linux-kernel - Re: [RFC PATCH] kvm: x86/vmx: Use kzalloc for cached

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Date:   Wed, 23 Jan 2019 10:25:40 -0800
From:   Tom Roeder <tmroeder@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Liran Alon <liran.alon@...cle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzbot+ded1696f6b50b615b630@...kaller.appspotmail.com
Subject: Re: [RFC PATCH] kvm: x86/vmx: Use kzalloc for cached_vmcs12

On Tue, Jan 15, 2019 at 11:15:51AM +0100, Paolo Bonzini wrote:
> On 15/01/19 03:43, Sean Christopherson wrote:
> >> -	vmx->nested.cached_vmcs12 = kmalloc(VMCS12_SIZE, GFP_KERNEL);
> >> +	vmx->nested.cached_vmcs12 = kzalloc(VMCS12_SIZE, GFP_KERNEL);
> >>  	if (!vmx->nested.cached_vmcs12)
> >>  		goto out_cached_vmcs12;
> > Obviously not your code, but why do we allocate VMCS12_SIZE instead of
> > sizeof(struct vmcs12)?  I get why we require userspace to reserve the
> > full 4k, but I don't understand why KVM needs to allocate the reserved
> > bytes internally.
> 
> It's just cleaner and shorter code to copy everything in and out,
> instead of having to explicitly zero the slack.

Could you please clarify? I don't see code that copies everything in and
out, but it depends on what you mean by "everything". In the context of
this email exchange, I assumed that "everything" was "all 4k
(VMCS12_SIZE)".

But it looks to me like the code doesn't copy 4k in and out, but rather
only ever copies sizeof(struct vmcs12) in and out. The copy_from_user
and copy_to_user cases in nested.c use sizeof(*vmcs12), which is
sizeof(struct vmcs12).

So maybe can switch to allocating sizeof(struct vmcs12). Is this
correct, or is there some other reason to allocate the larger size?