linux-kernel - Re: [PATCH for-6.8 v3 1/3] LoongArch: KVM: Fix input validation of _kvm_get_cpucfg and kvm_check

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Message-ID: <37521d9a-7980-486b-8ab6-980a8ccb7bbf@xen0n.name>
Date: Thu, 22 Feb 2024 18:39:57 +0800
From: WANG Xuerui <kernel@...0n.name>
To: maobibo <maobibo@...ngson.cn>, Paolo Bonzini <pbonzini@...hat.com>,
 Huacai Chen <chenhuacai@...nel.org>
Cc: Tianrui Zhao <zhaotianrui@...ngson.cn>, kvm@...r.kernel.org,
 loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org,
 WANG Xuerui <git@...0n.name>
Subject: Re: [PATCH for-6.8 v3 1/3] LoongArch: KVM: Fix input validation of
 _kvm_get_cpucfg and kvm_check_cpucfg

On 2/22/24 18:22, maobibo wrote:
> 
> 
> On 2024/2/22 下午5:45, WANG Xuerui wrote:
>> Hi,
>>
>> On 2/17/24 11:03, maobibo wrote:
>>> Hi Xuerui,
>>>
>>> Good catch, and thank for your patch.
>>>
>>> On 2024/2/16 下午4:58, WANG Xuerui wrote:
>>>> [snip]
>>>> @@ -324,31 +319,33 @@ static int _kvm_get_cpucfg(int id, u64 *v)
>>>>           if (cpu_has_lasx)
>>>>               *v |= CPUCFG2_LASX;
>>>> -        break;
>>>> +        return 0;
>>>> +    case 0 ... 1:
>>>> +    case 3 ... KVM_MAX_CPUCFG_REGS - 1:
>>>> +        /* no restrictions on other CPUCFG IDs' values */
>>>> +        *v = U64_MAX;
>>>> +        return 0;
>>> how about something like this?
>>>      default:
>>>          /* no restrictions on other CPUCFG IDs' values */
>>>          *v = U64_MAX;
>>>          return 0;
>>
>> I don't think this version correctly expresses the intent. Note that 
>> the CPUCFG ID range check is squashed into the switch as well, so one 
>> switch conveniently expresses the three intended cases at once:
>>
>> * the special treatment of CPUCFG2,
> +    case 0 ... 1:
> +    case 3 ... KVM_MAX_CPUCFG_REGS - 1:
> +        /* no restrictions on other CPUCFG IDs' values */
> +        *v = U64_MAX;
> +        return 0;
> cpucfg6 checking will be added for PMU support soon. So it will be
>          case 6:
>              do something check for cpucfg6
>              return mask;
>          case 0 ... 1:
>          case 3 ... 5:
>          case 7 ... KVM_MAX_CPUCFG_REGS - 1:
>                *v = U64_MAX;
>              return 0;
> 
> If you think it is reasonable to add these separate "case" sentences, I 
> have no objection.
>> * all-allow rules for other in-range CPUCFG IDs, and
>> * rejection for out-of-range IDs.
>   static int kvm_check_cpucfg(int id, u64 val)
>   {
> -    u64 mask;
> -    int ret = 0;
> -
> -    if (id < 0 && id >= KVM_MAX_CPUCFG_REGS)
> -        return -EINVAL;
> you can modify && with ||, like this:
>      if (id < 0 || id >= KVM_MAX_CPUCFG_REGS)
>          return -EINVAL;

Yes I know. Personally I don't find the "three cases" that annoying, but 
I agree with you that it can be mildly frustrating if one case addition 
would split the ranges even more.

Given I'd like to also change the U64_MAX into U32_MAX (CPUCFG data is 
specified to be 32-bit wide), I'll send a v4 that keeps the "if" branch 
to make more people happy (Huacai privately also expressed preference 
for minimizing changes to the overall code shape). Thanks for your 
suggestion.

-- 
WANG "xen0n" Xuerui

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