[PATCH 5.4 50/70] ipv6: fix memory leak in fib6_rule_suppress

[Greg Kroah-Hartman]

From: msizanoen1 <msizanoen@xxxxxxxxxxx>

commit cdef485217d30382f3bf6448c54b4401648fe3f1 upstream.

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@xxxxxxxxxxxxxxx
Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 include/net/fib_rules.h |    2 +-
 net/core/fib_rules.c    |    2 +-
 net/ipv4/fib_rules.c    |    2 +-
 net/ipv6/fib6_rules.c   |    5 ++---
 4 files changed, 5 insertions(+), 6 deletions(-)

--- a/include/net/fib_rules.h
+++ b/include/net/fib_rules.h
@@ -68,7 +68,7 @@ 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);
--- a/net/core/fib_rules.c
+++ b/net/core/fib_rules.c
@@ -300,7 +300,7 @@ jumped:
 		else
 			err = ops->action(rule, fl, flags, arg);
 
-		if (!err && ops->suppress && ops->suppress(rule, arg))
+		if (!err && ops->suppress && ops->suppress(rule, flags, arg))
 			continue;
 
 		if (err != -EAGAIN) {
--- a/net/ipv4/fib_rules.c
+++ b/net/ipv4/fib_rules.c
@@ -137,7 +137,7 @@ static int fib4_rule_action(struct fib_r
 	return err;
 }
 
-static bool fib4_rule_suppress(struct fib_rule *rule, struct fib_lookup_arg *arg)
+static bool fib4_rule_suppress(struct fib_rule *rule, int flags, struct fib_lookup_arg *arg)
 {
 	struct fib_result *result = (struct fib_result *) arg->result;
 	struct net_device *dev = NULL;
--- a/net/ipv6/fib6_rules.c
+++ b/net/ipv6/fib6_rules.c
@@ -260,7 +260,7 @@ static int fib6_rule_action(struct fib_r
 	return __fib6_rule_action(rule, flp, flags, arg);
 }
 
-static bool fib6_rule_suppress(struct fib_rule *rule, struct fib_lookup_arg *arg)
+static bool fib6_rule_suppress(struct fib_rule *rule, int flags, struct fib_lookup_arg *arg)
 {
 	struct fib6_result *res = arg->result;
 	struct rt6_info *rt = res->rt6;
@@ -287,8 +287,7 @@ static bool fib6_rule_suppress(struct fi
 	return false;
 
 suppress_route:
-	if (!(arg->flags & FIB_LOOKUP_NOREF))
-		ip6_rt_put(rt);
+	ip6_rt_put_flags(rt, flags);
 	return true;
 }