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Date: Fri, 2 Mar 2018 10:07:37 +0100
From: Joerg Roedel <joro@8bytes.org>
Subject: Re: [PATCH 12/31] x86/entry/32: Add PTI cr3 switch to non-NMI
 entry/exit points
Message-ID: <20180302090737.GO16484@8bytes.org>
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To: Brian Gerst <brgerst@gmail.com>
Cc: Joerg Roedel <jroedel@suse.de>, Waiman Long <longman@redhat.com>, 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>, Linus Torvalds <torvalds@linux-foundation.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 01, 2018 at 01:24:39PM -0500, Brian Gerst 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.

Yeah, okay, thanks for the clarification. This also means the #DB
handler needs to leave with the same cr3 as it entered. I'll work that
into my patches.

Thanks,

	Joerg

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