linux-kernel - [PATCH AUTOSEL 6.11 061/244] virt: sev-guest: Ensure the SNP guest messages do not exceed a page

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Message-ID: <20240925113641.1297102-61-sashal@kernel.org>
Date: Wed, 25 Sep 2024 07:24:42 -0400
From: Sasha Levin <sashal@...nel.org>
To: linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Cc: Nikunj A Dadhania <nikunj@....com>,
	Tom Lendacky <thomas.lendacky@....com>,
	Borislav Petkov <bp@...en8.de>,
	Sasha Levin <sashal@...nel.org>,
	tglx@...utronix.de,
	mingo@...hat.com,
	dave.hansen@...ux.intel.com,
	x86@...nel.org,
	ashish.kalra@....com,
	michael.roth@....com,
	brijesh.singh@....com,
	ardb@...nel.org,
	kevinloughlin@...gle.com,
	tzimmermann@...e.de,
	dan.j.williams@...el.com,
	sathyanarayanan.kuppuswamy@...ux.intel.com,
	u.kleine-koenig@...gutronix.de
Subject: [PATCH AUTOSEL 6.11 061/244] virt: sev-guest: Ensure the SNP guest messages do not exceed a page

From: Nikunj A Dadhania <nikunj@....com>

[ Upstream commit 2b9ac0b84c2cae91bbaceab62df4de6d503421ec ]

Currently, struct snp_guest_msg includes a message header (96 bytes) and
a payload (4000 bytes). There is an implicit assumption here that the
SNP message header will always be 96 bytes, and with that assumption the
payload array size has been set to 4000 bytes - a magic number. If any
new member is added to the SNP message header, the SNP guest message
will span more than a page.

Instead of using a magic number for the payload, declare struct
snp_guest_msg in a way that payload plus the message header do not
exceed a page.

  [ bp: Massage. ]

Suggested-by: Tom Lendacky <thomas.lendacky@....com>
Signed-off-by: Nikunj A Dadhania <nikunj@....com>
Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
Acked-by: Borislav Petkov (AMD) <bp@...en8.de>
Link: https://lore.kernel.org/r/20240731150811.156771-5-nikunj@amd.com
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 arch/x86/include/asm/sev.h              | 2 +-
 drivers/virt/coco/sev-guest/sev-guest.c | 2 ++
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/sev.h b/arch/x86/include/asm/sev.h
index 79bbe2be900eb..ee34ab00a8d6d 100644
--- a/arch/x86/include/asm/sev.h
+++ b/arch/x86/include/asm/sev.h
@@ -164,7 +164,7 @@ struct snp_guest_msg_hdr {
 
 struct snp_guest_msg {
 	struct snp_guest_msg_hdr hdr;
-	u8 payload[4000];
+	u8 payload[PAGE_SIZE - sizeof(struct snp_guest_msg_hdr)];
 } __packed;
 
 struct sev_guest_platform_data {
diff --git a/drivers/virt/coco/sev-guest/sev-guest.c b/drivers/virt/coco/sev-guest/sev-guest.c
index 6fc7884ea0a11..c86be0cd8ecd2 100644
--- a/drivers/virt/coco/sev-guest/sev-guest.c
+++ b/drivers/virt/coco/sev-guest/sev-guest.c
@@ -1090,6 +1090,8 @@ static int __init sev_guest_probe(struct platform_device *pdev)
 	void __iomem *mapping;
 	int ret;
 
+	BUILD_BUG_ON(sizeof(struct snp_guest_msg) > PAGE_SIZE);
+
 	if (!cc_platform_has(CC_ATTR_GUEST_SEV_SNP))
 		return -ENODEV;
 
-- 
2.43.0