CVE-2021-47546
Assigner: Linux
Published: 2024-05-24T15:09:51.286Z
State: PUBLISHED
descriptions: value: 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
affected: product: Linux
vendor: Linux
defaultStatus: unaffected
repo: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git
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
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
title: ipv6: fix memory leak in fib6_rule_suppress
x_generator: engine: bippy-d175d3acf727