a16383
https://git.kernel.org/stable/c/cdef485217d30382f3bf6448c54b4401648fe3f1
https://git.kernel.org/stable/c/ee38eb8cf9a7323884c2b8e0adbbeb2192d31e29
https://git.kernel.org/pub/scm/linux/security/vulns.git/plain/cve/published/2021/CVE-2021-47546.mbox
https://bugzilla.redhat.com/show_bug.cgi?id=2283403