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

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Date:   Thu, 10 Jun 2021 18:06:15 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        linux-efi@...r.kernel.org, platform-driver-x86@...r.kernel.org,
        linux-coco@...ts.linux.dev, linux-mm@...ck.org,
        linux-crypto@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>, tony.luck@...el.com,
        npmccallum@...hat.com
Subject: Re: [PATCH Part1 RFC v3 12/22] x86/kernel: Make the bss.decrypted
 section shared in RMP table

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@....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.

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