linux-kernel - Re: [PATCH Part2 v6 14/49] crypto: ccp: Handle the legacy TMR allocation when SNP is enabled

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Message-ID: <57c77407-d0bc-5e74-490b-f604b4f97691@amd.com>
Date:   Wed, 23 Nov 2022 12:32:19 -0600
From:   "Kalra, Ashish" <ashish.kalra@....com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-crypto@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
        jroedel@...e.de, thomas.lendacky@....com, hpa@...or.com,
        ardb@...nel.org, pbonzini@...hat.com, seanjc@...gle.com,
        vkuznets@...hat.com, jmattson@...gle.com, luto@...nel.org,
        dave.hansen@...ux.intel.com, slp@...hat.com, pgonda@...gle.com,
        peterz@...radead.org, srinivas.pandruvada@...ux.intel.com,
        rientjes@...gle.com, dovmurik@...ux.ibm.com, tobin@....com,
        michael.roth@....com, vbabka@...e.cz, kirill@...temov.name,
        ak@...ux.intel.com, tony.luck@...el.com, marcorr@...gle.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com, alpergun@...gle.com,
        dgilbert@...hat.com, jarkko@...nel.org
Subject: Re: [PATCH Part2 v6 14/49] crypto: ccp: Handle the legacy TMR
 allocation when SNP is enabled

Hello Boris,

On 11/23/2022 5:40 AM, Borislav Petkov wrote:
> On Tue, Nov 22, 2022 at 05:44:47AM -0600, Kalra, Ashish wrote:
>> It is important to note that if invalid address/len are supplied, the
>> failure will happen at the initial stage itself of transitioning these pages
>> to firmware state.
> 
> /me goes and checks out your v6 tree based on 5.18.
> 
> Lemme choose one:
> 
> static int snp_launch_update(struct kvm *kvm, struct kvm_sev_cmd *argp)
> {
> 	...
> 
>          inpages = sev_pin_memory(kvm, params.uaddr, params.len, &npages, 1);
> 
> 	...
> 
>          for (i = 0; i < npages; i++) {
>                  pfn = page_to_pfn(inpages[i]);
> 
> 		...
> 
>                  ret = __sev_issue_cmd(argp->sev_fd, SEV_CMD_SNP_LAUNCH_UPDATE, &data, error);
>                  if (ret) {
>                          /*
>                           * If the command failed then need to reclaim the page.
>                           */
>                          snp_page_reclaim(pfn);
> 
> and here it would leak the pages if it cannot reclaim them.
> 
> Now how did you get those?
> 
> Through params.uaddr and params.len which come from userspace:
> 
>          if (copy_from_user(&params, (void __user *)(uintptr_t)argp->data, sizeof(params)))
>                  return -EFAULT;
> 
> 
> Now, think about it, can userspace be trusted?
> 
> Exactly.
> 
> Yeah, yeah, I see it does is_hva_registered() but userspace can just as
> well supply the wrong region which fits.

Yes, that's right.

Also, before sev_issue_cmd() above, there is a call to 
rmp_make_private() to make these pages transition to firmware state 
before we issue the LAUNCH_UPDATE command as below:

            ret = rmp_make_private(pfn, gfn << PAGE_SHIFT, level,
				  sev_get_asid(kvm), true);
                 if (ret) {
                         ret = -EFAULT;
                         goto e_unpin;

                 }
...
  	ret = __sev_issue_cmd(argp->sev_fd, SEV_CMD_SNP_LAUNCH_UPDATE,
			      &data, error);

So in case, the userspace provided an invalid/incorrect range, this
transition would have failed and there would not have been a need to do 
any reclaim, so there are no pages leaked here.

This is also the reason why we need to reclaim pages if the subsequent 
LAUNCH_UPDATE command fails, as now the pages are in F/W state because 
of the rmp_make_private() call and they are now unsafe to be used by the 
host.

> 
>> In such a case the kernel panic is justifiable,
> 
> So userspace can supply whatever it wants and you'd panic?
> 
> You surely don't mean that.
> 

No, we don't want to do that.

>> but again if incorrect addresses are supplied, the failure will happen
>> at the initial stage of transitioning these pages to firmware state
>> and there is no need to reclaim.

This is the case i mentioned above, rmp_make_private() before the 
firmware command is the initial stage of transitioning the pages to 
firmware state before issuing the firmware command.

> 
> See above.
> 
>> Or, otherwise dump a warning and let the pages not be freed/returned
>> back to the page allocator.
>>
>> It is either innocent pages or kernel panic or an innocent host
>> process crash (these are the choices to make).
> 
> No, it is make the kernel as resilient as possible. Which means, no
> panic, add the pages to a not-to-be-used-anymore list and scream loudly
> with warning messages when it must leak pages so that people can fix the
> issue.
> 
> Ok?
> 

Right, yes, i totally agree.

So now we are adding these pages to an internal not-to-be-used-anymore 
list and printing warnings and ensuring no panics as we don't allow 
these pages to be released back to the page allocator.

Thanks,
Ashish