Re: [PATCH v9 17/43] x86/kernel: Make the .bss..decrypted section shared in RMP table

[Borislav Petkov]

On Fri, Jan 28, 2022 at 11:17:38AM -0600, Brijesh Singh wrote:
> The encryption attribute for the .bss..decrypted section is cleared in the
> initial page table build. This is because the section contains the data
> that need to be shared between the guest and the hypervisor.
> 
> When SEV-SNP is active, just clearing the encryption attribute in the
> page table is not enough. The page state need to be updated in the RMP
> table.
> 
> Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
> ---
>  arch/x86/kernel/head64.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
> index 8075e91cff2b..1239bc104cda 100644
> --- a/arch/x86/kernel/head64.c
> +++ b/arch/x86/kernel/head64.c
> @@ -143,7 +143,21 @@ static unsigned long sme_postprocess_startup(struct boot_params *bp, pmdval_t *p
>  	if (sme_get_me_mask()) {
>  		vaddr = (unsigned long)__start_bss_decrypted;
>  		vaddr_end = (unsigned long)__end_bss_decrypted;
> +
>  		for (; vaddr < vaddr_end; vaddr += PMD_SIZE) {
> +			/*
> +			 * When SEV-SNP is active then transition the page to
> +			 * shared in the RMP table so that it is consistent with
> +			 * the page table attribute change.
> +			 *
> +			 * At this point, kernel is running in identity mapped mode.
> +			 * The __start_bss_decrypted is a regular kernel address. The
> +			 * early_snp_set_memory_shared() requires a valid virtual
> +			 * address, so use __pa() against __start_bss_decrypted to
> +			 * get valid virtual address.
> +			 */

How's that?

                        /*
                         * On SNP, transition the page to shared in the RMP table so that
                         * it is consistent with the page table attribute change.
                         *
                         * __start_bss_decrypted has a virtual address in the high range
                         * mapping (kernel .text). PVALIDATE, by way of
                         * early_snp_set_memory_shared(), requires a valid virtual
                         * address but the kernel is currently running off of the identity
                         * mapping so use __pa() to get a *currently* valid virtual address.
                         */

-- 
Regards/Gruss,
    Boris.

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