linux-kernel - Re: [PATCHv4 10/30] x86/tdx: Handle CPUID via #VE

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Date:   Sun, 27 Feb 2022 04:07:33 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        luto@...nel.org, peterz@...radead.org,
        sathyanarayanan.kuppuswamy@...ux.intel.com, aarcange@...hat.com,
        ak@...ux.intel.com, dan.j.williams@...el.com, david@...hat.com,
        hpa@...or.com, jgross@...e.com, jmattson@...gle.com,
        joro@...tes.org, jpoimboe@...hat.com, knsathya@...nel.org,
        pbonzini@...hat.com, sdeep@...are.com, seanjc@...gle.com,
        tony.luck@...el.com, vkuznets@...hat.com, wanpengli@...cent.com,
        thomas.lendacky@....com, brijesh.singh@....com, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv4 10/30] x86/tdx: Handle CPUID via #VE

On Thu, Feb 24, 2022 at 11:04:04AM -0800, Dave Hansen wrote:
> On 2/24/22 07:56, Kirill A. Shutemov wrote:
> >  static bool virt_exception_user(struct pt_regs *regs, struct ve_info *ve)
> >  {
> > -	pr_warn("Unexpected #VE: %lld\n", ve->exit_reason);
> > -	return false;
> > +	switch (ve->exit_reason) {
> > +	case EXIT_REASON_CPUID:
> > +		return handle_cpuid(regs);
> > +	default:
> > +		pr_warn("Unexpected #VE: %lld\n", ve->exit_reason);
> > +		return false;
> > +	}
> >  }
> 
> What does this mean for userspace?  What kinds of things are we ceding
> to the (untrusted) VMM to supply to userspace?

Here's what I see called from userspace.
CPUID(AX=0x2)
CPUID(AX=0xb, CX=0x0)
CPUID(AX=0xb, CX=0x1)
CPUID(AX=0x40000000, CX=0xfffaba17)
CPUID(AX=0x80000007, CX=0x121)

> >  /* Handle the kernel #VE */
> > @@ -200,6 +235,8 @@ static bool virt_exception_kernel(struct pt_regs *regs, struct ve_info *ve)
> >  		return read_msr(regs);
> >  	case EXIT_REASON_MSR_WRITE:
> >  		return write_msr(regs);
> > +	case EXIT_REASON_CPUID:
> > +		return handle_cpuid(regs);
> >  	default:
> >  		pr_warn("Unexpected #VE: %lld\n", ve->exit_reason);
> >  		return false;
> What kinds of random CPUID uses in the kernel at runtime need this
> handling?

CPUID(AX=0x2)
CPUID(AX=0x6, CX=0x0)
CPUID(AX=0xb, CX=0x0)
CPUID(AX=0xb, CX=0x1)
CPUID(AX=0xb, CX=0x2)
CPUID(AX=0xf, CX=0x0)
CPUID(AX=0xf, CX=0x1)
CPUID(AX=0x10, CX=0x0)
CPUID(AX=0x10, CX=0x1)
CPUID(AX=0x10, CX=0x2)
CPUID(AX=0x10, CX=0x3)
CPUID(AX=0x16, CX=0x0)
CPUID(AX=0x1f, CX=0x0)
CPUID(AX=0x40000000, CX=0x0)
CPUID(AX=0x40000000, CX=0xfffaba17)
CPUID(AX=0x40000001, CX=0x0)
CPUID(AX=0x80000002, CX=0x0)
CPUID(AX=0x80000003, CX=0x0)
CPUID(AX=0x80000004, CX=0x0)
CPUID(AX=0x80000007, CX=0x0)
CPUID(AX=0x80000007, CX=0x121)

> Is it really OK that we let the VMM inject arbitrary CPUID
> values into random CPUID uses in the kernel... silently?

We realise that this is possible vector of attack and plan to implement
proper filtering. But it is beyon core enabling.

> Is this better than just returning 0's, for instance?

Plain 0 injection breaks the boot. More complicated solution is need.


-- 
 Kirill A. Shutemov