From ffb5a4843c5bde702ed17cbcdbda98b37f7a6dad Mon Sep 17 00:00:00 2001 From: Hangbin Liu Date: Tue, 31 Mar 2026 12:17:18 +0800 Subject: [PATCH] ipv6: fix data race in fib6_metric_set() using cmpxchg 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: - Set val for p->metrics before published via cmpxchg() so the metrics value is ready before the pointer becomes visible to other CPUs. - Replace the plain pointer store with cmpxchg() and free the allocation safely when competition failed. - Add READ_ONCE()/WRITE_ONCE() for metrics[] setting in the non-default metrics path to prevent compiler-based data races. Fixes: d4ead6b34b67 ("net/ipv6: move metrics from dst to rt6_info") Reported-by: Fei Liu Reviewed-by: Jiayuan Chen Signed-off-by: Hangbin Liu Reviewed-by: Eric Dumazet Link: https://patch.msgid.link/20260331-b4-fib6_metric_set-kmemleak-v3-1-88d27f4d8825@gmail.com Signed-off-by: Jakub Kicinski --- net/ipv6/ip6_fib.c | 14 +++++++++++--- 1 file changed, 11 insertions(+), 3 deletions(-) diff --git a/net/ipv6/ip6_fib.c b/net/ipv6/ip6_fib.c index dd26657b6a4a..45ef4d65dcbc 100644 --- a/net/ipv6/ip6_fib.c +++ b/net/ipv6/ip6_fib.c @@ -727,20 +727,28 @@ unlock: void fib6_metric_set(struct fib6_info *f6i, int metric, u32 val) { + struct dst_metrics *m; + 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; + p->metrics[metric - 1] = val; refcount_set(&p->refcnt, 1); - f6i->fib6_metrics = p; + if (cmpxchg(&f6i->fib6_metrics, dflt, p) != dflt) + kfree(p); + else + return; } - f6i->fib6_metrics->metrics[metric - 1] = val; + m = READ_ONCE(f6i->fib6_metrics); + WRITE_ONCE(m->metrics[metric - 1], val); } /*