linux-kernel - [PATCH] x86/reboot: Don't corrupt memory on non-BIOS systems

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Message-Id: <20250109204352.1720337-1-romank@linux.microsoft.com>
Date: Thu,  9 Jan 2025 12:43:52 -0800
From: Roman Kisel <romank@...ux.microsoft.com>
To: akpm@...ux-foundation.org,
	bhe@...hat.com,
	bp@...en8.de,
	dave.hansen@...ux.intel.com,
	hpa@...or.com,
	kai.huang@...el.com,
	kirill.shutemov@...ux.intel.com,
	mingo@...hat.com,
	pbonzini@...hat.com,
	tglx@...utronix.de,
	x86@...nel.org,
	linux-kernel@...r.kernel.org
Cc: apais@...rosoft.com,
	benhill@...rosoft.com,
	ssengar@...rosoft.com,
	sunilmut@...rosoft.com,
	vdso@...bites.dev
Subject: [PATCH] x86/reboot: Don't corrupt memory on non-BIOS systems

native_machine_emergency_restart() writes unconditionally
to the physical address of 0x472 to pass the warm reboot
flags to BIOS. The BIOS reads this on booting to bypass memory
test and do the warm boot. On the non-BIOS systems, other
means have to be employed, and this write is a memory corruption.

Fix that by moving the offending write into the case where
the machine is rebooted via BIOS.

Signed-off-by: Roman Kisel <romank@...ux.microsoft.com>
---
 arch/x86/kernel/reboot.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/reboot.c b/arch/x86/kernel/reboot.c
index 615922838c51..6eec8653493f 100644
--- a/arch/x86/kernel/reboot.c
+++ b/arch/x86/kernel/reboot.c
@@ -637,9 +637,8 @@ static void native_machine_emergency_restart(void)
 
 	tboot_shutdown(TB_SHUTDOWN_REBOOT);
 
-	/* Tell the BIOS if we want cold or warm reboot */
+	/* Tell the firmware if we want cold or warm reboot */
 	mode = reboot_mode == REBOOT_WARM ? 0x1234 : 0;
-	*((unsigned short *)__va(0x472)) = mode;
 
 	/*
 	 * If an EFI capsule has been registered with the firmware then
@@ -681,6 +680,7 @@ static void native_machine_emergency_restart(void)
 			break;
 
 		case BOOT_BIOS:
+			*((unsigned short *)__va(0x472)) = mode;
 			machine_real_restart(MRR_BIOS);
 
 			/* We're probably dead after this, but... */

base-commit: eea6e4b4dfb8859446177c32961c96726d0117be
-- 
2.34.1