# CVE-2021-47546: linux: In the Linux kernel, the following vulnerability has been resolved:  ipv6: fix memory leak in fib6_rule_suppress  The...

CVE-2021-47546 is fixed in linux on 1 platform, first by DEBIAN-CVE-2021-47546, released May 24, 2024. It is not reported as exploited. Patch Radar priority: Routine.

- Web page: https://patchblox.com/security/cve/CVE-2021-47546
- Priority: Routine
- CVSS: 5.5 (medium)
- EPSS: 0.0022
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: May 24, 2024

## Description

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

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | linux Debian 12.15 (bookworm) | 5.15.15-1 | [DEBIAN-CVE-2021-47546](https://patchblox.com/security/data/fix/linux/debian-cve-2021-47546.md) | May 24, 2024 |
| Linux | linux Debian 13.5 (trixie) | 5.15.15-1 | [DEBIAN-CVE-2021-47546](https://patchblox.com/security/data/fix/linux/debian-cve-2021-47546.md) | May 24, 2024 |
| Linux | linux Debian 14 | 5.15.15-1 | [DEBIAN-CVE-2021-47546](https://patchblox.com/security/data/fix/linux/debian-cve-2021-47546.md) | May 24, 2024 |

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