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

[Thomas Gleixner <tglx@linutronix.de>]

On Mon, Mar 21 2022 at 15:29, Andy Lutomirski wrote:
> On Thu, Mar 10, 2022, at 3:15 AM, Bharata B Rao wrote:
>> This patchset makes use of Upper Address Ignore (UAI) feature available
>> on upcoming AMD processors to provide user address tagging support for x86/AMD.
>>
>> 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
>>
>> https://bugzilla.kernel.org/attachment.cgi?id=300549
>
> I hate to be a pain, but I'm really not convinced that this feature is
> suitable for Linux.  There are a few reasons:

Abusing bit 63 is not suitable for any OS in my opinion.

> Right now, the concept that the high bit of an address determines
> whether it's a user or a kernel address is fairly fundamental to the
> x86_64 (and x86_32!) code.  It may not be strictly necessary to
> preserve this, but violating it would require substantial thought.
> With UAI enabled, kernel and user addresses are, functionally,
> interleaved.  This makes things like access_ok checks, and more
> generally anything that operates on a range of addresses, behave
> potentially quite differently.  A lot of auditing of existing code
> would be needed to make it safe.

Which might be finished ten years from now....

Seriously there is no justification for the bit 63 abuse. This has been
pointed out by various people to AMD before this saw the public. Other
vendors seem to have gotten the memo.

The proper solution here is to issue an erratum and fix this nonsense in
microcode for the already taped out silicon and get rid of it in the
design of future ones completely. Anything else is just wishful
thinking.

Thanks,

        tglx