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

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Message-ID: <03D69594-6E5E-47B1-ACF7-B0222F642DE5@zytor.com>
Date: Tue, 25 Feb 2025 12:28:11 -0800
From: "H. Peter Anvin" <hpa@...or.com>
To: Ingo Molnar <mingo@...nel.org>, Roman Kisel <romank@...ux.microsoft.com>
CC: 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,
        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 February 25, 2025 12:25:12 PM PST, Ingo Molnar <mingo@...nel.org> wrote:
>
>* 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);
>
>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?