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Date: Wed, 17 Jan 2018 10:24:42 +0100
From: Joerg Roedel <joro@8bytes.org>
Subject: Re: [PATCH 03/16] x86/entry/32: Leave the kernel via the trampoline
 stack
Message-ID: <20180117092442.GJ28161@8bytes.org>
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To: Andy Lutomirski <luto@kernel.org>
Cc: 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@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>, Brian Gerst <brgerst@gmail.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>

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.


	Joerg

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