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 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