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

["H. Peter Anvin" <hpa@zytor.com>]

On February 25, 2025 12:39:59 PM PST, Ingo Molnar <mingo@kernel.org> wrote:
>
>* H. Peter Anvin <hpa@zytor.com> wrote:
>
>> On February 25, 2025 12:25:12 PM PST, Ingo Molnar <mingo@kernel.org> wrote:
>> >
>> >* Roman Kisel <romank@linux.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@linux.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);
>> >
>> >If the value of 0x472 is only meaningful in the legacy 'BOOT_BIOS' 
>> >case, then at minimum the whole block should be moved to that case, not 
>> >just the setting.
>> >
>> >Thanks,
>> >
>> >	Ingo
>> 
>> Does the memory corruption actually matter, though?
>
>I presume the issue came up by auditing & reviewing host writes to a 
>barebones non-legacy VM?
>
>I'd argue that we shouldn't be writing random values into random legacy 
>addresses, especially if that special address doesnt seem to be covered 
>by any modern spec? Basic defensive coding practices and such.
>
>Thanks,
>
>	Ingo

The ultimate answer to the question is probably WWWD.