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

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Date:   Mon, 5 Apr 2021 16:33:02 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Tom Lendacky <thomas.lendacky@....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 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?