CVE-2021-47546 in Linux
Summary
by MITRE • 05/24/2024
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
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Analysis
by VulDB Data Team • 05/24/2024
The vulnerability CVE-2021-47546 represents a critical memory leak within the Linux kernel's IPv6 forwarding implementation that specifically affects systems utilizing nftables with fib rules and suppress prefix functionality. This issue manifests when IPv6 nftables firewall rules contain fib lookup operations combined with IPv6 routing rules that implement suppress_prefix functionality, commonly used by tools like wg-quick for WireGuard configurations. The memory leak occurs in the ip6_dst_cache slab cache, where each incoming IPv6 packet triggers an allocation that is never properly freed, leading to progressive memory consumption over time.
The root cause of this vulnerability stems from a problematic commit ca7a03c41753 that altered the behavior of fib6_rule_suppress function. The implementation incorrectly uses generic FIB_LOOKUP_NOREF flag for all protocol contexts instead of properly distinguishing between generic and protocol-specific flags. While the generic args->flags always contain FIB_LOOKUP_NOREF, the IPv6-specific RT6_LOOKUP_F_DST_NOREF flag may not be set, causing the suppress function to fail in properly decrementing reference counts when needed. This fundamental mismatch between generic and protocol-specific flag handling creates a scenario where memory allocations remain unreleased indefinitely.
The operational impact of this vulnerability is significant for network infrastructure running Linux kernels with affected versions, particularly in environments utilizing complex IPv6 routing policies combined with nftables firewall rules. The memory leak grows continuously with each incoming packet, potentially leading to system performance degradation, memory exhaustion, and eventual system instability or crashes. Network devices that handle substantial IPv6 traffic volumes, such as routers, firewalls, and VPN gateways, face the highest risk of experiencing this issue. The vulnerability is particularly concerning because it operates silently in the background, with memory consumption increasing gradually without obvious system indicators until it reaches critical levels.
The mitigation strategy involves applying the kernel patch that properly exposes protocol-specific flags to the suppress function and ensures that RT6_LOOKUP_F_DST_NOREF flag is checked instead of the generic FIB_LOOKUP_NOREF when determining whether to decrement reference counts. This fix aligns with the established security principle of proper resource management and prevents the accumulation of unreleased memory allocations. System administrators should prioritize updating their Linux kernel versions to include this fix, particularly in production environments handling IPv6 traffic with complex routing policies. The vulnerability demonstrates the importance of careful flag management in kernel code and aligns with CWE-401 (Improper Release of Memory) and ATT&CK technique T1499.001 (Network Denial of Service) through its potential for resource exhaustion and service disruption. Organizations should also monitor their systems for memory usage patterns and implement proper alerting mechanisms to detect similar memory leak behaviors in other kernel components.