Attacker Value
Unknown
(0 users assessed)
Exploitability
Unknown
(0 users assessed)
User Interaction
None
Privileges Required
Low
Attack Vector
Local
0

CVE-2021-47546

Disclosure Date: May 24, 2024 •
CVE-2021-47546 CVSS v3 Base Score: 5.5
Add MITRE ATT&CK tactics and techniques that apply to this CVE.

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 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.

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CVSS V3 Severity and Metrics
Base Score:
5.5 Medium
Impact Score:
3.6
Exploitability Score:
1.8
Vector:
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Attack Vector (AV):
Local
Attack Complexity (AC):
Low
Privileges Required (PR):
Low
User Interaction (UI):
None
Scope (S):
Unchanged
Confidentiality (C):
None
Integrity (I):
None
Availability (A):
High

General Information

Offensive Application
Unknown
Utility Class
Unknown
Ports
Unknown
OS
Unknown
Vulnerable Versions
Linux ee38eb8cf9a7
Linux not down converted
Prerequisites
Unknown
Discovered By
Unknown
PoC Author
Unknown
Metasploit Module
Unknown
Reporter
Unknown

Vendors

  • linux

Products

  • linux kernel
Technical Analysis