Re: [PATCH Part1 RFC v3 12/22] x86/kernel: Make the bss.decrypted section shared in RMP table

Borislav Petkov
2021-06-10T16:06:26Z

On Wed, Jun 02, 2021 at 09:04:06AM -0500, Brijesh Singh wrote:
> The encryption attribute for the bss.decrypted region 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@amd.com>
> ---
> arch/x86/kernel/head64.c | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
> index de01903c3735..f4c3e632345a 100644
> --- a/arch/x86/kernel/head64.c
> +++ b/arch/x86/kernel/head64.c
> @@ -288,7 +288,14 @@ unsigned long __head __startup_64(unsigned long physaddr,
> if (mem_encrypt_active()) {
> 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.
> + */
> + early_snp_set_memory_shared(__pa(vaddr), __pa(vaddr), PTRS_PER_PMD);
> +
> i = pmd_index(vaddr);
> pmd[i] -= sme_get_me_mask();
> }
> --

It seems to me that all that code from the sme_encrypt_kernel(bp); call
to the end of the function should be in a separate function in sev.c
called sev_prepare_kernel(...args...) to be at least abstracted away
from the main boot path.

Thx.

--
Regards/Gruss,
Boris.