linux-kernel - Re: [PATCH] x86/boot: skip realmode init code when running as Xen PV guest

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <99a452d0-18f2-f4ab-522a-dec15d913d3b@suse.com>
Date:   Tue, 22 Nov 2022 07:28:21 +0100
From:   Juergen Gross <jgross@...e.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86/boot: skip realmode init code when running as Xen PV
 guest

On 21.11.22 20:08, Borislav Petkov wrote:
> On Mon, Nov 21, 2022 at 05:24:33PM +0100, Juergen Gross wrote:
>> When running as a Xen PV guest there is no need for setting up the
>> realmode trampoline, as realmode isn't supported in this environment.
>>
>> Trying to setup the trampoline has been proven to be problematic in
>> some cases, especially when trying to debug early boot problems with
>> Xen requiring to keep the EFI boot-services memory mapped (some
>> firmware variants seem to claim basically all memory below 1M for boot
>> services).
>>
>> Skip the trampoline setup code for Xen PV guests.
>>
>> Fixes: 084ee1c641a0 ("x86, realmode: Relocator for realmode code")
>> Signed-off-by: Juergen Gross <jgross@...e.com>
>> ---
>>   arch/x86/include/asm/realmode.h | 4 ++--
>>   arch/x86/realmode/init.c        | 3 +++
>>   2 files changed, 5 insertions(+), 2 deletions(-)
>   
>> diff --git a/arch/x86/include/asm/realmode.h b/arch/x86/include/asm/realmode.h
>> index fd6f6e5b755a..5bfce58f1bab 100644
>> --- a/arch/x86/include/asm/realmode.h
>> +++ b/arch/x86/include/asm/realmode.h
>> @@ -78,8 +78,8 @@ extern unsigned char secondary_startup_64_no_verify[];
>>   
>>   static inline size_t real_mode_size_needed(void)
>>   {
>> -	if (real_mode_header)
>> -		return 0;	/* already allocated. */
>> +	if (real_mode_header || cpu_feature_enabled(X86_FEATURE_XENPV))
>> +		return 0;	/* already allocated or not needed. */
>>   
>>   	return ALIGN(real_mode_blob_end - real_mode_blob, PAGE_SIZE);
>>   }
>> diff --git a/arch/x86/realmode/init.c b/arch/x86/realmode/init.c
>> index 41d7669a97ad..1826700b156e 100644
>> --- a/arch/x86/realmode/init.c
>> +++ b/arch/x86/realmode/init.c
>> @@ -202,6 +202,9 @@ static void __init set_real_mode_permissions(void)
>>   
>>   static int __init init_real_mode(void)
>>   {
>> +	if (cpu_feature_enabled(X86_FEATURE_XENPV))a
> 
> This reminds me of the notorious if (xen) sprinkling from years ago.
> Please don't do that.
> 

Okay, what about plan B:

- rework realmode/rm to:
   + replace header.S with main.c making it possible to initialize
     struct real_mode_header using the struct definition
   + optional: merge stack.S into main.c
- include realmode/rm addresses needed outside of it in struct
   real_mode_header
- setup a dummy struct real_mode_header in Xen PV code removing the
   need to skip init_real_mode(), but making it basically a nop

Would you be fine with that?


Juergen

Download attachment "OpenPGP_0xB0DE9DD628BF132F.asc" of type "application/pgp-keys" (3099 bytes)

Download attachment "OpenPGP_signature" of type "application/pgp-signature" (496 bytes)