[MODERATED] Re: [PATCH v6 1/9] TAAv6 1

On 10/10/2019 00:38, speck for Andrew Cooper wrote:
> On 10/10/2019 00:22, speck for Pawan Gupta wrote:
>> Transactional Synchronization Extensions (TSX) may be used on certain
>> processors as part of a speculative side channel attack.  A microcode
>> update for existing processors that are vulnerable to this attack will
>> add a new MSR, IA32_TSX_CTRL to allow the system administrator the
>> option to disable TSX as one of the possible mitigations.  [Note that
>> future processors that are not vulnerable will also support the
>> IA32_TSX_CTRL MSR].  Add defines for the new IA32_TSX_CTRL MSR and its
>> bits.
>>
>> TSX has two sub-features:
>>
>> 1. Restricted Transactional Memory (RTM) is an explicitly-used feature
>>    where new instructions begin and end TSX transactions.
>> 2. Hardware Lock Elision (HLE) is implicitly used when certain kinds of
>>    "old" style locks are used by software.
>>
>> Bit 7 of the IA32_ARCH_CAPABILITIES indicates the presence of the
>> IA32_TSX_CTRL MSR.
>>
>> There are two control bits in IA32_TSX_CTRL MSR:
>>
>>   Bit 0: When set it disables the Restricted Transactional Memory (RTM)
>>          sub-feature of TSX (will force all transactions to abort on the
>> 	 XBEGIN instruction).
>>
>>   Bit 1: When set it disables the enumeration of the RTM and HLE feature
>>          (i.e. it will make CPUID(EAX=7).EBX{bit4} and
>>          CPUID(EAX=7).EBX{bit11} read as 0).
>>
>> The other TSX sub-feature, Hardware Lock Elision (HLE), is unconditionally
>> disabled but still enumerated as present by CPUID(EAX=7).EBX{bit4}.
> So one paragraph was changed, but not this one it seems.
>
> As for HLE itself, bit 0 is specified to disable it, along with RTM. 
> (Or at least, it says so in the latest doc I have on the subject).
>
> I don't know what the enabled status of HLE is on the MDS_NO, TAA parts,
> and whether it is statically disabled with the TSX_CTRL microcode, but
> if it isn't statically disabled then it needs to be dynamically disabled
> by bit 0, or a 'CLFLUSH; XBEGIN ...; MOV secret' can still be used to
> exploit TAA.

Apologies.  That is the RTM sequence.

For HLE, I meant 'CLFLUSH; XAQUIRE ...; MOV secret'.

~Andrew