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From: Brian Gerst <brgerst@gmail.com>
Date: Wed, 17 Jan 2018 06:00:07 -0800
Message-ID: <CAMzpN2hXjHhx_9GDih8r808dYemYcy02f+LeXfG_8iuJkN82gA@mail.gmail.com>
Subject: Re: [PATCH 03/16] x86/entry/32: Leave the kernel via the trampoline stack
To: Joerg Roedel <joro@8bytes.org>
Cc: Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        "H . Peter Anvin" <hpa@zytor.com>, X86 ML <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Juergen Gross <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 KH <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>, Joerg Roedel <jroedel@suse.de>
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On Wed, Jan 17, 2018 at 5:57 AM, Brian Gerst <brgerst@gmail.com> wrote:
> On Wed, Jan 17, 2018 at 1:24 AM, Joerg Roedel <joro@8bytes.org> wrote:
>> On Tue, Jan 16, 2018 at 02:48:43PM -0800, Andy Lutomirski wrote:
>>> On Tue, Jan 16, 2018 at 8:36 AM, Joerg Roedel <joro@8bytes.org> wrote:
>>> > +       /* Restore user %edi and user %fs */
>>> > +       movl (%edi), %edi
>>> > +       popl %fs
>>>
>>> Yikes!  We're not *supposed* to be able to observe an asynchronous
>>> descriptor table change, but if the LDT changes out from under you,
>>> this is going to blow up badly.  It would be really nice if you could
>>> pull this off without percpu access or without needing to do this
>>> dance where you load user FS, then kernel FS, then user FS.  If that's
>>> not doable, then you should at least add exception handling -- look at
>>> the other 'pop %fs' instructions in entry_32.S.
>>
>> You are right! This also means I need to do the 'popl %fs' before the
>> cr3-switch. I'll fix it in the next version.
>>
>> I have no real idea on how to switch back to the entry stack without
>> access to per_cpu variables. I also can't access the cpu_entry_area for
>> the cpu yet, because for that we need to be on the entry stack already.
>
> Switch to the trampoline stack before loading user segments.

But then again, you could take a fault on the trampoline stack if you
get a bad segment.  Perhaps just pushing the new stack pointer onto
the process stack before user segment loads will be the right move.

--
Brian Gerst