linux-kernel - Re: [PATCH 07/34] x86/entry/32: Restore segments before int registers

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Date:   Mon, 5 Mar 2018 10:23:59 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Joerg Roedel <joro@...tes.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Peter Anvin <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        Andrew Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jürgen Groß <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>, Jiri Kosina <jkosina@...e.cz>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Brian Gerst <brgerst@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Eduardo Valentin <eduval@...zon.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Will Deacon <will.deacon@....com>,
        "Liguori, Anthony" <aliguori@...zon.com>,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        Hugh Dickins <hughd@...gle.com>,
        Kees Cook <keescook@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Waiman Long <llong@...hat.com>, Pavel Machek <pavel@....cz>,
        Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH 07/34] x86/entry/32: Restore segments before int registers

On Mon, Mar 5, 2018 at 5:12 AM, Joerg Roedel <joro@...tes.org> wrote:
>
>> The things is, we *know* that we will restore two segment registers with the
>> user cr3 already loaded: CS and SS get restored with the final iret.
>
> Yeah, I know, but the iret-exception path is fine because it will
> deliver a SIGILL and doesn't return to the faulting iret.

That's not so much my worry, as just getting %cr3 wrong. The fact is,
we still take the exception, and we still have to handle it, and that
still needs to get the user<->kernel cr3 right.

So then the whole "restore segments early" must be wrong, because
*that* path must get it all right too, no?

And it appears that the code *does* get it right, and you can just
avoid this patch entirely?

> The iret-exception case is tested by the ldt_gdt selftest (the
> do_multicpu_tests subtest). But I didn't actually tested single-stepping
> through sysenter yet. I just re-ran the same tests I did with v2 on this
> patch-set.

Ok. Maybe we should have a test for the "take DB on first instruction
of sysenter".

           Linus