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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
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>,
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        "Liguori, Anthony" <aliguori@amazon.com>,
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        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>
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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