linux-kernel - Re: [PATCH 12/31] x86/entry/32: Add PTI cr3 switch to non-NMI entry/exit points

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Date:   Thu, 1 Mar 2018 10:38:21 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Brian Gerst <brgerst@...il.com>
Cc:     Joerg Roedel <jroedel@...e.de>, Waiman Long <longman@...hat.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>, Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>, Jiri Kosina <jkosina@...e.cz>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        David Laight <David.Laight@...lab.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Eduardo Valentin <eduval@...zon.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Will Deacon <will.deacon@....com>,
        "Liguori, Anthony" <aliguori@...zon.com>,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        Hugh Dickins <hughd@...gle.com>,
        Kees Cook <keescook@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Waiman Long <llong@...hat.com>, Pavel Machek <pavel@....cz>
Subject: Re: [PATCH 12/31] x86/entry/32: Add PTI cr3 switch to non-NMI
 entry/exit points

On Thu, Mar 1, 2018 at 10:24 AM, Brian Gerst <brgerst@...il.com> wrote:
>
> The IF flag only affects external maskable interrupts, not traps or
> faults.  You do need to check CR3 because SYSENTER does not clear TF
> and will immediately cause a debug trap on kernel entry (with user
> CR3) if set.  That is why the code existed before to check for the
> entry stack for debug/NMI.

Note that debug traps can happen regardless of TF, Think kgdb etc.
Arguably kgdb users get what they deserve, but still.. I think root
can set kernel breakpoints too.

          Linus