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From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Thu, 1 Mar 2018 10:38:21 -0800
Message-ID: <CA+55aFynSZgf1wnWrweODJiRw5hLkUPzFF7T8QJ7vBo=zWTfqw@mail.gmail.com>
Subject: Re: [PATCH 12/31] x86/entry/32: Add PTI cr3 switch to non-NMI
 entry/exit points
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To: Brian Gerst <brgerst@gmail.com>
Cc: Joerg Roedel <jroedel@suse.de>, Waiman Long <longman@redhat.com>, Joerg Roedel <joro@8bytes.org>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@kernel.org>, "H . Peter Anvin" <hpa@zytor.com>, the arch/x86 maintainers <x86@kernel.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, Linux-MM <linux-mm@kvack.org>, Andy Lutomirski <luto@kernel.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>, 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>, Pavel Machek <pavel@ucw.cz>

On Thu, Mar 1, 2018 at 10:24 AM, Brian Gerst <brgerst@gmail.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

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