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

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Message-ID: <4dcd23cb-eb55-42e7-aa76-dbaf2e2a7e07@citrix.com>
Date: Thu, 3 Jul 2025 00:42:27 +0100
From: Andrew Cooper <andrew.cooper3@...rix.com>
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Subject: Re: [PATCHv8 14/17] x86/traps: Handle LASS thrown #SS

> 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