Subject: Re: [PATCH net] ipv6: use READ_ONCE() in ipv6_flowlabel_get()

From: David Laight <[email-masked]>
Date: Mon, 1 Jun 2026 13:22:45 +0100

On Sun, 31 May 2026 23:39:46 +0800
Runyu Xiao <[email-masked]> 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: [email-masked]
> Signed-off-by: Runyu Xiao <[email-masked]>
> ---
>  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