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

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Date:   Tue, 14 Jun 2022 16:13:33 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Tom Lendacky <thomas.lendacky@....com>
Cc:     Michael Roth <michael.roth@....com>, 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>,
        "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>,
        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

On Tue, Jun 14, 2022, Tom Lendacky wrote:
> On 6/14/22 10:43, Sean Christopherson wrote:
> > On Tue, Jun 14, 2022, Sean Christopherson wrote:
> > > 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 SEV INIT_EX was a PEBKAC issue.  An SEV guest boots just fine with a clang-built
> > kernel, so either it's a finnicky relocation issue or something specific to SME.
> 
> I just built and booted 5.19-rc2 with clang-13 and SME enabled without issue:
> 
> [    4.118226] Memory Encryption Features active: AMD SME

Phooey. 

> Maybe something with your kernel config? Can you send me your config?

Attached.  If you can't repro, I'll find someone on our end to work on this.

Thanks!

Download attachment ".config.bz2" of type "application/octet-stream" (28503 bytes)