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ipv6: fix memory leak in fib6_rule_suppress
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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 ca7a03c ("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

Link: https://bugzilla.kernel.org/show_bug.cgi?id=215105
Fixes: ca7a03c ("ipv6: do not free rt if FIB_LOOKUP_NOREF is set on suppress rule")
Cc: [email protected]
Signed-off-by: Jason A. Donenfeld <[email protected]>
Signed-off-by: David S. Miller <[email protected]>
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msizanoen1 authored and davem330 committed Nov 29, 2021
1 parent d00a50c commit cdef485
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Showing 4 changed files with 7 additions and 4 deletions.
4 changes: 3 additions & 1 deletion include/net/fib_rules.h
Original file line number Diff line number Diff line change
Expand Up @@ -69,7 +69,7 @@ struct fib_rules_ops {
int (*action)(struct fib_rule *,
struct flowi *, int,
struct fib_lookup_arg *);
bool (*suppress)(struct fib_rule *,
bool (*suppress)(struct fib_rule *, int,
struct fib_lookup_arg *);
int (*match)(struct fib_rule *,
struct flowi *, int);
Expand Down Expand Up @@ -218,7 +218,9 @@ INDIRECT_CALLABLE_DECLARE(int fib4_rule_action(struct fib_rule *rule,
struct fib_lookup_arg *arg));

INDIRECT_CALLABLE_DECLARE(bool fib6_rule_suppress(struct fib_rule *rule,
int flags,
struct fib_lookup_arg *arg));
INDIRECT_CALLABLE_DECLARE(bool fib4_rule_suppress(struct fib_rule *rule,
int flags,
struct fib_lookup_arg *arg));
#endif
2 changes: 1 addition & 1 deletion net/core/fib_rules.c
Original file line number Diff line number Diff line change
Expand Up @@ -323,7 +323,7 @@ int fib_rules_lookup(struct fib_rules_ops *ops, struct flowi *fl,
if (!err && ops->suppress && INDIRECT_CALL_MT(ops->suppress,
fib6_rule_suppress,
fib4_rule_suppress,
rule, arg))
rule, flags, arg))
continue;

if (err != -EAGAIN) {
Expand Down
1 change: 1 addition & 0 deletions net/ipv4/fib_rules.c
Original file line number Diff line number Diff line change
Expand Up @@ -141,6 +141,7 @@ INDIRECT_CALLABLE_SCOPE int fib4_rule_action(struct fib_rule *rule,
}

INDIRECT_CALLABLE_SCOPE bool fib4_rule_suppress(struct fib_rule *rule,
int flags,
struct fib_lookup_arg *arg)
{
struct fib_result *result = (struct fib_result *) arg->result;
Expand Down
4 changes: 2 additions & 2 deletions net/ipv6/fib6_rules.c
Original file line number Diff line number Diff line change
Expand Up @@ -267,6 +267,7 @@ INDIRECT_CALLABLE_SCOPE int fib6_rule_action(struct fib_rule *rule,
}

INDIRECT_CALLABLE_SCOPE bool fib6_rule_suppress(struct fib_rule *rule,
int flags,
struct fib_lookup_arg *arg)
{
struct fib6_result *res = arg->result;
Expand Down Expand Up @@ -294,8 +295,7 @@ INDIRECT_CALLABLE_SCOPE bool fib6_rule_suppress(struct fib_rule *rule,
return false;

suppress_route:
if (!(arg->flags & FIB_LOOKUP_NOREF))
ip6_rt_put(rt);
ip6_rt_put_flags(rt, flags);
return true;
}

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