[PATCH net] ipv4: stop PMTU walk when nexthop group shrinks

[Daehyeon Ko <4ncienth@gmail.com>]

Commit 7d3f3b4367f3 ("net: ipv4: Cache pmtu for all packet paths if
multipath enabled") made __ip_rt_update_pmtu() update every path.

For nexthop objects, fib_info_num_path() and fib_info_nhc()
independently load nh->nh_grp.  RCU protects each group's lifetime but
does not make the loads observe the same group.  If replacement shrinks
the group after the loop accepts an index, fib_info_nhc() returns NULL
and update_or_create_fnhe() dereferences it.

A deterministic probe only widened the existing window between the real
operations.  On v7.2, an RTM_NEWNEXTHOP replacement published a
one-member group after the PMTU reader accepted index 1 from a two-member
group, producing:

  KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
  RIP: update_or_create_fnhe+0x45/0x15b0

Stop when the indexed path is absent.  Groups are dense, so no later
index exists in that snapshot.  The same real-writer window completed
114,423 PMTU iterations without an oops with this check.  A reproducer
is available on request.

Replacement requires CAP_NET_ADMIN in the network namespace.  Where
unprivileged user namespaces are permitted, a local user can obtain it
in a new user and network namespace.

Fixes: 7d3f3b4367f3 ("net: ipv4: Cache pmtu for all packet paths if multipath enabled")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Daehyeon Ko <4ncienth@gmail.com>
---
 net/ipv4/route.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index 37674d76f90f0..5f2197874bebc 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -1082,6 +1082,8 @@ static void __ip_rt_update_pmtu(struct rtable *rt, struct flowi4 *fl4, u32 mtu)
 
 			for (nhsel = 0; nhsel < fib_info_num_path(res.fi); nhsel++) {
 				nhc = fib_info_nhc(res.fi, nhsel);
+				if (!nhc)
+					break;
 				update_or_create_fnhe(nhc, fl4->daddr, 0, mtu, lock,
 						      jiffies + net->ipv4.ip_rt_mtu_expires);
 			}

base-commit: 4f1da630d13de0370e2f6361f961f1c8b384af1c
-- 
2.55.0