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

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Date:   Thu, 24 Jan 2019 02:17:27 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Tom Roeder <tmroeder@...gle.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 23/01/19 19:25, Tom Roeder wrote:
> 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)".

I was thinking of vmx_get_nested_state, but actually it only copies
sizeof(*vmcs12).  However, that is the place where we should copy 4k out
of it, including the zeroes.  Otherwise, our userspace clients (which
doesn't know sizeof(*vmcs12) could leak uninitialized data of their own.

Paolo

> 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?
>