linux-kernel - RE: [PATCH] LDT improvements

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Date:   Fri, 8 Dec 2017 13:55:24 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Andy Lutomirski' <luto@...capital.net>,
        Thomas Gleixner <tglx@...utronix.de>
CC:     Ingo Molnar <mingo@...nel.org>, Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>, X86 ML <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: RE: [PATCH] LDT improvements

From: Andy Lutomirski
> Sent: 08 December 2017 13:20
...
> >> - It would also be a cleaner approach all around, and would avoid the fixmap
> >>   complications and the scheduler muckery.
> >
> > The error code of such an access is always 0x03. So I added a special
> > handler, which checks whether the address is in the LDT map range and
> > verifies that the access bit in the descriptor is 0. If that's the case it
> > sets it and returns. If not, the thing dies. That works.
> 
> What if you are in kernel mode and try to return to a context with SS or CS pointing to a non-accessed
> segment?
> Or what if you try to schedule to a context with fs or, worse, gs pointing to such a segment?

Well, the cpu will fault in kernel on the 'pop %xs' or 'iret' instruction.
These all (probably) happen on the kernel stack with the usergs loaded.
So the fault handler has to look at the opcodes and/or %pc value, sort
out the stack (etc) and then generate SIGSEGV.

I'm not sure the kernel needs to know why the segment selector is invalid.

	David