Re: [RFC PATCH v0 0/6] x86/AMD: Userspace address tagging

[Bharata B Rao]

On 3/11/2022 1:45 PM, David Laight wrote:
> From: Bharata B Rao
>> Sent: 11 March 2022 05:43
>> On 3/10/2022 10:49 PM, David Laight wrote:
>>> From: Dave Hansen <dave.hansen@xxxxxxxxx>
>>>> Sent: 10 March 2022 16:46
>>>>
>>>> On 3/10/22 06:32, David Laight wrote:
>>>>>> UAI allows software to store a tag in the upper 7 bits of a logical
>>>>>> address [63:57]. When enabled, the processor will suppress the
>>>>>> traditional canonical address checks on the addresses. More information
>>>>>> about UAI can be found in section 5.10 of 'AMD64 Architecture
>>>>>> Programmer's Manual, Vol 2: System Programming' which is available from
>>>>>>
> ,,,
>>>>> Is that really allowing bit 63 to be used?
>>>>> That is normally the user-kernel bit.
>>>>> I can't help feeling that will just badly break things.
>>>>
>>>> Yeah, this does seem worrisome.  The LAM approach[1] retains
>>>> canonicality checking for bit 63.
>>>
>>> Actually it is rather worse than 'worrisome'.
>>> Allowing the user all address upto the base of the valid
>>> kernel addresses (probably tags to 3e, but not 3f)
>>> means that you can't use a fast address check in access_ok().
>>> You are forced to use the strict test that 32bit kernels use.
>>
>> From what I see, there is a single implementation of access_ok()
>> in arch/x86/asm/include/uaccess.h that does check if the user
>> address+size exceeds the limit.
>>
>> Guess I am missing something, but can you please point me to the fast
>> implementation(that benefits from bit 63 being user/kernel address
>> disambiguation bit) and the strict checking in 32bit kernels that
>> are you are referring to?
> 
> You can just check ((address | size) >> 62) on 64bit arch that
> use bit 63 to select user/kernel and have a massive address
> hole near the boundary.
> The compiler optimises out constant size from that calculation.
> On x86-64 non-canonical addresses give a different fault
> to 'page not present' - but that can be handled.

Ok, so are you mentioning about a future optimization to access_ok()
that could benefit by retaining bit 63 as kernel/user bit?

Since you said using bit 63 to store metadata will break things,
I was trying to understand how and where does it break.

Regards,
Bharata.