Re: [PATCH v12 22/46] x86/sev: Use SEV-SNP AP creation to start secondary CPUs

[Sean Christopherson <seanjc@google.com>]

On Mon, Mar 07, 2022, Brijesh Singh wrote:
> From: Tom Lendacky <thomas.lendacky@amd.com>
> 
> To provide a more secure way to start APs under SEV-SNP, use the SEV-SNP
> AP Creation NAE event. This allows for guest control over the AP register
> state rather than trusting the hypervisor with the SEV-ES Jump Table
> address.
> 
> During native_smp_prepare_cpus(), invoke an SEV-SNP function that, if
> SEV-SNP is active, will set/override apic->wakeup_secondary_cpu. This
> will allow the SEV-SNP AP Creation NAE event method to be used to boot
> the APs. As a result of installing the override when SEV-SNP is active,
> this method of starting the APs becomes the required method. The override
> function will fail to start the AP if the hypervisor does not have
> support for AP creation.

...

> @@ -823,6 +843,230 @@ void snp_set_memory_private(unsigned long vaddr, unsigned int npages)
>  	pvalidate_pages(vaddr, npages, true);
>  }
>  
> +static int snp_set_vmsa(void *va, bool vmsa)
> +{
> +	u64 attrs;
> +
> +	/*
> +	 * Running at VMPL0 allows the kernel to change the VMSA bit for a page
> +	 * using the RMPADJUST instruction. However, for the instruction to
> +	 * succeed it must target the permissions of a lesser privileged
> +	 * VMPL level, so use VMPL1 (refer to the RMPADJUST instruction in the
> +	 * AMD64 APM Volume 3).
> +	 */
> +	attrs = 1;
> +	if (vmsa)
> +		attrs |= RMPADJUST_VMSA_PAGE_BIT;
> +
> +	return rmpadjust((unsigned long)va, RMP_PG_SIZE_4K, attrs);
> +}
> +
> +#define __ATTR_BASE		(SVM_SELECTOR_P_MASK | SVM_SELECTOR_S_MASK)
> +#define INIT_CS_ATTRIBS		(__ATTR_BASE | SVM_SELECTOR_READ_MASK | SVM_SELECTOR_CODE_MASK)
> +#define INIT_DS_ATTRIBS		(__ATTR_BASE | SVM_SELECTOR_WRITE_MASK)
> +
> +#define INIT_LDTR_ATTRIBS	(SVM_SELECTOR_P_MASK | 2)
> +#define INIT_TR_ATTRIBS		(SVM_SELECTOR_P_MASK | 3)
> +
> +static void *snp_alloc_vmsa_page(void)
> +{
> +	struct page *p;
> +
> +	/*
> +	 * Allocate VMSA page to work around the SNP erratum where the CPU will
> +	 * incorrectly signal an RMP violation #PF if a large page (2MB or 1GB)
> +	 * collides with the RMP entry of VMSA page. The recommended workaround
> +	 * is to not use a large page.
> +	 */
> +
> +	/* Allocate an 8k page which is also 8k-aligned */
> +	p = alloc_pages(GFP_KERNEL_ACCOUNT | __GFP_ZERO, 1);
> +	if (!p)
> +		return NULL;
> +
> +	split_page(p, 1);
> +
> +	/* Free the first 4k. This page may be 2M/1G aligned and cannot be used. */
> +	__free_page(p);
> +
> +	return page_address(p + 1);
> +}
> +
> +static void snp_cleanup_vmsa(struct sev_es_save_area *vmsa)
> +{
> +	int err;
> +
> +	err = snp_set_vmsa(vmsa, false);

Uh, so what happens if a malicious guest does RMPADJUST to convert a VMSA page
back to a "normal" page while the host is trying to VMRUN that VMSA?  Does VMRUN
fault?

Can Linux refuse to support this madness and instead require the ACPI MP wakeup
protocol being proposed/implemented for TDX?  That would allow KVM to have at
least a chance of refusing to support AP "creation", which IMO is a CVE or three
waiting to happen.  From a KVM perspective, I don't ever want to be running a
guest-defined VMSA.

https://lore.kernel.org/all/YWnbfCet84Vup6q9@google.com

> +	if (err)
> +		pr_err("clear VMSA page failed (%u), leaking page\n", err);
> +	else
> +		free_page((unsigned long)vmsa);