linux-kernel - Re: [PATCHv8 14/17] x86/traps: Handle LASS thrown #SS

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Message-ID: <5F0DC7C6-58D9-4316-AFCB-3F002601DA9F@zytor.com>
Date: Wed, 02 Jul 2025 17:44:27 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Andrew Cooper <andrew.cooper3@...rix.com>
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Subject: Re: [PATCHv8 14/17] x86/traps: Handle LASS thrown #SS

On July 2, 2025 4:42:27 PM PDT, Andrew Cooper <andrew.cooper3@...rix.com> wrote:
>> Note: for a FRED system, ERETU can generate #SS for a non-canonical user space RSP
>
>How?  Or to phrase it differently, I hope not.
>
>%rsp is a 64bit value and does not have canonical restrictions elsewhere
>in the architecture, so far as I'm aware.  IRET really can restore a
>non-canonical %rsp, and userspace can run for an indeterminate period of
>time with a non-canonical %rsp as long as there are no stack accesses.
>
>Accesses relative to the the stack using a non-canonical pointer will
>suffer #SS, but ERETU doesn't modify the userspace stack AFAICT.  I
>can't see anything in the ERETU pseudocode in the FRED spec that
>mentions a canonical check or memory access using %rsp.
>
>~Andrew

You are right of course. Brainfart on my part.