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Wed, 25 Mar 2026 21:22:49 -0700 (PDT) Received: from 1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.ip6.arpa ([240e:34c:576a:8120:24cb:c7d9:bd46:8af]) by smtp.gmail.com with ESMTPSA id 41be03b00d2f7-c7673978668sm911369a12.27.2026.03.25.21.22.39 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 25 Mar 2026 21:22:48 -0700 (PDT) From: Hangbin Liu Date: Thu, 26 Mar 2026 12:22:20 +0800 Subject: [PATCH net] ipv6: fix data race in fib6_metric_set() using cmpxchg Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20260326-b4-fib6_metric_set-kmemleak-v1-1-c89fc1b312c0@gmail.com> X-B4-Tracking: v=1; b=H4sIAAAAAAAC/x3MQQqDMBAF0KvIrDugqUbbq5Qio351sIklCaUg3 r2hy7d5B0UERaR7cVDAR6PuPqO6FDSu4hewTtlkSmPLq7E81DzrYHuHFHTsIxJvDu4F2bgVaap b25muFsrDO2DW739/kEei53n+AB/sg01yAAAA X-Change-ID: 20260326-b4-fib6_metric_set-kmemleak-7aa51978284a To: "David S. Miller" , David Ahern , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman Cc: David Ahern , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Fei Liu , Hangbin Liu X-Mailer: b4 0.14.3 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 Signed-off-by: Hangbin Liu --- 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