Re: [PATCH net 1/5] ipv6: restrict gc_elasticity to 31

[Kuniyuki Iwashima <kuniyu@google.com>]

On Fri, 23 Oct 2026 17:13:27 +0800, Jiayuan Chen wrote:
> Every counted IPv6 dst allocation runs ip6_dst_gc() once the number of
> live dst goes over gc_thresh (1024 by default). ip6_dst_gc() decays
> ip6_rt_gc_expire by gc_elasticity: the smaller gc_elasticity is, the
> smaller ip6_rt_gc_expire gets, i.e. the more aggressive the gc.
> 
> ip6_rt_gc_expire is a u32, but gc_elasticity has no upper bound, so
> writing >= 32 shifts a 32-bit value by >= 32 bits, which is UB:
> 
> 	val = atomic_read(&net->ipv6.ip6_rt_gc_expire);
> 	atomic_set(&net->ipv6.ip6_rt_gc_expire, val - (val >> rt_elasticity));
> 
>   UBSAN: shift-out-of-bounds in net/ipv6/route.c:3369:53
>   shift exponent 32 is too large for 32-bit type 'unsigned int'
>   CPU: 2 UID: 0 PID: 615 Comm: ping6 Not tainted 7.3.0-rc7+ #364 PREEMPT
>   Call Trace:
>    <IRQ>
>    dump_stack_lvl+0xc1/0xf0
>    dump_stack+0x10/0x20
>    __ubsan_handle_shift_out_of_bounds+0x1d1/0x390
>    ip6_dst_gc.cold+0x15/0x1a
>    dst_alloc+0x4e0/0x620
>    ip6_rt_cache_alloc+0x189/0x7e0
>    __ip6_rt_update_pmtu+0x68e/0x1160
>    ip6_update_pmtu+0x30c/0x3a0
>    icmpv6_err+0x1fa/0x330
>    icmpv6_notify+0x303/0x770
>    icmpv6_rcv+0xc12/0x1670
>    ip6_protocol_deliver_rcu+0xb4a/0x1520
>    ip6_input_finish+0x19c/0x400
>    ip6_input+0xd4/0x240
>    ipv6_rcv+0x1fd/0x330
>    __netif_receive_skb_one_core+0x118/0x1c0
>    __netif_receive_skb+0x21/0x150
>    process_backlog+0x37e/0x1200
>    __napi_poll.constprop.0+0xa5/0x5e0
>    net_rx_action+0x4c0/0xf70
>    handle_softirqs+0x204/0x9b0
>    __do_softirq+0x10/0x18
> 
> Cap it at 31. 31 and 32 are equivalent anyway: ip6_rt_gc_expire starts
> at 30*HZ (~30000 with HZ=1000), and such a value shifted right by 15 or
> more is already 0, so val - (val >> rt_elasticity) stops decaying well
> before 31. Values above 31 add no new behaviour, they only hit the UB.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Cc: stable@vger.kernel.org
> Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
> ---
>  net/ipv6/route.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
> index 94e5ef1c4f83..6f2efc219f39 100644
> --- a/net/ipv6/route.c
> +++ b/net/ipv6/route.c
> @@ -6579,6 +6579,8 @@ static int ipv6_sysctl_rtcache_flush(const struct ctl_table *ctl, int write,
>  	return 0;
>  }
>  
> +static int ip6_rt_gc_elasticity_max = 31;
> +
>  static const struct ctl_table ipv6_route_table_template[] = {
>  	{
>  		.procname	=	"max_size",
> @@ -6627,7 +6629,9 @@ static const struct ctl_table ipv6_route_table_template[] = {
>  		.data		=	&init_net.ipv6.sysctl.ip6_rt_gc_elasticity,
>  		.maxlen		=	sizeof(int),
>  		.mode		=	0644,
> -		.proc_handler	=	proc_dointvec,
> +		.proc_handler	=	proc_dointvec_minmax,

If you need v2 or as follow-up in net-next, please make it u8.

Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>




> +		.extra1		=	SYSCTL_ZERO,
> +		.extra2		=	&ip6_rt_gc_elasticity_max,
>  	},
>  	{
>  		.procname	=	"mtu_expires",
> -- 
> 2.43.0
>