linux-kernel - Re: [PATCH v8 1/2] x86/boot/compressed: Skip Video Memory access in Decompressor for SEV-ES/SNP.

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Date: Wed, 19 Jun 2024 12:22:06 +0200
From: Borislav Petkov <bp@...en8.de>
To: Ashish Kalra <Ashish.Kalra@....com>
Cc: tglx@...utronix.de, mingo@...hat.com, dave.hansen@...ux.intel.com,
	x86@...nel.org, rafael@...nel.org, hpa@...or.com,
	peterz@...radead.org, adrian.hunter@...el.com,
	sathyanarayanan.kuppuswamy@...ux.intel.com, jun.nakajima@...el.com,
	rick.p.edgecombe@...el.com, thomas.lendacky@....com,
	michael.roth@....com, seanjc@...gle.com, kai.huang@...el.com,
	bhe@...hat.com, kirill.shutemov@...ux.intel.com, bdas@...hat.com,
	vkuznets@...hat.com, dionnaglaze@...gle.com, anisinha@...hat.com,
	jroedel@...e.de, ardb@...nel.org, dyoung@...hat.com,
	kexec@...ts.infradead.org, linux-coco@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 1/2] x86/boot/compressed: Skip Video Memory access in
 Decompressor for SEV-ES/SNP.

On Mon, Jun 17, 2024 at 09:15:12PM +0000, Ashish Kalra wrote:
> From: Ashish Kalra <ashish.kalra@....com>
> 
> Accessing guest video memory/RAM during kernel decompressor
> causes guest termination as boot stage2 #VC handler for
> SEV-ES/SNP systems does not support MMIO handling.
> 
> This issue is observed with SEV-ES/SNP guest kexec as
> kexec -c adds screen_info to the boot parameters
> passed to the kexec kernel, which causes console output to
> be dumped to both video and serial.
> 
> As the decompressor output gets cleared really fast, it is
> preferable to get the console output only on serial, hence,
> skip accessing video RAM during decompressor stage to
> prevent guest termination.
> 
> Add early_sev_detect() to detect SEV-ES/SNP guest and skip
> accessing video RAM during decompressor stage.
> 
> Serial console output during decompressor stage works as
> boot stage2 #VC handler already supports handling port I/O.
> 
> Suggested-by: Borislav Petkov <Borislav.Petkov@....com>
> Suggested-by: Thomas Lendacy <thomas.lendacky@....com>
> Signed-off-by: Ashish Kalra <ashish.kalra@....com>
> Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>
> ---
>  arch/x86/boot/compressed/misc.c | 23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)

Use this massaged version for your next submission:

From: Ashish Kalra <ashish.kalra@....com>
Date: Mon, 17 Jun 2024 21:15:12 +0000
Subject: [PATCH] x86/boot: Skip video memory access in the decompressor for SEV-ES/SNP

Accessing guest video memory/RAM in the decompressor causes guest
termination as the boot stage2 #VC handler for SEV-ES/SNP systems does
not support MMIO handling.

This issue is observed during a SEV-ES/SNP guest kexec as kexec -c adds
screen_info to the boot parameters passed to the second kernel, which
causes console output to be dumped to both video and serial.

As the decompressor output gets cleared really fast, it is preferable to
get the console output only on serial, hence, skip accessing the video
RAM during decompressor stage to prevent guest termination.

Serial console output during decompressor stage works as boot stage2 #VC
handler already supports handling port I/O.

  [ bp: Massage. ]

Suggested-by: Borislav Petkov (AMD) <bp@...en8.de>
Suggested-by: Thomas Lendacy <thomas.lendacky@....com>
Signed-off-by: Ashish Kalra <ashish.kalra@....com>
Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@...ux.intel.com>
---
 arch/x86/boot/compressed/misc.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
index 944454306ef4..826b4d5cb1f0 100644
--- a/arch/x86/boot/compressed/misc.c
+++ b/arch/x86/boot/compressed/misc.c
@@ -385,6 +385,19 @@ static void parse_mem_encrypt(struct setup_header *hdr)
 		hdr->xloadflags |= XLF_MEM_ENCRYPTION;
 }
 
+static void early_sev_detect(void)
+{
+	/*
+	 * Accessing video memory causes guest termination because
+	 * the boot stage2 #VC handler of SEV-ES/SNP guests does not
+	 * support MMIO handling and kexec -c adds screen_info to the
+	 * boot parameters passed to the kexec kernel, which causes
+	 * console output to be dumped to both video and serial.
+	 */
+	if (sev_status & MSR_AMD64_SEV_ES_ENABLED)
+		lines = cols = 0;
+}
+
 /*
  * The compressed kernel image (ZO), has been moved so that its position
  * is against the end of the buffer used to hold the uncompressed kernel
@@ -440,6 +453,8 @@ asmlinkage __visible void *extract_kernel(void *rmode, unsigned char *output)
 	 */
 	early_tdx_detect();
 
+	early_sev_detect();
+
 	console_init();
 
 	/*
-- 
2.43.0

-- 
Regards/Gruss,
    Boris.

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