Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751742AbdHQKfR (ORCPT ); Thu, 17 Aug 2017 06:35:17 -0400 Received: from foss.arm.com ([217.140.101.70]:48024 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750775AbdHQKfP (ORCPT ); Thu, 17 Aug 2017 06:35:15 -0400 Date: Thu, 17 Aug 2017 11:35:15 +0100 From: Will Deacon To: Mark Rutland Cc: Andy Lutomirski , Matt Fleming , Ard Biesheuvel , Sai Praneeth Prakhya , Peter Zijlstra , "linux-efi@vger.kernel.org" , "linux-kernel@vger.kernel.org" , joeyli , Borislav Petkov , "Michael S. Tsirkin" , "Neri, Ricardo" , "Ravi V. Shankar" Subject: Re: [PATCH 3/3] x86/efi: Use efi_switch_mm() rather than manually twiddling with cr3 Message-ID: <20170817103514.GC27872@arm.com> References: <1502824706-30762-1-git-send-email-sai.praneeth.prakhya@intel.com> <1502824706-30762-4-git-send-email-sai.praneeth.prakhya@intel.com> <20170816095338.GB17270@leverpostej> <20170816100709.GG12845@arm.com> <20170816110321.GC17270@leverpostej> <20170816125715.GB3384@codeblueprint.co.uk> <20170815223541.GA25778@remoulade> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170815223541.GA25778@remoulade> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1825 Lines: 39 On Tue, Aug 15, 2017 at 11:35:41PM +0100, Mark Rutland wrote: > On Wed, Aug 16, 2017 at 09:14:41AM -0700, Andy Lutomirski wrote: > > On Wed, Aug 16, 2017 at 5:57 AM, Matt Fleming wrote: > > > On Wed, 16 Aug, at 12:03:22PM, Mark Rutland wrote: > > >> > > >> I'd expect we'd abort at a higher level, not taking any sample. i.e. > > >> we'd have the core overflow handler check in_funny_mm(), and if so, skip > > >> the sample, as with the skid case. > > > > > > FYI, this is my preferred solution for x86 too. > > > > One option for the "funny mm" flag would be literally the condition > > current->mm != current->active_mm. I *think* this gets all the cases > > right as long as efi_switch_mm is careful with its ordering and that > > the arch switch_mm() code can handle the resulting ordering. (x86's > > can now, I think, or at least will be able to in 4.14 -- not sure > > about other arches). > > For arm64 we'd have to rework things a bit to get the ordering right > (especially when we flip to/from the idmap), but otherwise this sounds sane to > me. > > > That being said, there's a totally different solution: run EFI > > callbacks in a kernel thread. This has other benefits: we could run > > those callbacks in user mode some day, and doing *that* in a user > > thread seems like a mistake. > > I think that wouldn't work for CPU-bound perf events (which are not > ctx-switched with the task). > > It might be desireable to do that anyway, though. I'm still concerned that we're treating perf specially here -- are we absolutely sure that nobody else is going to attempt user accesses off the back of an interrupt? If not, then I'd much prefer a solution that catches anybody doing that with the EFI page table installed, rather than trying to play whack-a-mole like this. Will