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

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Message-ID: <35864797-77DF-434C-A912-F4B5FA03E863@zytor.com>
Date: Thu, 09 Jan 2025 19:23:31 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Roman Kisel <romank@...ux.microsoft.com>, akpm@...ux-foundation.org,
        bhe@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.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: Re: [PATCH] x86/reboot: Don't corrupt memory on non-BIOS systems

On January 9, 2025 12:43:52 PM PST, Roman Kisel <romank@...ux.microsoft.com> wrote:
>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

Which system have you seen where this "corrupts" memory?