[PATCH net] ipv6: fix data race in fib6_metric_set() using cmpxchg

[Hangbin Liu <liuhangbin@gmail.com>]

fib6_metric_set() may be called concurrently from softirq context without
holding the FIB table lock. A typical path is:

  ndisc_router_discovery()
    spin_unlock_bh(&table->tb6_lock)        <- lock released
    fib6_metric_set(rt, RTAX_HOPLIMIT, ...) <- lockless call

When two CPUs process Router Advertisement packets for the same router
simultaneously, they can both arrive at fib6_metric_set() with the same
fib6_info pointer whose fib6_metrics still points to dst_default_metrics.

  if (f6i->fib6_metrics == &dst_default_metrics) {   /* both CPUs: true */
      struct dst_metrics *p = kzalloc_obj(*p, GFP_ATOMIC);
      refcount_set(&p->refcnt, 1);
      f6i->fib6_metrics = p;   /* CPU1 overwrites CPU0's p -> p0 leaked */
  }

The dst_metrics allocated by the losing CPU has refcnt=1 but no pointer
to it anywhere in memory, producing a kmemleak report:

  unreferenced object 0xff1100025aca1400 (size 96):
    comm "softirq", pid 0, jiffies 4299271239
    backtrace:
      kmalloc_trace+0x28a/0x380
      fib6_metric_set+0xcd/0x180
      ndisc_router_discovery+0x12dc/0x24b0
      icmpv6_rcv+0xc16/0x1360

Fix this by replacing the plain pointer store with cmpxchg() and free
the allocation safely when competition failed.

Fixes: d4ead6b34b67 ("net/ipv6: move metrics from dst to rt6_info")
Reported-by: Fei Liu <feliu@redhat.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
---
 net/ipv6/ip6_fib.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c
index dd26657b6a4a..64de761f40d5 100644
--- a/net/ipv6/ip6_fib.c
+++ b/net/ipv6/ip6_fib.c
@@ -730,14 +730,16 @@ void fib6_metric_set(struct fib6_info *f6i, int metric, u32 val)
 	if (!f6i)
 		return;
 
-	if (f6i->fib6_metrics == &dst_default_metrics) {
+	if (READ_ONCE(f6i->fib6_metrics) == &dst_default_metrics) {
+		struct dst_metrics *dflt = (struct dst_metrics *)&dst_default_metrics;
 		struct dst_metrics *p = kzalloc_obj(*p, GFP_ATOMIC);
 
 		if (!p)
 			return;
 
 		refcount_set(&p->refcnt, 1);
-		f6i->fib6_metrics = p;
+		if (cmpxchg(&f6i->fib6_metrics, dflt, p) != dflt)
+			kfree(p);
 	}
 
 	f6i->fib6_metrics->metrics[metric - 1] = val;

---
base-commit: c4ea7d8907cf72b259bf70bd8c2e791e1c4ff70f
change-id: 20260326-b4-fib6_metric_set-kmemleak-7aa51978284a

Best regards,
-- 
Hangbin Liu <liuhangbin@gmail.com>