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Re: [committed, PATCH 1/3] x86: CET v2.0: Update NOTRACK prefix

On Mon, Jul 3, 2017 at 12:18 AM, Jan Beulich <> wrote:
>>>> On 21.06.17 at 17:35, <> wrote:
>> Update NOTRACK prefix handling to support memory indirect branch for
>> CET v2.0:
>> rcement-technology-preview.pdf
> It is pretty hard to judge whether the changes here are correct
> without the doc being really precise on the obvious corner case:
> How does one correctly use a segment override and a notrack
> prefix on an indirect call/jmp with a memory operand? Most
> importantly, how does one use %ds: but not notrack, notrack
> but not %ds:, or %ds and notrack?

The spec has

No-track Prefix for Near Indirect Call/Jmp
Near indirect call and jmp instructions when prefixed with 3EH are
termed “non-tracked indirect control
transfer instructions” and do not modify the CET indirect branch
tracker. Far call and jmp are always tracked
and ignore the 3EH prefix. The NO_TRACK_EN control in the
IA32_U_CET/IA32_S_CET MSR enables this
no-track prefix treatment. When this control is 0, the near indirect
call and jmp are always tracked irrespec-
tive of the presence of the 3EH prefix.
In 64-bit mode, the 3EH prefix on an indirect call or jmp is
recognized as a no-track prefix when the following
conditions are satisfied.
3EH must be the last legacy prefix of any group (except any REX).
There must not be a 64H/65H prefix on the instruction.
In legacy/compatibility mode, the 3EH prefix on an indirect call or
jmp is recognized as a no-track prefix
when it is the last group 2 prefix on the instruction.

That means you can't use the notrack pefix with a segment override
and you can't use %ds segment override on indirect branch.


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