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Mon, 30 Mar 2026 21:17:40 -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:5765:500:c92f:4f4e:9953:45b7]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2b24265a954sm99614135ad.18.2026.03.30.21.17.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 30 Mar 2026 21:17:40 -0700 (PDT) From: Hangbin Liu Date: Tue, 31 Mar 2026 12:17:18 +0800 Subject: [PATCH net v3] ipv6: fix data race in fib6_metric_set() using cmpxchg Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Message-Id: <20260331-b4-fib6_metric_set-kmemleak-v3-1-88d27f4d8825@gmail.com> X-B4-Tracking: v=1; b=H4sIAAAAAAAC/43NQQqDMBQE0KtI1k0xX422q96jFEnSr340WpIgL eLdG7JyVbqcGXizMY+O0LNrtjGHK3la5hiKU8bMoOYeOT1jZpCDzAuQXJe8Iy1bi8GRaT0GPlq 0E6qR10pV4lI30JSKReHlsKN30u9sxsAesRzIh8V90uMq0vQXvgouuGkunRG6EGDyW28VTWez2 OSucLTq3xZEq5BSg6kbXRk4Wvu+fwF5ei2wFwEAAA== X-Change-ID: 20260326-b4-fib6_metric_set-kmemleak-7aa51978284a To: "David S. Miller" , David Ahern , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Jiayuan Chen 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 =3D=3D &dst_default_metrics) { /* both CPUs: true= */ struct dst_metrics *p =3D kzalloc_obj(*p, GFP_ATOMIC); refcount_set(&p->refcnt, 1); f6i->fib6_metrics =3D p; /* CPU1 overwrites CPU0's p -> p0 leaked */ } The dst_metrics allocated by the losing CPU has refcnt=3D1 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 --- Changes in v3: - move variable declarations to the top of function (Jiayuan Chen) - Link to v2: https://lore.kernel.org/r/20260327-b4-fib6_metric_set-kmemlea= k-v2-1-366b2c78b5c2@gmail.com Changes in v2: - Set val for p->metrics before published via cmpxchg() (Eric Dumazet) - Add READ_ONCE()/WRITE_ONCE() for metrics[] setting (Jiayuan Chen) - Link to v1: https://lore.kernel.org/r/20260326-b4-fib6_metric_set-kmemlea= k-v1-1-c89fc1b312c0@gmail.com --- 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 @@ static int inet6_dump_fib(struct sk_buff *skb, struct= netlink_callback *cb) =20 void fib6_metric_set(struct fib6_info *f6i, int metric, u32 val) { + struct dst_metrics *m; + if (!f6i) return; =20 - if (f6i->fib6_metrics =3D=3D &dst_default_metrics) { + if (READ_ONCE(f6i->fib6_metrics) =3D=3D &dst_default_metrics) { + struct dst_metrics *dflt =3D (struct dst_metrics *)&dst_default_metrics; struct dst_metrics *p =3D kzalloc_obj(*p, GFP_ATOMIC); =20 if (!p) return; =20 + p->metrics[metric - 1] =3D val; refcount_set(&p->refcnt, 1); - f6i->fib6_metrics =3D p; + if (cmpxchg(&f6i->fib6_metrics, dflt, p) !=3D dflt) + kfree(p); + else + return; } =20 - f6i->fib6_metrics->metrics[metric - 1] =3D val; + m =3D READ_ONCE(f6i->fib6_metrics); + WRITE_ONCE(m->metrics[metric - 1], val); } =20 /* --- base-commit: c4ea7d8907cf72b259bf70bd8c2e791e1c4ff70f change-id: 20260326-b4-fib6_metric_set-kmemleak-7aa51978284a Best regards, --=20 Hangbin Liu