From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Kirill A. Shutemov" Subject: Re: [PATCH 04/13] Always expose MAP_UNINITIALIZED to userspace Date: Tue, 15 Sep 2015 03:23:58 +0300 Message-ID: <20150915002358.GA12618@node.dhcp.inet.fi> References: <1441832902-28993-1-git-send-email-palmer@dabbelt.com> <1442271047-4908-1-git-send-email-palmer@dabbelt.com> <1442271047-4908-5-git-send-email-palmer@dabbelt.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <1442271047-4908-5-git-send-email-palmer@dabbelt.com> Sender: linux-kernel-owner@vger.kernel.org To: Palmer Dabbelt Cc: arnd@arndb.de, dhowells@redhat.com, viro@zeniv.linux.org.uk, ast@plumgrid.com, aishchuk@linux.vnet.ibm.com, aarcange@redhat.com, akpm@linux-foundation.org, luto@kernel.org, acme@kernel.org, bhe@redhat.com, 3chas3@gmail.com, chris@zankel.net, dave@sr71.net, dyoung@redhat.com, drysdale@google.com, davem@davemloft.net, ebiederm@xmission.com, geoff@infradead.org, gregkh@linuxfoundation.org, hpa@zytor.com, mingo@kernel.org, iulia.manda21@gmail.com, plagnioj@jcrosoft.com, jikos@kernel.org, josh@joshtriplett.org, kexec@lists.infradead.org, linux-api@vger.kernel.org, linux-arch@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-xtensa@linux-xtensa.org, mathieu.desnoyers@efficios.com, jcmvbkbc@gmail.com, paulmck@linux.vnet.ibm.com, a.p.zijlstra@chello.nl, tglx@linutronix.de, tomi.valkeinen@ti.com, vgoyal@redhat.com, x86@kernel.org List-Id: linux-arch.vger.kernel.org On Mon, Sep 14, 2015 at 03:50:38PM -0700, Palmer Dabbelt wrote: > This used to be hidden behind CONFIG_MMAP_ALLOW_UNINITIALIZED, so > userspace wouldn't actually ever see it be non-zero. While I could > have kept hiding it, the man pages seem to indicate that > MAP_UNINITIALIZED should be visible: > > mmap(2) > MAP_UNINITIALIZED (since Linux 2.6.33) > Don't clear anonymous pages. This flag is intended to improve > performance on embedded devices. This flag is honored only if the > kernel was configured with the CONFIG_MMAP_ALLOW_UNINITIALIZED > option. Because of the security implications, that option is > normally enabled only on embedded devices (i.e., devices where one > has complete control of the contents of user memory). > > and since the only time it shows up in my /usr/include is in this > header I believe this should have been visible to userspace (as > non-zero, which wouldn't do anything when or'd into the flags) all > along. Are you sure about "wouldn't do anything"? Suspiciously, 0x4000000 is also (1 << MAP_HUGE_SHIFT). I'm not sure if any architecture has order-1 huge pages, but still looks like we have conflict here. I think it's harmful to expose non-zero MAP_UNINITIALIZED to system which potentially can handle multiple users. Or non-trivial user space in general. Should we leave it at least under '#ifndef CONFIG_MMU'? I don't think it's possible to have single ABI for MMU and MMU-less systems anyway. And we can avoid conflict with MAP_HUGE_SHIFT this way. P.S. MAP_UNINITIALIZED itself looks very broken to me. I probably need dig mailing list on why it was allowed. But that's other topic. -- Kirill A. Shutemov From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wi0-f176.google.com ([209.85.212.176]:37445 "EHLO mail-wi0-f176.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751600AbbIOAYD (ORCPT ); Mon, 14 Sep 2015 20:24:03 -0400 Received: by wicfx3 with SMTP id fx3so5632914wic.0 for ; Mon, 14 Sep 2015 17:24:00 -0700 (PDT) Date: Tue, 15 Sep 2015 03:23:58 +0300 From: "Kirill A. Shutemov" Subject: Re: [PATCH 04/13] Always expose MAP_UNINITIALIZED to userspace Message-ID: <20150915002358.GA12618@node.dhcp.inet.fi> References: <1441832902-28993-1-git-send-email-palmer@dabbelt.com> <1442271047-4908-1-git-send-email-palmer@dabbelt.com> <1442271047-4908-5-git-send-email-palmer@dabbelt.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1442271047-4908-5-git-send-email-palmer@dabbelt.com> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Palmer Dabbelt Cc: arnd@arndb.de, dhowells@redhat.com, viro@zeniv.linux.org.uk, ast@plumgrid.com, aishchuk@linux.vnet.ibm.com, aarcange@redhat.com, akpm@linux-foundation.org, luto@kernel.org, acme@kernel.org, bhe@redhat.com, 3chas3@gmail.com, chris@zankel.net, dave@sr71.net, dyoung@redhat.com, drysdale@google.com, davem@davemloft.net, ebiederm@xmission.com, geoff@infradead.org, gregkh@linuxfoundation.org, hpa@zytor.com, mingo@kernel.org, iulia.manda21@gmail.com, plagnioj@jcrosoft.com, jikos@kernel.org, josh@joshtriplett.org, kexec@lists.infradead.org, linux-api@vger.kernel.org, linux-arch@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-xtensa@linux-xtensa.org, mathieu.desnoyers@efficios.com, jcmvbkbc@gmail.com, paulmck@linux.vnet.ibm.com, a.p.zijlstra@chello.nl, tglx@linutronix.de, tomi.valkeinen@ti.com, vgoyal@redhat.com, x86@kernel.org Message-ID: <20150915002358.EjYNkZ24QJFzgE0NF__2JyQFir9da7Lhu1bpetCC-u8@z> On Mon, Sep 14, 2015 at 03:50:38PM -0700, Palmer Dabbelt wrote: > This used to be hidden behind CONFIG_MMAP_ALLOW_UNINITIALIZED, so > userspace wouldn't actually ever see it be non-zero. While I could > have kept hiding it, the man pages seem to indicate that > MAP_UNINITIALIZED should be visible: > > mmap(2) > MAP_UNINITIALIZED (since Linux 2.6.33) > Don't clear anonymous pages. This flag is intended to improve > performance on embedded devices. This flag is honored only if the > kernel was configured with the CONFIG_MMAP_ALLOW_UNINITIALIZED > option. Because of the security implications, that option is > normally enabled only on embedded devices (i.e., devices where one > has complete control of the contents of user memory). > > and since the only time it shows up in my /usr/include is in this > header I believe this should have been visible to userspace (as > non-zero, which wouldn't do anything when or'd into the flags) all > along. Are you sure about "wouldn't do anything"? Suspiciously, 0x4000000 is also (1 << MAP_HUGE_SHIFT). I'm not sure if any architecture has order-1 huge pages, but still looks like we have conflict here. I think it's harmful to expose non-zero MAP_UNINITIALIZED to system which potentially can handle multiple users. Or non-trivial user space in general. Should we leave it at least under '#ifndef CONFIG_MMU'? I don't think it's possible to have single ABI for MMU and MMU-less systems anyway. And we can avoid conflict with MAP_HUGE_SHIFT this way. P.S. MAP_UNINITIALIZED itself looks very broken to me. I probably need dig mailing list on why it was allowed. But that's other topic. -- Kirill A. Shutemov