linux-kernel - Re: [PATCH 2/3] x86/cpu: fix unbootable VMs by inlining memcmp in hypervisor_cpuid

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Message-ID: <877ccz12ab.ffs@tglx>
Date: Fri, 02 Aug 2024 15:29:16 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Nikolay Borisov <nik.borisov@...e.com>, Alexey Dobriyan
 <adobriyan@...il.com>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov
 <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>
Cc: xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
 x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>, Juergen Gross
 <jgross@...e.com>, Boris Ostrovsky <boris.ostrovsky@...cle.com>
Subject: Re: [PATCH 2/3] x86/cpu: fix unbootable VMs by inlining memcmp in
 hypervisor_cpuid_base

On Fri, Aug 02 2024 at 16:25, Nikolay Borisov wrote:
> On 2.08.24 г. 11:50 ч., Alexey Dobriyan wrote:
>> If this memcmp() is not inlined then PVH early boot code can call
>> into KASAN-instrumented memcmp() which results in unbootable VMs:
>> 
>> 	pvh_start_xen
>> 	xen_prepare_pvh
>> 	xen_cpuid_base
>> 	hypervisor_cpuid_base
>> 	memcmp
>> 
>> Ubuntu's gcc version 11.4.0 (Ubuntu 11.4.0-1ubuntu1~22.04) inlines
>> memcmp with patch and the bug is partially fixed.
>> 
>> Leave FIXME just in case someone cares enough to compare 3 pairs of
>> integers like 3 pairs of integers.
>> 
>> Signed-off-by: Alexey Dobriyan <adobriyan@...il.com>
>> ---
>> 
>>   arch/x86/include/asm/cpuid.h | 15 ++++++++++++++-
>>   1 file changed, 14 insertions(+), 1 deletion(-)
>> 
>> diff --git a/arch/x86/include/asm/cpuid.h b/arch/x86/include/asm/cpuid.h
>> index 6b122a31da06..3eca7824430e 100644
>> --- a/arch/x86/include/asm/cpuid.h
>> +++ b/arch/x86/include/asm/cpuid.h
>> @@ -196,7 +196,20 @@ static inline uint32_t hypervisor_cpuid_base(const char *sig, uint32_t leaves)
>>   	for_each_possible_hypervisor_cpuid_base(base) {
>>   		cpuid(base, &eax, &signature[0], &signature[1], &signature[2]);
>>   
>> -		if (!memcmp(sig, signature, 12) &&
>> +		/*
>> +		 * FIXME rewrite cpuid comparators to accept uint32_t[3].
>> +		 *
>> +		 * This memcmp()
>> +		 * a) is called from PVH early boot code
>> +		 *    before instrumentation is set up,
>> +		 * b) may be compiled to "call memcmp" (not inlined),
>> +		 * c) memcmp() itself may be instrumented.
>> +		 *
>> +		 * Any combination of 2 is fine, but all 3 aren't.
>> +		 *
>> +		 * Force inline this function call.
>> +		 */
>> +		if (!__builtin_memcmp(sig, signature, 12) &&
>
> Instead of putting this giant FIXME, why not simply do the comparison as 
> ints, i.e ((uint32_t)&sig[0]) == signature1 && ((uitn32_t)&sig[4]) == 
> signature2 && ((uint32_t)&sig[8] == signature_3  and be done with it?

Because a smart compiler might turn it into a memcmp() :