linux-kernel - Re: [PATCH v12 19/46] x86/kernel: Make the .bss..decrypted section shared in RMP table

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Message-ID: <YqfabnTRxFSM+LoX@google.com>
Date:   Tue, 14 Jun 2022 00:46:38 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Michael Roth <michael.roth@....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,
        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>,
        Vitaly Kuznetsov <vkuznets@...hat.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>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Borislav Petkov <bp@...en8.de>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        brijesh.ksingh@...il.com, tony.luck@...el.com, marcorr@...gle.com,
        sathyanarayanan.kuppuswamy@...ux.intel.com
Subject: Re: [PATCH v12 19/46] x86/kernel: Make the .bss..decrypted section
 shared in RMP table

s/Brijesh/Michael

On Mon, Mar 07, 2022, 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@....com>
> ---
>  arch/x86/kernel/head64.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
> 
> diff --git a/arch/x86/kernel/head64.c b/arch/x86/kernel/head64.c
> index 83514b9827e6..656d2f3e2cf0 100644
> --- a/arch/x86/kernel/head64.c
> +++ b/arch/x86/kernel/head64.c
> @@ -143,7 +143,20 @@ static unsigned long __head sme_postprocess_startup(struct boot_params *bp, pmdv
>  	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) {
> +			/*
> +			 * 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.
> +			 */
> +			early_snp_set_memory_shared(__pa(vaddr), __pa(vaddr), PTRS_PER_PMD);

This breaks SME on Rome and Milan when compiling with clang-13.  I haven't been
able to figure out exactly what goes wrong.  printk isn't functional at this point,
and interactive debug during boot on our test systems is beyond me.  I can't even
verify that the bug is specific to clang because the draconian build system for our
test systems apparently is stuck pointing at gcc-4.9.

I suspect the issue is related to relocation and/or encrypting memory, as skipping
the call to early_snp_set_memory_shared() if SNP isn't active masks the issue.
I've dug through the assembly and haven't spotted a smoking gun, e.g. no obvious
use of absolute addresses.

Forcing a VM through the same path doesn't fail.  I can't test an SEV guest at the
moment because INIT_EX is also broken.

The crash incurs a very, very slow reboot, and I was out of cycles to work on this
about three hours ago.  If someone on the AMD side can repro, it would be much
appreciated.