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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-45ef354cd87sm23765184f8f.24.2026.06.01.05.22.46 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 01 Jun 2026 05:22:46 -0700 (PDT) Date: Mon, 1 Jun 2026 13:22:45 +0100 From: David Laight To: Runyu Xiao Cc: "David S . Miller" , Jakub Kicinski , netdev@vger.kernel.org, Paolo Abeni , Eric Dumazet , David Ahern , Ido Schimmel , Simon Horman , linux-kernel@vger.kernel.org, jianhao.xu@seu.edu.cn, stable@vger.kernel.org Subject: Re: [PATCH net] ipv6: use READ_ONCE() in ipv6_flowlabel_get() Message-ID: <20260601132245.4be1b32a@pumpkin> In-Reply-To: <20260531153946.1627418-1-runyu.xiao@seu.edu.cn> References: <20260531153946.1627418-1-runyu.xiao@seu.edu.cn> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) 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-Transfer-Encoding: 7bit On Sun, 31 May 2026 23:39:46 +0800 Runyu Xiao wrote: > ipv6_flowlabel_get() still reads the shared per-net sysctl fields > flowlabel_consistency and flowlabel_state_ranges with plain loads, > while writers update them through proc_dou8vec_minmax(). These checks > run in the live IPV6_FLOWLABEL_MGR path, so lockless plain reads leave > KCSAN-visible data races and can make the policy checks observe stale or > inconsistent values. > > The race can be reached on a running system by toggling > /proc/sys/net/ipv6/flowlabel_consistency and > /proc/sys/net/ipv6/flowlabel_state_ranges while another task repeatedly > issues IPV6_FLOWLABEL_MGR requests with IPV6_FL_F_REFLECT or a > state-ranges flow label. > > This issue was first flagged by our static analysis tool while scanning > lockless IPv6 sysctl readers, then manually audited on Linux v6.18.21. > The IPV6_FLOWLABEL_MGR paths were runtime-reproduced with QEMU/KCSAN by > concurrently flipping the two sysctls while TCP reflect and UDP > state-ranges setsockopt actors exercised ipv6_flowlabel_get(). KCSAN > reported races between proc_dou8vec_minmax() and the two plain-load > sites in ipv6_flowlabel_get(). > > A narrower second-round UDPv6 + IPV6_AUTOFLOWLABEL send-side reproducer > also hit the inline ip6_make_flowlabel() reader through > __ip6_make_skb() / proc_dou8vec_minmax(), but that site is already > fixed in this tree by commit ded139b59b5d > ("ipv6: annotate data-races from ip6_make_flowlabel()"). The remaining > plain readers in this tree are both in ipv6_flowlabel_get(). > > Use READ_ONCE() for those remaining sysctl reads so they follow the same > lockless reader contract already used by other IPv6 sysctl readers. > > Build-tested by compiling net/ipv6/ip6_flowlabel.o on x86_64. > > Representative QEMU/KCSAN reports from the two target reader paths: > > BUG: KCSAN: data-race in ipv6_flowlabel_opt / proc_dou8vec_minmax > write: proc_dou8vec_minmax+0x206/0x220 > read: ipv6_flowlabel_opt+0x6d8/0xd20 > do_ipv6_setsockopt+0x873/0x2220 > tcp_setsockopt+0x72/0xb0 > > BUG: KCSAN: data-race in ipv6_flowlabel_opt / proc_dou8vec_minmax > write: proc_dou8vec_minmax+0x206/0x220 > read: ipv6_flowlabel_opt+0x129/0xd20 > do_ipv6_setsockopt+0x873/0x2220 > udpv6_setsockopt+0x21/0x40 > > Fixes: 6444f72b4b74 ("ipv6: add flowlabel_consistency sysctl") > Fixes: 82a584b7cd36 ("ipv6: Flow label state ranges") > Cc: stable@vger.kernel.org > Signed-off-by: Runyu Xiao > --- > net/ipv6/ip6_flowlabel.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/net/ipv6/ip6_flowlabel.c b/net/ipv6/ip6_flowlabel.c > index b1ccdf0dc646..1ab5ad0dcf24 100644 > --- a/net/ipv6/ip6_flowlabel.c > +++ b/net/ipv6/ip6_flowlabel.c > @@ -620,7 +620,7 @@ static int ipv6_flowlabel_get(struct sock *sk, struct in6_flowlabel_req *freq, > int err; > > if (freq->flr_flags & IPV6_FL_F_REFLECT) { > - if (net->ipv6.sysctl.flowlabel_consistency) { > + if (READ_ONCE(net->ipv6.sysctl.flowlabel_consistency)) { That can't actually fix anything. If the value can be written concurrently it will still be zero or non-zero even if the write gets split. So it can only ever be the same as the write happening a bit earlier or a bit later. There might be a real bug if the code looks at net->ipv6.sysctl.flowlabel_consistency again. But a READ_ONCE() in an if won't fix anything. > net_info_ratelimited("Can not set IPV6_FL_F_REFLECT if flowlabel_consistency sysctl is enable\n"); > return -EPERM; > } > @@ -633,7 +633,7 @@ static int ipv6_flowlabel_get(struct sock *sk, struct in6_flowlabel_req *freq, > > if (freq->flr_label & ~IPV6_FLOWLABEL_MASK) > return -EINVAL; > - if (net->ipv6.sysctl.flowlabel_state_ranges && > + if (READ_ONCE(net->ipv6.sysctl.flowlabel_state_ranges) && Ditto. > (freq->flr_label & IPV6_FLOWLABEL_STATELESS_FLAG)) > return -ERANGE; > -- David