[PATCH] ipv6: do not free rt if FIB_LOOKUP_NOREF is set on suppress rule

["Jason A. Donenfeld" <Jason@zx2c4.com>]

Commit 7d9e5f422150 removed references from certain dsts, but accounting
for this never translated down into the fib6 suppression code. This bug
was triggered by WireGuard users who use wg-quick(8), which uses the
"suppress-prefix" directive to ip-rule(8) for routing all of their
internet traffic without routing loops. The test case in the link of
this commit reliably triggers various crashes due to the use-after-free
caused by the reference underflow.

Cc: stable@vger.kernel.org
Fixes: 7d9e5f422150 ("ipv6: convert major tx path to use RT6_LOOKUP_F_DST_NOREF")
Test-case: https://git.zx2c4.com/WireGuard/commit/?id=ad66532000f7a20b149e47c5eb3a957362c8e161
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
---
 net/ipv6/fib6_rules.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/ipv6/fib6_rules.c b/net/ipv6/fib6_rules.c
index d22b6c140f23..f9e8fe3ff0c5 100644
--- a/net/ipv6/fib6_rules.c
+++ b/net/ipv6/fib6_rules.c
@@ -287,7 +287,8 @@ static bool fib6_rule_suppress(struct fib_rule *rule, struct fib_lookup_arg *arg
 	return false;
 
 suppress_route:
-	ip6_rt_put(rt);
+	if (!(arg->flags & FIB_LOOKUP_NOREF))
+		ip6_rt_put(rt);
 	return true;
 }
 
-- 
2.21.0