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

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Date:   Thu, 7 Jul 2022 20:43:40 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Tom Lendacky <thomas.lendacky@....com>,
        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>,
        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 Fri, Jul 01, 2022, Borislav Petkov wrote:
> On Thu, Jun 16, 2022 at 04:41:05PM +0000, Sean Christopherson wrote:
> > > I worry that another use of cc_platform_has() could creep in at some point
> > > and cause the same issue. Not sure how bad it would be, performance-wise, to
> > > remove the jump table optimization for arch/x86/coco/core.c.
> 
> Is there a gcc switch for that?

I believe -fno-jump-tables will do the trick.  That also reminds me exactly why
CONFIG_RETPOLINE=y isn't broken, jump tables are disabled when retpolines are enabled[*].

[*] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86952

> > One thought would be to initialize "vendor" to a bogus value, disallow calls to
> > cc_set_vendor() until after the kernel as gotten to a safe point, and then WARN
> > (or panic?) if cc_platform_has() is called before "vendor" is explicitly set.
> > New calls can still get in, but they'll be much easier to detect and less likely
> > to escape initial testing.
> 
> The invalid vendor thing makes sense but I don't think it'll help in
> this case.
> 
> We set vendor in sme_enable() which comes before the
> 
> __startup_64 -> sme_postprocess_startup
> 
> path you're hitting.

Right, but that's easily solved, no?  E.g.

diff --git a/arch/x86/mm/mem_encrypt_amd.c b/arch/x86/mm/mem_encrypt_amd.c
index e8f7953fda83..ed3118f5bf62 100644
--- a/arch/x86/mm/mem_encrypt_amd.c
+++ b/arch/x86/mm/mem_encrypt_amd.c
@@ -487,6 +487,8 @@ void __init sme_early_init(void)
        if (!sme_me_mask)
                return;

+       cc_set_vendor(CC_VENDOR_AMD);
+
        early_pmd_flags = __sme_set(early_pmd_flags);

        __supported_pte_mask = __sme_set(__supported_pte_mask);
diff --git a/arch/x86/mm/mem_encrypt_identity.c b/arch/x86/mm/mem_encrypt_identity.c
index f415498d3175..6b1c60032400 100644
--- a/arch/x86/mm/mem_encrypt_identity.c
+++ b/arch/x86/mm/mem_encrypt_identity.c
@@ -611,7 +611,6 @@ void __init sme_enable(struct boot_params *bp)
 out:
        if (sme_me_mask) {
                physical_mask &= ~sme_me_mask;
-               cc_set_vendor(CC_VENDOR_AMD);
                cc_set_mask(sme_me_mask);
        }
 }

And disallow cc_set_vendor() before x86_64_start_kernel(), then fix any fallout.

> We could do only the aspect of checking whether it hasn't been set yet
> and warn then, in order to make the usage more robust...
> 
> -- 
> Regards/Gruss,
>     Boris.
> 
> https://people.kernel.org/tglx/notes-about-netiquette