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.

Weakness

The product does not sufficiently track and release allocated memory after it has been used, which slowly consumes remaining memory.

Affected Software

Potential Mitigations

• Choose a language or tool that provides automatic memory management, or makes manual memory management less error-prone.
• For example, glibc in Linux provides protection against free of invalid pointers.
• When using Xcode to target OS X or iOS, enable automatic reference counting (ARC) [REF-391].
• To help correctly and consistently manage memory when programming in C++, consider using a smart pointer class such as std::auto_ptr (defined by ISO/IEC ISO/IEC 14882:2003), std::shared_ptr and std::unique_ptr (specified by an upcoming revision of the C++ standard, informally referred to as C++ 1x), or equivalent solutions such as Boost.

References

• 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