Re: [PATCH] net: prevent user from passing illegal stab size

[王贇 <yun.wang@linux.alibaba.com>]

On 2021/9/24 上午12:00, Jakub Kicinski wrote:
> On Thu, 23 Sep 2021 17:08:13 +0800 王贇 wrote:
>> We observed below report when playing with netlink sock:
>>
>>   UBSAN: shift-out-of-bounds in net/sched/sch_api.c:580:10
>>   shift exponent 249 is too large for 32-bit type
>>   CPU: 0 PID: 685 Comm: a.out Not tainted
>>   Call Trace:
>>    dump_stack_lvl+0x8d/0xcf
>>    ubsan_epilogue+0xa/0x4e
>>    __ubsan_handle_shift_out_of_bounds+0x161/0x182
>>    __qdisc_calculate_pkt_len+0xf0/0x190
>>    __dev_queue_xmit+0x2ed/0x15b0
>>
>> it seems like kernel won't check the stab size_log passing from
>> user, and will use the insane value later to calculate pkt_len.
>>
>> This patch just add a check on the size_log to avoid insane
>> calculation.
>>
>> Reported-by: Abaci <abaci@linux.alibaba.com>
>> Signed-off-by: Michael Wang <yun.wang@linux.alibaba.com>
>> ---
>>  include/uapi/linux/pkt_sched.h | 1 +
>>  net/sched/sch_api.c            | 3 +++
>>  2 files changed, 4 insertions(+)
>>
>> diff --git a/include/uapi/linux/pkt_sched.h b/include/uapi/linux/pkt_sched.h
>> index ec88590..fa194a0 100644
>> --- a/include/uapi/linux/pkt_sched.h
>> +++ b/include/uapi/linux/pkt_sched.h
>> @@ -98,6 +98,7 @@ struct tc_ratespec {
>>  };
>>
>>  #define TC_RTAB_SIZE	1024
>> +#define TC_LOG_MAX	30
> 
> Adding a uAPI define is not necessary.

Yes, I'll move this to somewhere else.

> 
>>  struct tc_sizespec {
>>  	unsigned char	cell_log;
>> diff --git a/net/sched/sch_api.c b/net/sched/sch_api.c
>> index 5e90e9b..1b6b8f8 100644
>> --- a/net/sched/sch_api.c
>> +++ b/net/sched/sch_api.c
>> @@ -513,6 +513,9 @@ static struct qdisc_size_table *qdisc_get_stab(struct nlattr *opt,
>>  		return stab;
>>  	}
>>
>> +	if (s->size_log > TC_LOG_MAX)
>> +		return ERR_PTR(-EINVAL);
> 
> Looks sane, please add an extack message.
> 
> Why not cover cell_log as well while at it? 

You're right, will add message and check this one too in v2 :-)

Regards,
Michael Wang

> 
>>  	stab = kmalloc(sizeof(*stab) + tsize * sizeof(u16), GFP_KERNEL);
>>  	if (!stab)
>>  		return ERR_PTR(-ENOMEM);