linux-kernel - Re: [PATCH] tty: vt: add a bounds checking in vt_do_kdgkb

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Date:   Fri, 9 Sep 2022 10:02:41 +0800
From:   Hangyu Hua <hbh25y@...il.com>
To:     Jiri Slaby <jirislaby@...nel.org>, gregkh@...uxfoundation.org,
        changlianzhi@...ontech.com, dmitry.torokhov@...il.com
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] tty: vt: add a bounds checking in vt_do_kdgkb_ioctl()

On 8/9/2022 16:10, Jiri Slaby wrote:
> On 08. 09. 22, 9:54, Hangyu Hua wrote:
>> As array_index_nospec's comments indicate，a bounds checking need to add
>> before calling array_index_nospec.
>>
>> Signed-off-by: Hangyu Hua <hbh25y@...il.com>
>> ---
>>   drivers/tty/vt/keyboard.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/drivers/tty/vt/keyboard.c b/drivers/tty/vt/keyboard.c
>> index be8313cdbac3..b9845455df79 100644
>> --- a/drivers/tty/vt/keyboard.c
>> +++ b/drivers/tty/vt/keyboard.c
>> @@ -2067,6 +2067,9 @@ int vt_do_kdgkb_ioctl(int cmd, struct kbsentry 
>> __user *user_kdgkb, int perm)
>>       if (get_user(kb_func, &user_kdgkb->kb_func))
>>           return -EFAULT;
>> +    if (kb_func >= MAX_NR_FUNC)
> 
> kb_func is unsigned char and MAX_NR_FUNC is 256. So this should be 
> eliminated by the compiler anyway.
> 
> But the check might be a good idea if we ever decide to support more 
> keys. But will/can we? I am not so sure, so adding it right now is kind 
> of superfluous. In any way we'd need to introduce a completely different 
> iterface/ioctls.

If you say so, I think greg should be right. We don't need any bounds 
checking here. It may be a good idea to delete redundant
array_index_nospec. Do i need to make a new patch?

> 
>> +        return -EFAULT;
> 
> EINVAL would be more appropriate, IMO.
> 
>> +
>>       kb_func = array_index_nospec(kb_func, MAX_NR_FUNC);
>>       switch (cmd) {
> 
> thanks,

Thanks,
Hangyu