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

[Dave Hansen]

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?