Re: [PATCH RFC v8 15/56] x86/sev: Invalidate pages from the direct map when adding them to the RMP table

[Tom Dohrmann]

On Mon, Feb 20, 2023 at 12:38:06PM -0600, Michael Roth wrote:
> From: Brijesh Singh <brijesh.singh@xxxxxxx>
>
> The integrity guarantee of SEV-SNP is enforced through the RMP table.
> The RMP is used with standard x86 and IOMMU page tables to enforce
> memory restrictions and page access rights. The RMP check is enforced as
> soon as SEV-SNP is enabled globally in the system. When hardware
> encounters an RMP-check failure, it raises a page-fault exception.
>
> The rmp_make_private() and rmp_make_shared() helpers are used to add
> or remove the pages from the RMP table. Improve the rmp_make_private()
> to invalidate state so that pages cannot be used in the direct-map after
> they are added the RMP table, and restored to their default valid
> permission after the pages are removed from the RMP table.
>
> Co-developed-by: Ashish Kalra <ashish.kalra@xxxxxxx>
> Signed-off-by: Ashish Kalra <ashish.kalra@xxxxxxx>
> Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
> Signed-off-by: Michael Roth <michael.roth@xxxxxxx>
> ---
>  arch/x86/kernel/sev.c | 57 +++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 57 insertions(+)
>
> diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
> index a49f30c10dc1..3e5ff5934e83 100644
> --- a/arch/x86/kernel/sev.c
> +++ b/arch/x86/kernel/sev.c
> @@ -2595,6 +2595,37 @@ int psmash(u64 pfn)
>  }
>  EXPORT_SYMBOL_GPL(psmash);
>
> +static int restore_direct_map(u64 pfn, int npages)
> +{
> +	int i, ret = 0;
> +
> +	for (i = 0; i < npages; i++) {
> +		ret = set_direct_map_default_noflush(pfn_to_page(pfn + i));
> +		if (ret)
> +			goto cleanup;
> +	}
> +
> +cleanup:
> +	WARN(ret > 0, "Failed to restore direct map for pfn 0x%llx\n", pfn + i);
> +	return ret;
> +}
> +
> +static int invalidate_direct_map(u64 pfn, int npages)
> +{
> +	int i, ret = 0;
> +
> +	for (i = 0; i < npages; i++) {
> +		ret = set_direct_map_invalid_noflush(pfn_to_page(pfn + i));
> +		if (ret)
> +			goto cleanup;
> +	}
> +
> +cleanup:
> +	WARN(ret > 0, "Failed to invalidate direct map for pfn 0x%llx\n", pfn + i);
> +	restore_direct_map(pfn, i);

This immediately restores the direct map after invalidating it. It
probably needs to put behind if(ret).

Regards, Tom

> +	return ret;
> +}
> +
>  static int rmpupdate(u64 pfn, struct rmp_state *val)
>  {
>  	int max_attempts = 4 * num_present_cpus();
> @@ -2605,6 +2636,21 @@ static int rmpupdate(u64 pfn, struct rmp_state *val)
>  	if (!cpu_feature_enabled(X86_FEATURE_SEV_SNP))
>  		return -ENXIO;
>
> +	level = RMP_TO_X86_PG_LEVEL(val->pagesize);
> +	npages = page_level_size(level) / PAGE_SIZE;
> +
> +	/*
> +	 * If page is getting assigned in the RMP table then unmap it from the
> +	 * direct map.
> +	 */
> +	if (val->assigned) {
> +		if (invalidate_direct_map(pfn, npages)) {
> +			pr_err("Failed to unmap %d pages at pfn 0x%llx from the direct_map\n",
> +			       npages, pfn);
> +			return -EFAULT;
> +		}
> +	}
> +
>  	do {
>  		/* Binutils version 2.36 supports the RMPUPDATE mnemonic. */
>  		asm volatile(".byte 0xF2, 0x0F, 0x01, 0xFE"
> @@ -2630,6 +2676,17 @@ static int rmpupdate(u64 pfn, struct rmp_state *val)
>  			 attempts, val->asid, ret, pfn, npages);
>  	}
>
> +	/*
> +	 * Restore the direct map after the page is removed from the RMP table.
> +	 */
> +	if (!val->assigned) {
> +		if (restore_direct_map(pfn, npages)) {
> +			pr_err("Failed to map %d pages at pfn 0x%llx into the direct_map\n",
> +			       npages, pfn);
> +			return -EFAULT;
> +		}
> +	}
> +
>  	return 0;
>  }
>
> --
> 2.25.1
>