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: <Y34GuNXaa3csthJY@zn.tnic>
Date:   Wed, 23 Nov 2022 12:40:40 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     "Kalra, Ashish" <ashish.kalra@....com>
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

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.

> 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.

> 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.

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?

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette