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From: Brian Gerst <brgerst@gmail.com>
Date: Mon, 5 Mar 2018 15:38:02 -0500
Message-ID: <CAMzpN2hscOXJFzm07Hk=2Ttr3wQFSisxP=EZhRMtAU6xSm8zSw@mail.gmail.com>
Subject: Re: [PATCH 07/34] x86/entry/32: Restore segments before int registers
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To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Joerg Roedel <joro@8bytes.org>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@kernel.org>, Peter Anvin <hpa@zytor.com>, the arch/x86 maintainers <x86@kernel.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>, Andrew Lutomirski <luto@kernel.org>, Dave Hansen <dave.hansen@intel.com>, Josh Poimboeuf <jpoimboe@redhat.com>, =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= <jgross@suse.com>, Peter Zijlstra <peterz@infradead.org>, Borislav Petkov <bp@alien8.de>, Jiri Kosina <jkosina@suse.cz>, Boris Ostrovsky <boris.ostrovsky@oracle.com>, David Laight <David.Laight@aculab.com>, Denys Vlasenko <dvlasenk@redhat.com>, Eduardo Valentin <eduval@amazon.com>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Will Deacon <will.deacon@arm.com>, "Liguori, Anthony" <aliguori@amazon.com>, Daniel Gruss <daniel.gruss@iaik.tugraz.at>, Hugh Dickins <hughd@google.com>, Kees Cook <keescook@google.com>, Andrea Arcangeli <aarcange@redhat.com>, Waiman Long <llong@redhat.com>, Pavel Machek <pavel@ucw.cz>, Joerg Roedel <jroedel@suse.de>

On Mon, Mar 5, 2018 at 1:23 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> 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

There already is a test: single_step_syscall.c

--
Brian Gerst

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