Re: [PATCH net] ipv6: fix memory leak in fib6_rule_suppress

[msizanoen <msizanoen@qtmlabs.xyz>]

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On 11/23/21 19:48, Jason A. Donenfeld wrote:

> From: msizanoen1 <msizanoen@qtmlabs.xyz>
>
> 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
>
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=215105
> Fixes: ca7a03c41753 ("ipv6: do not free rt if FIB_LOOKUP_NOREF is set on suppress rule")
> Cc: stable@vger.kernel.org
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
> The original author of this commit and commit message is anonymous and
> is therefore unable to sign off on it. Greg suggested that I do the sign
> off, extracting it from the bugzilla entry above, and post it properly.
> The patch "seems to work" on first glance, but I haven't looked deeply
> at it yet and therefore it doesn't have my Reviewed-by, even though I'm
> submitting this patch on the author's behalf. And it should probably get
> a good look from the v6 fib folks. The original author should be on this
> thread to address issues that come off, and I'll shephard additional
> versions that he has.
This patch has been running on my personal laptop since I debugged the 
issue, so you can also add a `Tested-by: <msizanoen@qtmlabs.xyz>`.
>
>   include/net/fib_rules.h | 4 +++-
>   net/core/fib_rules.c    | 2 +-
>   net/ipv4/fib_rules.c    | 1 +
>   net/ipv6/fib6_rules.c   | 4 ++--
>   4 files changed, 7 insertions(+), 4 deletions(-)
>
> diff --git a/include/net/fib_rules.h b/include/net/fib_rules.h
> index 4b10676c69d1..bd07484ab9dd 100644
> --- a/include/net/fib_rules.h
> +++ b/include/net/fib_rules.h
> @@ -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);
> @@ -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
> diff --git a/net/core/fib_rules.c b/net/core/fib_rules.c
> index 79df7cd9dbc1..1bb567a3b329 100644
> --- a/net/core/fib_rules.c
> +++ b/net/core/fib_rules.c
> @@ -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) {
> diff --git a/net/ipv4/fib_rules.c b/net/ipv4/fib_rules.c
> index ce54a30c2ef1..364ad3446b2f 100644
> --- a/net/ipv4/fib_rules.c
> +++ b/net/ipv4/fib_rules.c
> @@ -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;
> diff --git a/net/ipv6/fib6_rules.c b/net/ipv6/fib6_rules.c
> index 40f3e4f9f33a..dcedfe29d9d9 100644
> --- a/net/ipv6/fib6_rules.c
> +++ b/net/ipv6/fib6_rules.c
> @@ -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;
> @@ -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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