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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Mon, 5 Mar 2018 13:58:32 -0800
Message-ID: <CA+55aFx2dxZmL487CnhV6rWRiqmJwZNAspyPqCD4Hwqxwncs6Q@mail.gmail.com>
Subject: Re: [PATCH 07/34] x86/entry/32: Restore segments before int registers
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To: Joerg Roedel <joro@8bytes.org>
Cc: Brian Gerst <brgerst@gmail.com>, 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 1:35 PM, Joerg Roedel <joro@8bytes.org> wrote:
> On Mon, Mar 05, 2018 at 12:50:33PM -0800, Linus Torvalds wrote:
>>
>> Ahh, good. So presumably Joerg actually did check it, just didn't even notice ;)
>
> Yeah, sort of. I ran the test, but it didn't catch the failure case in
> previous versions which was return to user with kernel-cr3 :)

Ahh. Yes, that's bad. The NX protection to guarantee that you don't
return to user mode was really good on x86-64.

So some other case could slip through, because user code can happily
run with the kernel page tables.

> I could probably add some debug instrumentation to check for that in my
> future testing, as there is no NX protection in the user address-range
> for the kernel-cr3.

Does not NX work with PAE?

Oh, it looks like the NX bit is marked as "RSVD (must be 0)" in the
PDPDT. Oh well.

                  Linux

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