CVE-2021-47546 ipv6: fix memory leak in fib6_rule_suppress

2024-05-2415:09:51

Linux

www.cve.org

linux kernel

ipv6

memory leak

nftables

firewall rules

routing rules

wg-quick

bpftrace

ip6_dst_cache

slab cache

rt6_lookup_f_dst_noref

suppress function

protocol-specific flags

ipv6 packet

6.6 Medium

AI Score

Confidence

Low

0.0004 Low

EPSS

Percentile

5.1%

JSON

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.

CNA Affected

[
  {
    "product": "Linux",
    "vendor": "Linux",
    "defaultStatus": "unaffected",
    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
    "programFiles": [
      "include/net/fib_rules.h",
      "net/core/fib_rules.c",
      "net/ipv4/fib_rules.c",
      "net/ipv6/fib6_rules.c"
    ],
    "versions": [
      {
        "version": "ca7a03c41753",
        "lessThan": "ee38eb8cf9a7",
        "status": "affected",
        "versionType": "git"
      },
      {
        "version": "ca7a03c41753",
        "lessThan": "209d35ee34e2",
        "status": "affected",
        "versionType": "git"
      },
      {
        "version": "ca7a03c41753",
        "lessThan": "8ef8a76a340e",
        "status": "affected",
        "versionType": "git"
      },
      {
        "version": "ca7a03c41753",
        "lessThan": "cdef485217d3",
        "status": "affected",
        "versionType": "git"
      }
    ]
  },
  {
    "product": "Linux",
    "vendor": "Linux",
    "defaultStatus": "affected",
    "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
    "programFiles": [
      "include/net/fib_rules.h",
      "net/core/fib_rules.c",
      "net/ipv4/fib_rules.c",
      "net/ipv6/fib6_rules.c"
    ],
    "versions": [
      {
        "version": "5.4",
        "status": "affected"
      },
      {
        "version": "0",
        "lessThan": "5.4",
        "status": "unaffected",
        "versionType": "custom"
      },
      {
        "version": "5.4.164",
        "lessThanOrEqual": "5.4.*",
        "status": "unaffected",
        "versionType": "custom"
      },
      {
        "version": "5.10.84",
        "lessThanOrEqual": "5.10.*",
        "status": "unaffected",
        "versionType": "custom"
      },
      {
        "version": "5.15.7",
        "lessThanOrEqual": "5.15.*",
        "status": "unaffected",
        "versionType": "custom"
      },
      {
        "version": "5.16",
        "lessThanOrEqual": "*",
        "status": "unaffected",
        "versionType": "original_commit_for_fix"
      }
    ]
  }
]