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Miller" , David Ahern , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , David Ahern , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Fei Liu Subject: Re: [PATCH net] ipv6: fix data race in fib6_metric_set() using cmpxchg Message-ID: References: <20260326-b4-fib6_metric_set-kmemleak-v1-1-c89fc1b312c0@gmail.com> 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=us-ascii Content-Disposition: inline In-Reply-To: On Thu, Mar 26, 2026 at 06:44:43AM +0000, Hangbin Liu wrote: > On Thu, Mar 26, 2026 at 02:23:15PM +0800, Jiayuan Chen wrote: > > > > On 3/26/26 12:22 PM, Hangbin Liu wrote: > > > 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; > > > > Suggest using marked accessors to suppress KCSAN warnings: > > > > struct dst_metrics *m = READ_ONCE(f6i->fib6_metrics); > > WRITE_ONCE(m->metrics[metric - 1], val); > > Thanks, I will update this in next version. BTW, do we really need to WRITE_ONCE here? What if the `val` are different on 2 CPUs? This would hide the problem, right? Thanks Hangbin