linux-kernel - Re: [PATCH] KVM: SVM: Fix sparse: incorrect type in argument 1 (different base types)

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Date:   Thu, 22 Feb 2018 09:56:57 -0600
From:   Brijesh Singh <brijesh.singh@....com>
To:     Al Viro <viro@...IV.linux.org.uk>
Cc:     brijesh.singh@....com, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Borislav Petkov <bp@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        linux-kernel@...r.kernel.org, Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH] KVM: SVM: Fix sparse: incorrect type in argument 1
 (different base types)



On 02/21/2018 02:18 PM, Al Viro wrote:
> On Wed, Feb 21, 2018 at 01:59:55PM -0600, Brijesh Singh wrote:
> 
>> Sure, checking access_ok() does not guarantee that later
>> copy_from_user() will not fail. But it does eliminate one possible
>> reason for the failure. We are trying to validate most of the user
>> inputs before we invoke  SEV command.
> 
> That makes no sense whatsoever.  If user is deliberately fuzzing
> your code or trying to DoS it, that "validation" doesn't buy you
> anything - they can just as well feed you NULL, after all.
> 


Currently, we let user query the blob length with params.len == 0 || 
param.uaddr == NULL. We could limit it to just params.len == 0.


> What is the rationale for that?  "Userland is accidentally feeding
> us garbage pointers" is the case where slowness is the least of your
> concerns...
> 

My intent was to do some obvious failure checks on user inputs before 
invoking the HW. I do see your point that if userspace is feeding us 
garbage then slowness is least of our concern. If you think that we 
should not be using access_ok() in this particular case then I am okay 
with it.