linux-kernel - Re: [PATCHv6 07/30] x86/traps: Add #VE support for TDX guest

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Date:   Thu, 17 Mar 2022 19:18:15 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     mingo@...hat.com, bp@...en8.de, dave.hansen@...el.com,
        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,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCHv6 07/30] x86/traps: Add #VE support for TDX guest

On Thu, Mar 17 2022 at 20:33, Kirill A. Shutemov wrote:
> On Thu, Mar 17, 2022 at 01:48:54AM +0100, Thomas Gleixner wrote:
>> On Wed, Mar 16 2022 at 05:08, Kirill A. Shutemov wrote:
>> Hmm?
>
> Does the changed version below address your concerns?
>
> 	void tdx_get_ve_info(struct ve_info *ve)
> 	{
> 		struct tdx_module_output out;
>
> 		/*
> 		 * Called during #VE handling to retrieve the #VE info from the
> 		 * TDX module.
> 		 *
> 		 * This has to be called early in #VE handling.  A "nested" #VE which
> 		 * occurs before this will raise a #DF and is not recoverable.
> 		 *
> 		 * The call retrieves the #VE info from the TDX module, which also
> 		 * clears the "#VE valid" flag. This must be done before anything else
> 		 * because any #VE that occurs while the valid flag is set will lead to
> 		 * #DF.
> 		 *
> 		 * Note, the TDX module treats virtual NMIs as inhibited if the #VE
> 		 * valid flag is set. It means that NMI=>#VE will not result in a #DF.
> 		 */
> 		tdx_module_call(TDX_GET_VEINFO, 0, 0, 0, 0, &out);
>
> 		/* Transfer the output parameters */
> 		ve->exit_reason = out.rcx;
> 		ve->exit_qual   = out.rdx;
> 		ve->gla         = out.r8;
> 		ve->gpa         = out.r9;
> 		ve->instr_len   = lower_32_bits(out.r10);
> 		ve->instr_info  = upper_32_bits(out.r10);
> 	}

Nice.

>> The point is that any #VE in such a code path is fatal and you better
>> come up with some reasonable explanation why this is not the case in
>> those code pathes and how a potential violation of that assumption might
>> be detected especially in rarely used corner cases. If such a violation
>> is not detectable by audit, CI, static code analysis or whatever then
>> document the consequences instead of pretending that the problem does
>> not exist and the kernel is perfect today and forever.
>
> It is detectable by audit. The critical windows very limited and located
> in the highly scrutinized entry code. But, yes, I cannot guarantee that
> this code will be perfect forever.

Fair enough.

> Consequences of #VE in these critical windows are mentioned in the
> comment:
>
> 	Any exception in this window leads to hard to debug issues and can
> 	be exploited for privilege escalation. 
>
> I have hard time understanding what I has to change here. Do you want
> details of audit to be documented? Make consequences of #VE at the wrong
> point to be more prominent in the comment? 

So having something like this in the comment would be helpful:

        *
	* The entry code has been audited carefuly for following these
        * expectations. Changes in the entry code have to be audited for
        * correctness vs. this aspect.  #VE in these places will cause
        * [an instant kernel panic | whatever | fill the blanks ]
        *

Thanks,

        tglx