CVE-2021-47546

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 set1 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.