Re: [PATCH net v2 1/5] ipv6: restrict gc_elasticity to 31

[Jiayuan Chen <jiayuan.chen@linux.dev>]

On 10/29/26 9:53 AM, Kuniyuki Iwashima wrote:
> On Wed, Oct 28, 2026 at 6:47 PM Jiayuan Chen <jiayuan.chen@linux.dev> wrote:
>>
>> On 10/29/26 7:58 AM, Kuniyuki Iwashima wrote:
>>> On Tue, Oct 27, 2026 at 3:48 AM Jiayuan Chen <jiayuan.chen@linux.dev> 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_dst_gc() reads ip6_rt_gc_expire into a u32, but gc_elasticity has no
>>>> bounds at all, so writing >= 32 shifts a 32-bit value by >= 32 bits, and
>>>> a negative value gives a negative shift count. Both are 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
>>>>
>>>> Make it a u8 and reject writes outside 0..31 with proc_dou8vec_minmax().
>>>>
>>>> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
>>>> Cc: stable@vger.kernel.org
>>>> Reviewed-by: Fernando Fernandez Mancera <fmancera@suse.de>
>>>> Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
>>>> Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
>>>> ---
>>>> v2: switched to u8 and proc_dou8vec_minmax(), as suggested by Kuniyuki
>>>> Iwashima.
>>>> ---
>>>>    include/net/netns/ipv6.h | 2 +-
>>>>    net/ipv6/route.c         | 7 +++++--
>>>>    2 files changed, 6 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/include/net/netns/ipv6.h b/include/net/netns/ipv6.h
>>>> index ffda15318be59..aefce37c04a4f 100644
>>>> --- a/include/net/netns/ipv6.h
>>>> +++ b/include/net/netns/ipv6.h
>>>> @@ -25,11 +25,11 @@ struct netns_sysctl_ipv6 {
>>>>           int ip6_rt_gc_min_interval;
>>>>           int ip6_rt_gc_timeout;
>>>>           int ip6_rt_gc_interval;
>>>> -       int ip6_rt_gc_elasticity;
>>>>           int ip6_rt_mtu_expires;
>>>>           int ip6_rt_min_advmss;
>>>>           u32 multipath_hash_fields;
>>>>           u8 multipath_hash_policy;
>>>> +       u8 ip6_rt_gc_elasticity;
>>>>
>>>>           __cacheline_group_begin(sysctl_ipv6_flowlabel);
>>>>           u8 flowlabel_consistency;
>>>> diff --git a/net/ipv6/route.c b/net/ipv6/route.c
>>>> index 94e5ef1c4f83e..abc8b811cd423 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;
>>> as you need v3, s/int/u8/
>>>
>> Hi Kuniyuki,
>>
>> proc_dou8vec_minmax() put extra1/extra2 into proc_uint_conv(), which
>> reads them as unsigned int, so the max has to stay an int like
>> tcp_syn_retries_max and friends; a u8 there would read
>> three bytes past the variable.
> Ah exactly, then please use unsigned int.
>
> Thanks


Makes sense, unsigned int in v3.