linux-kernel - Re: [patch 13/16] x86/ldt: Introduce LDT write fault handler

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Date:   Tue, 12 Dec 2017 11:51:43 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Andy Lutomirsky <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Borislav Petkov <bpetkov@...e.de>,
        Greg KH <gregkh@...uxfoundation.org>,
        Kees Cook <keescook@...gle.com>,
        Hugh Dickins <hughd@...gle.com>,
        Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        David Laight <David.Laight@...lab.com>,
        Eduardo Valentin <eduval@...zon.com>,
        "Liguori, Anthony" <aliguori@...zon.com>,
        Will Deacon <will.deacon@....com>,
        linux-mm <linux-mm@...ck.org>
Subject: Re: [patch 13/16] x86/ldt: Introduce LDT write fault handler

On Tue, Dec 12, 2017 at 11:21 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
>
> That has nothing to do with the user installed LDT. The kernel does not use
> and rely on LDT at all.

Sure it does. We end up loading the selector for %gs and %fs, and
those selectors end up being connected with whatever user-mode has set
up for them.

We then set the FS/GS base pointer to a kernel-specific value, but
that is _separately_ from the actual accessed bit that is in the
selector.

So the kernel doesn't care, but the kernel definitely uses them.

                 Linus