linux-kernel - Re: What exactly do 32-bit x86 exceptions push on the stack in the CS slot?

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Message-ID: <CA+55aFwH8zQNj42LyqFTbXqU+nUbDgDhqMW+so_-OPpM5SRwuQ@mail.gmail.com>
Date:   Mon, 21 Nov 2016 10:00:56 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Brian Gerst <brgerst@...il.com>,
        Andy Lutomirski <luto@...nel.org>, tedheadster@...il.com,
        "H. Peter Anvin" <hpa@...or.com>,
        George Spelvin <linux@...izon.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        X86 ML <x86@...nel.org>
Subject: Re: What exactly do 32-bit x86 exceptions push on the stack in the CS slot?

On Sun, Nov 20, 2016 at 11:13 PM, Ingo Molnar <mingo@...nel.org> wrote:
>
> So I have applied your fix that addresses the worst fallout directly:
>
>   fc0e81b2bea0 x86/traps: Ignore high word of regs->cs in early_fixup_exception()
>
> ... but otherwise we might be better off zeroing out the high bits of segment
> registers stored on the stack, in all entry code pathways

Ugh.

I'd much rather we go back to just making the "cs" entry explicitly
16-bit, and have a separate padding entry, the way we used to long
long ago.

Or just rename it to something that you're not supposed to access
directly, and a helper accessor function that masks off the high bits.

The entry code-paths are *much* more critical than any of the few user
codepaths. Let's not add complexity to entry. Make the structure
actually reflect reality instead.

             Linus