linux-kernel - Re: [PATCH] KVM: sev: Fix int overflow in send|recieve_update

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Message-ID: <d6c2455c-aff5-a135-2610-53dd6b586b59@amd.com>
Date:   Tue, 10 Jan 2023 11:16:30 -0600
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Peter Gonda <pgonda@...gle.com>
Cc:     kvm@...r.kernel.org, Andy Nguyen <theflow@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        stable@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: sev: Fix int overflow in send|recieve_update_data
 ioctls

On 1/10/23 10:44, Peter Gonda wrote:
>>>
>>> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
>>> index 273cba809328..9451de72f917 100644
>>> --- a/arch/x86/kvm/svm/sev.c
>>> +++ b/arch/x86/kvm/svm/sev.c
>>> @@ -1294,7 +1294,7 @@ static int sev_send_update_data(struct kvm *kvm, struct kvm_sev_cmd *argp)
>>>
>>>        /* Check if we are crossing the page boundary */
>>>        offset = params.guest_uaddr & (PAGE_SIZE - 1);
>>> -     if ((params.guest_len + offset > PAGE_SIZE))
>>> +     if (params.guest_len > PAGE_SIZE || (params.guest_len + offset > PAGE_SIZE))
>>
>> I see the original if statement had double parentheses, which looks
>> strange. Should this if (and the one below) be:
>>
>>          if (params.guest_len > PAGE_SIZE || (params.guest_len + offset) > PAGE_SIZE)
> 
> Isn't the order of operations here: '+' and then '>'. So is the patch
> correct and matches the old conditional? I am fine adding additional

But what was the purpose of them in the old conditional? They weren't
necessary.

But, yes, that order of operations is correct and those are both before
'||'. So the extra parentheses around the second condition check are still
strange then, right?

Given that, then:

	if (params.guest_len > PAGE_SIZE || params.guest_len + offset > PAGE_SIZE)

> () for clarity though.

I do like the look and clarity of the parentheses around the addition.

Thanks,
Tom