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

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

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.