linux-kernel - Re: [PATCH] LDT improvements

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Date:   Fri, 8 Dec 2017 15:06:54 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        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>,
        David Laight <David.Laight@...lab.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] LDT improvements

On Fri, Dec 08, 2017 at 05:20:00AM -0800, Andy Lutomirski wrote:
> > 
> > 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?

How would that be different from setting a 'crap' GS in modify_ldt() and
then returning from the syscall? That is something we should be able to
deal with already, no?

Is this something ldt_gdt.c already tests? The current "Test GS" is in
test_gdt_invalidation() which seems to suggest not.

Could we get a testcase for the exact situation you worry about? I'm not
sure I'd trust myself to get it right, all this LDT magic is new to me.