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Subject: Re: [PATCH 07/34] x86/entry/32: Restore segments before int registers
To:     Linus Torvalds <torvalds@linux-foundation.org>,
        Joerg Roedel <joro@8bytes.org>
Cc:     Brian Gerst <brgerst@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        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>,
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        Peter Zijlstra <peterz@infradead.org>,
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        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>,
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        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>
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From:   "H. Peter Anvin" <hpa@zytor.com>
Message-ID: <12c11262-5e0f-2987-0a74-3bde4b66c352@zytor.com>
Date:   Mon, 5 Mar 2018 14:03:49 -0800
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On 03/05/18 13:58, Linus Torvalds wrote:
> 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.
> 

On NX-enabled hardware NX works with PDE, but the PDPDT in general
doesn't have permission bits (it's really more of a set of four CR3s
than a page table level.)

	-hpa