linux-kernel - Re: [PATCH 2/5] crypto: ccp: Reject SEV commands with mismatching command buffer

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Message-ID: <d79ac2a4-10f7-2ff1-fb66-d246348493fa@amd.com>
Date:   Mon, 5 Apr 2021 11:37:13 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Brijesh Singh <brijesh.singh@....com>,
        John Allen <john.allen@....com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
        Borislav Petkov <bp@...e.de>
Subject: Re: [PATCH 2/5] crypto: ccp: Reject SEV commands with mismatching
 command buffer



On 4/5/21 11:33 AM, Sean Christopherson wrote:
> On Mon, Apr 05, 2021, Tom Lendacky wrote:
>> On 4/2/21 6:36 PM, Sean Christopherson wrote:
>>> diff --git a/drivers/crypto/ccp/sev-dev.c b/drivers/crypto/ccp/sev-dev.c
>>> index 6556d220713b..4c513318f16a 100644
>>> --- a/drivers/crypto/ccp/sev-dev.c
>>> +++ b/drivers/crypto/ccp/sev-dev.c
>>> @@ -141,6 +141,7 @@ static int __sev_do_cmd_locked(int cmd, void *data, int *psp_ret)
>>>  	struct sev_device *sev;
>>>  	unsigned int phys_lsb, phys_msb;
>>>  	unsigned int reg, ret = 0;
>>> +	int buf_len;
>>>  
>>>  	if (!psp || !psp->sev_data)
>>>  		return -ENODEV;
>>> @@ -150,7 +151,11 @@ static int __sev_do_cmd_locked(int cmd, void *data, int *psp_ret)
>>>  
>>>  	sev = psp->sev_data;
>>>  
>>> -	if (data && WARN_ON_ONCE(is_vmalloc_addr(data)))
>>> +	buf_len = sev_cmd_buffer_len(cmd);
>>> +	if (WARN_ON_ONCE(!!data != !!buf_len))
>>
>> Seems a bit confusing to me.  Can this just be:
>>
>> 	if (WARN_ON_ONCE(data && !buf_len))
> 
> Or as Christophe pointed out, "!data != !buf_len".
> 
>> Or is this also trying to catch the case where buf_len is non-zero but
>> data is NULL?
> 
> Ya.  It's not necessary to detect "buf_len && !data", but it doesn't incur
> additional cost.  Is there a reason _not_ to disallow that?

Nope, no reason. I was just trying to process all the not signs :)

Thanks,
Tom

>