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

[H. Peter Anvin]

On January 9, 2025 12:43:52 PM PST, Roman Kisel <romank@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxxxxx>
>---
> 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?