From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx205.postini.com [74.125.245.205]) by kanga.kvack.org (Postfix) with SMTP id 112846B0074 for ; Wed, 3 Oct 2012 00:44:31 -0400 (EDT) Message-ID: <506BC2A0.8060500@zytor.com> Date: Tue, 02 Oct 2012 21:44:16 -0700 From: "H. Peter Anvin" MIME-Version: 1.0 Subject: Re: [PATCH] Fix devmem_is_allowed for below 1MB accesses for an efi machine References: <1349213536-3436-1-git-send-email-tmac@hp.com> <506B6191.6080605@zytor.com> <20121003043116.GA26241@srcf.ucam.org> In-Reply-To: <20121003043116.GA26241@srcf.ucam.org> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Matthew Garrett Cc: T Makphaibulchoke , tglx@linutronix.de, mingo@redhat.com, x86@kernel.org, akpm@linux-foundation.org, yinghai@kernel.org, tiwai@suse.de, viro@zeniv.linux.org.uk, aarcange@redhat.com, tony.luck@intel.com, mgorman@suse.de, weiyang@linux.vnet.ibm.com, octavian.purdila@intel.com, paul.gortmaker@windriver.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org On 10/02/2012 09:31 PM, Matthew Garrett wrote: > On Tue, Oct 02, 2012 at 02:50:09PM -0700, H. Peter Anvin wrote: > >> That sounds like exactly the opposite of normal /dev/mem behavior... we >> allow access to non-memory resources (which really could do anything if >> misused), but not memory. > > From arch/x86/mm/init.c: > > * On x86, access has to be given to the first megabyte of ram because that area > * contains bios code and data regions used by X and dosemu and similar apps. > > Limiting this to just RAM would be safer than it currently is. I'm not > convinced that there's any good reason to allow *any* access down there > for EFI systems, though. > Sorry, fail. We *always* expose the I/O regions to /dev/mem. That is what /dev/mem *does*. The above is an exception (which is really obsolete, too: we should simply disallow access to anything which is treated as system RAM, which doesn't include the BIOS regions in question; the only reason we don't is that some versions of X take a checksum of the RAM in the first megabyte as some kind of idiotic random seed.) -hpa -- 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