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

[Andrew Cooper <andrew.cooper3@citrix.com>]

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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.

~Andrew