From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755826AbdKMVOv (ORCPT ); Mon, 13 Nov 2017 16:14:51 -0500 Received: from Galois.linutronix.de ([146.0.238.70]:35133 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755796AbdKMVOs (ORCPT ); Mon, 13 Nov 2017 16:14:48 -0500 Date: Mon, 13 Nov 2017 22:14:36 +0100 (CET) From: Thomas Gleixner To: "Kirill A. Shutemov" cc: "Kirill A. Shutemov" , Ingo Molnar , Linus Torvalds , x86@kernel.org, "H. Peter Anvin" , Andy Lutomirski , Cyrill Gorcunov , Nicholas Piggin , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] x86/mm: Do not allow non-MAP_FIXED mapping across DEFAULT_MAP_WINDOW border In-Reply-To: <20171113200657.pk56mxofg2t2xbi6@node.shutemov.name> Message-ID: References: <20171107130539.52676-1-kirill.shutemov@linux.intel.com> <20171113164154.fp5fd2seozbmxcbs@node.shutemov.name> <20171113200657.pk56mxofg2t2xbi6@node.shutemov.name> User-Agent: Alpine 2.20 (DEB 67 2015-01-07) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 13 Nov 2017, Kirill A. Shutemov wrote: > On Mon, Nov 13, 2017 at 08:14:54PM +0100, Thomas Gleixner wrote: > > > > It will succeed with 5-level paging. > > > > > > And why is this allowed? > > > > > > > It should be safe as with 4-level paging such request would fail and it's > > > > reasonable to expect that userspace is not relying on the failure to > > > > function properly. > > > > > > Huch? > > > > > > The first rule when looking at user space is that is broken or > > > hostile. Reasonable and user space are mutually exclusive. > > > > Aside of that in case of get_unmapped_area: > > > > If va_unmapped_area() fails, then the address and the len which caused the > > overlap check to trigger are handed in to arch_get_unmapped_area(), which > > again can create an invalid mapping if I'm not missing something. > > > > If mappings which overlap the boundary are invalid then we have to make > > sure at all ends that they wont happen. > > They are not invalid. > > The patch tries to address following theoretical issue: > > We have an application that tries, for some reason, to allocate memory > with mmap(addr), without MAP_FIXED, where addr is near the borderline of > 47-bit address space and addr+len is above the border. > > On 4-level paging machine this request would succeed, but the address will > always be within 47-bit VA -- cannot allocate by hint address, ignore it. > > If the application cannot handle high address this might be an issue on > 5-level paging machine as such call would succeed *and* allocate memory by > the specified hint address. In this case part of the mapping would be > above the border line and may lead to misbehaviour. > > I hope this makes any sense :) I can see where you are heading to. Now the case I was looking at is: arch_get_unmapped_area_topdown() addr0 = addr; .... if (addr) { if (cross_border(addr, len)) goto get_unmapped_area; ... } get_unmapped_area: ... if (addr > DEFAULT_MAP_WINDOW && !in_compat_syscall()) ^^^ evaluates to false because addr < DEFAULT_MAP_WINDOW addr - vm_unmapped_area(&info); ^^^ fails for whatever reason. bottomup: return arch_get_unmapped_area(.., addr0, len, ....); AFAICT arch_get_unmapped_area() can allocate a mapping which crosses the border, i.e. a mapping which you want to prevent for the !MAP_FIXED case. Thanks, tglx