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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Mon, 5 Mar 2018 10:23:59 -0800
Message-ID: <CA+55aFwn5EkHTfrUFww54CDWovoUornv6rSrao43agbLBQD6-Q@mail.gmail.com>
Subject: Re: [PATCH 07/34] x86/entry/32: Restore segments before int registers
To:     Joerg Roedel <joro@8bytes.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>, Peter Anvin <hpa@zytor.com>,
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        Andrea Arcangeli <aarcange@redhat.com>,
        Waiman Long <llong@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        Joerg Roedel <jroedel@suse.de>
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On Mon, Mar 5, 2018 at 5:12 AM, Joerg Roedel <joro@8bytes.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