linux-kernel - Re: [PATCH v2 1/2] x86/sev/vc: fix efi runtime instruction emulation

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Message-ID: <20250623154412.GHaFl2TJL0iGvy30JY@fat_crate.local>
Date: Mon, 23 Jun 2025 17:44:12 +0200
From: Borislav Petkov <bp@...en8.de>
To: Tom Lendacky <thomas.lendacky@....com>
Cc: Gerd Hoffmann <kraxel@...hat.com>, linux-coco@...ts.linux.dev,
	kvm@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	"open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/2] x86/sev/vc: fix efi runtime instruction emulation

I have this now:

From: Gerd Hoffmann <kraxel@...hat.com>
Date: Mon, 2 Jun 2025 12:50:48 +0200
Subject: [PATCH] x86/sev: Fix EFI runtime instruction emulation

In case efi_mm is active use the userspace instruction decoder which
supports fetching instructions from active_mm.  This is needed to make
instruction emulation work for EFI runtime code, so it can use CPUID and
RDMSR.

EFI runtime code uses the CPUID instruction to gather information about
the environment it is running in, such as SEV being enabled or not, and
choose (if needed) the SEV code path for ioport access.

EFI runtime code uses the RDMSR instruction to get the location of the
CAA page (see SVSM spec, section 4.2 - "Post Boot").

The big picture behind this is that the kernel needs to be able to
properly handle #VC exceptions that come from EFI runtime services.
Since EFI runtime services have a special page table mapping for the EFI
virtual address space, the efi_mm context must be used when decoding
instructions during #VC handling.

  [ bp: Massage and extend commit message with more backstory, add
    clarifying comment from Tom. ]

Signed-off-by: Gerd Hoffmann <kraxel@...hat.com>
Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
Link: https://lore.kernel.org/20250602105050.1535272-2-kraxel@redhat.com
---
 arch/x86/coco/sev/vc-handle.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/x86/coco/sev/vc-handle.c b/arch/x86/coco/sev/vc-handle.c
index 0989d98da130..e498a8965939 100644
--- a/arch/x86/coco/sev/vc-handle.c
+++ b/arch/x86/coco/sev/vc-handle.c
@@ -17,6 +17,7 @@
 #include <linux/mm.h>
 #include <linux/io.h>
 #include <linux/psp-sev.h>
+#include <linux/efi.h>
 #include <uapi/linux/sev-guest.h>
 
 #include <asm/init.h>
@@ -178,9 +179,14 @@ static enum es_result __vc_decode_kern_insn(struct es_em_ctxt *ctxt)
 		return ES_OK;
 }
 
+/*
+ * User instruction decoding is also required for the EFI runtime. Even though
+ * EFI runtime is running in kernel mode, it uses special EFI virtual address
+ * mappings that require the use of efi_mm to properly address and decode.
+ */
 static enum es_result vc_decode_insn(struct es_em_ctxt *ctxt)
 {
-	if (user_mode(ctxt->regs))
+	if (user_mode(ctxt->regs) || current->active_mm == &efi_mm)
 		return __vc_decode_user_insn(ctxt);
 	else
 		return __vc_decode_kern_insn(ctxt);
-- 
2.43.0


-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette