linux-kernel - Re: [PATCH v2 07/10] x86/entry: Vastly simplify SYSENTER TF handling

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Message-ID: <CAMzpN2gPY4xdR46kUPROx8RsoPO9v0JAuSwUDXzmdbutyjVkRw@mail.gmail.com>
Date:	Mon, 7 Mar 2016 12:17:46 -0500
From:	Brian Gerst <brgerst@...il.com>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>
Subject: Re: [PATCH v2 07/10] x86/entry: Vastly simplify SYSENTER TF handling

On Sun, Mar 6, 2016 at 12:52 AM, Andy Lutomirski <luto@...nel.org> wrote:
> Due to a blatant design error, SYSENTER doesn't clear TF.  As a result,
> if a user does SYSENTER with TF set, we will single-step through the
> kernel until something clears TF.  There is absolutely nothing we can
> do to prevent this short of turning off SYSENTER [1].
>
> Simplify the handling considerably with two changes:
>
> 1. We already sanitize EFLAGS in SYSENTER to clear NT and AC.  We can
>    add TF to that list of flags to sanitize with no overhead whatsoever.
>
> 2. Teach do_debug to ignore single-step traps in the SYSENTER prologue.

What is wrong with the current method of clearing TF and setting
TIF_SINGLESTEP on the first debug trap?  This patch actually increases
complexity because it has to check for a range of addresses rather
than just the first instruction, plus it has to singlestep all the way
through the SYSENTER prologue.

Unless there is an actual issue with TIF_SINGLESTEP, I don't think
this patch is an improvement.

--
Brian Gerst