From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wi0-f175.google.com (mail-wi0-f175.google.com [209.85.212.175]) by kanga.kvack.org (Postfix) with ESMTP id 3742D82F64 for ; Thu, 5 Nov 2015 11:27:37 -0500 (EST) Received: by wicll6 with SMTP id ll6so13614076wic.0 for ; Thu, 05 Nov 2015 08:27:36 -0800 (PST) Received: from pandora.arm.linux.org.uk (pandora.arm.linux.org.uk. [2001:4d48:ad52:3201:214:fdff:fe10:1be6]) by mx.google.com with ESMTPS id m135si10895803wmb.68.2015.11.05.08.27.35 for (version=TLSv1 cipher=RC4-SHA bits=128/128); Thu, 05 Nov 2015 08:27:36 -0800 (PST) Date: Thu, 5 Nov 2015 16:27:19 +0000 From: Russell King - ARM Linux Subject: Re: [PATCH] arm: Use kernel mm when updating section permissions Message-ID: <20151105162719.GQ8644@n2100.arm.linux.org.uk> References: <1446685239-28522-1-git-send-email-labbott@fedoraproject.org> <20151105094615.GP8644@n2100.arm.linux.org.uk> <563B81DA.2080409@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <563B81DA.2080409@redhat.com> Sender: owner-linux-mm@kvack.org List-ID: To: Laura Abbott Cc: Laura Abbott , Kees Cook , Catalin Marinas , Will Deacon , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org On Thu, Nov 05, 2015 at 08:20:42AM -0800, Laura Abbott wrote: > On 11/05/2015 01:46 AM, Russell King - ARM Linux wrote: > >On Wed, Nov 04, 2015 at 05:00:39PM -0800, Laura Abbott wrote: > >>Currently, read only permissions are not being applied even > >>when CONFIG_DEBUG_RODATA is set. This is because section_update > >>uses current->mm for adjusting the page tables. current->mm > >>need not be equivalent to the kernel version. Use pgd_offset_k > >>to get the proper page directory for updating. > > > >What are you trying to achieve here? You can't use these functions > >at run time (after the first thread has been spawned) to change > >permissions, because there will be multiple copies of the kernel > >section mappings, and those copies will not get updated. > > > >In any case, this change will probably break kexec and ftrace, as > >the running thread will no longer see the updated page tables. > > > > I think I was hitting that exact problem with multiple copies > not getting updated. The section_update code was being called > and I was seeing the tables get updated but nothing was being > applied when I tried to write to text or check the debugfs > page table. The current flow is: > > rest_init -> kernel_thread(kernel_init) and from that thread > mark_rodata_ro. So mark_rodata_ro is always going to happen > in a thread. > > Do we need to update for both init_mm and the first running > thread? The "first running thread" is merely coincidental for things like kexec. Hmm. Actually, I think the existing code _should_ be fine. At the point where mark_rodata_ro() is, we should still be using init_mm, so updating the current threads page tables should actually be updating the swapper_pg_dir. The other cases (kexec and ftrace) I think are also fine as they stand - we want to be changing the currently active page tables there. So, I really think we do not want to be using pgd_offset_k() here at all - but we need to find out what's changed to cause (presumably) mark_rodata_ro() not to be hitting the swapper page tables. Maybe some debug in mark_rodata_ro() to find out what current->mm is? -- FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up according to speedtest.net. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org