Re: [RFC PATCH v9 01/27] Documentation/x86: Add CET description

Dave Hansen
2020-03-09T17:21:54Z

On 3/9/20 10:00 AM, Yu-cheng Yu wrote:
> On Wed, 2020-02-26 at 09:57 -0800, Dave Hansen wrote:
>>> index ade4e6ec23e0..8b69ebf0baed 100644
>>> --- a/Documentation/admin-guide/kernel-parameters.txt
>>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>>> @@ -3001,6 +3001,12 @@
>>> noexec=on: enable non-executable mappings (default)
>>> noexec=off: disable non-executable mappings
>>>
>>> + no_cet_shstk [X86-64] Disable Shadow Stack for user-mode
>>> + applications
>>
>> If we ever add kernel support, "no_cet_shstk" will mean "no cet shstk
>> for userspace"?
>
> What about no_user_shstk, no_kernel_shstk?

Those are better.

>>> + no_cet_ibt [X86-64] Disable Indirect Branch Tracking for user-mode
>>> + applications
>>> +
>>> nosmap [X86,PPC]
>>> Disable SMAP (Supervisor Mode Access Prevention)
>>> even if it is supported by processor.
>>
>> BTW, this documentation is misplaced. It needs to go to the spot where
>> you introduce the code for these options.
>
> We used to introduce the document later in the series. The feedback was to
> introduce it first so that readers know what to expect.

To me, that doesn't apply for things that are implemented in this
specific of a spot in the code and *ALSO* might not even make the final
series.

>>> +Note:
>>> + There is no CET-enabling arch_prctl function. By design, CET is
>>> + enabled automatically if the binary and the system can support it.
>>
>> This is kinda interesting. It means that a JIT couldn't choose to
>> protect the code it generates and have different rules from itself?
>
> JIT needs to be updated for CET first. Once that is done, it runs with CET
> enabled. It can use the NOTRACK prefix, for example.

Am I missing something?

What's the direct connection between shadow stacks and Indirect Branch
Tracking other than Intel marketing umbrellas?

>>> + The parameters passed are always unsigned 64-bit. When an IA32
>>> + application passing pointers, it should only use the lower 32 bits.
>>
>> Won't a 32-bit app calling prctl() use the 32-bit ABI? How would it
>> even know it's running on a 64-bit kernel?
>
> The 32-bit app is passing only a pointer to an array of 64-bit numbers.

Well, the documentation just talked about pointers and I naively assume
it means the "unsigned long *" you had in there.

Rather than make suggestions, just say that the ABI is universally
64-bit. Saying that the pointers must be valid is just kinda silly.
It's also not 100% clear what an "IA32 application" *MEANS* given fun
things like x32.

Also, I went to go find this implementation in your series. I couldn't
find it. Did I miss a patch? Or are you documenting things you didn't
even implement?