linux-kernel - Re: [PATCH 02/10] x86/entry/compat: In SYSENTER, sink AC clearing below the existing FLAGS test

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Message-ID: <20160229220903.GH3724@pd.tnic>
Date:	Mon, 29 Feb 2016 23:09:03 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Brian Gerst <brgerst@...il.com>
Subject: Re: [PATCH 02/10] x86/entry/compat: In SYSENTER, sink AC clearing
 below the existing FLAGS test

On Mon, Feb 29, 2016 at 12:45:58PM -0800, Andy Lutomirski wrote:
> I don't think so.  Sensible user programs shouldn't set AC in the first place.

Then I'm most likely missing something: so before this patch, we did
unconditionally CLAC thus disallowing kernel access to user pages. Why
don't we need it anymore and need to pay attention only to user rFLAGS?

Especially since we do:

do_fast_syscall_32
|-> __get_user
   |-> __get_user_nocheck
      |-> __uaccess_begin which is stac()

Or are we saying, we don't need that CLAC in the beginning of
entry_SYSENTER_compat() at all because we're going to STAC anyway in
__get_user() ?

Hmmm...

-- 
Regards/Gruss,
    Boris.

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