{"cve":"CVE-2021-47546","nvd":{"published":"2024-05-24T15:15:19.450","modified":"2026-06-17T04:17:43.990","status":"Modified","description":"In the Linux kernel, the following vulnerability has been resolved:\n\nipv6: fix memory leak in fib6_rule_suppress\n\nThe kernel leaks memory when a `fib` rule is present in IPv6 nftables\nfirewall rules and a suppress_prefix rule is present in the IPv6 routing\nrules (used by certain tools such as wg-quick). In such scenarios, every\nincoming packet will leak an allocation in `ip6_dst_cache` slab cache.\n\nAfter some hours of `bpftrace`-ing and source code reading, I tracked\ndown the issue to ca7a03c41753 (\"ipv6: do not free rt if\nFIB_LOOKUP_NOREF is set on suppress rule\").\n\nThe problem with that change is that the generic `args->flags` always have\n`FIB_LOOKUP_NOREF` set[1][2] but the IPv6-specific flag\n`RT6_LOOKUP_F_DST_NOREF` might not be, leading to `fib6_rule_suppress` not\ndecreasing the refcount when needed.\n\nHow to reproduce:\n - Add the following nftables rule to a prerouting chain:\n     meta nfproto ipv6 fib saddr . mark . iif oif missing drop\n   This can be done with:\n     sudo nft create table inet test\n     sudo nft create chain inet test test_chain '{ type filter hook prerouting priority filter + 10; policy accept; }'\n     sudo nft add rule inet test test_chain meta nfproto ipv6 fib saddr . mark . iif oif missing drop\n - Run:\n     sudo ip -6 rule add table main suppress_prefixlength 0\n - Watch `sudo slabtop -o | grep ip6_dst_cache` to see memory usage increase\n   with every incoming ipv6 packet.\n\nThis patch exposes the protocol-specific flags to the protocol\nspecific `suppress` function, and check the protocol-specific `flags`\nargument for RT6_LOOKUP_F_DST_NOREF instead of the generic\nFIB_LOOKUP_NOREF when decreasing the refcount, like this.\n\n[1]: https://github.com/torvalds/linux/blob/ca7a03c4175366a92cee0ccc4fec0038c3266e26/net/ipv6/fib6_rules.c#L71\n[2]: https://github.com/torvalds/linux/blob/ca7a03c4175366a92cee0ccc4fec0038c3266e26/net/ipv6/fib6_rules.c#L99","cvss_v4":null,"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H","score":5.5},"cwes":["CWE-401"],"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","https://git.kernel.org/stable/c/209d35ee34e25f9668c404350a1c86d914c54ffa"],"products":["linux:linux_kernel"],"source":"nvd","synced_at":"2026-10-19T21:50:30.954589733+00:00"},"epss":{"cve":"CVE-2021-47546","score":0.00222,"percentile":0.1282,"date":"2026-10-31"},"kev":null,"osv":[],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}