From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1751991AbdEJGyU (ORCPT <rfc822;w@1wt.eu>);
        Wed, 10 May 2017 02:54:20 -0400
Received: from bombadil.infradead.org ([65.50.211.133]:39670 "EHLO
        bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1751901AbdEJGyR (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Wed, 10 May 2017 02:54:17 -0400
Date: Tue, 9 May 2017 23:53:01 -0700
From: Christoph Hellwig <hch@infradead.org>
To: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Christoph Hellwig <hch@infradead.org>,
        Andy Lutomirski <luto@kernel.org>, Ingo Molnar <mingo@kernel.org>,
        Greg KH <greg@kroah.com>, Thomas Garnier <thgarnie@google.com>,
        Martin Schwidefsky <schwidefsky@de.ibm.com>,
        Heiko Carstens <heiko.carstens@de.ibm.com>,
        Dave Hansen <dave.hansen@intel.com>, Arnd Bergmann <arnd@arndb.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        David Howells <dhowells@redhat.com>,
        =?iso-8859-1?Q?Ren=E9?= Nyffenegger <mail@renenyffenegger.ch>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Paul E . McKenney" <paulmck@linux.vnet.ibm.com>,
        "Eric W . Biederman" <ebiederm@xmission.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Pavel Tikhomirov <ptikhomirov@virtuozzo.com>,
        Ingo Molnar <mingo@redhat.com>, "H . Peter Anvin" <hpa@zytor.com>,
        Paolo Bonzini <pbonzini@redhat.com>, Rik van Riel <riel@redhat.com>,
        Kees Cook <keescook@chromium.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Brian Gerst <brgerst@gmail.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Russell King <linux@armlinux.org.uk>,
        Will Deacon <will.deacon@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Mark Rutland <mark.rutland@arm.com>, James Morse <james.morse@arm.com>,
        linux-s390 <linux-s390@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        "linux-arm-kernel@lists.infradead.org" 
        <linux-arm-kernel@lists.infradead.org>,
        Kernel Hardening <kernel-hardening@lists.openwall.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: [kernel-hardening] Re: [PATCH v9 1/4] syscalls: Verify address
 limit before returning to user-mode
Message-ID: <20170510065301.GC4115@infradead.org>
References: <20170508073352.caqe3fqf7nuxypgi@gmail.com>
 <20170508124621.GA20705@kroah.com>
 <20170509064522.anusoikaalvlux3w@gmail.com>
 <20170509085659.GA32555@infradead.org>
 <CALCETrUh8NO2scaqEM48K70Fo2+V3=Cpyk4JurCDiCYp4nm_+g@mail.gmail.com>
 <20170509130250.GA11381@infradead.org>
 <20170509160322.GA15902@infradead.org>
 <20170510021118.GA390@ZenIV.linux.org.uk>
 <20170510024524.GB390@ZenIV.linux.org.uk>
 <20170510031254.GC390@ZenIV.linux.org.uk>
MIME-Version: 1.0
Content-Type: text/plain; charset=iso-8859-1
Content-Disposition: inline
Content-Transfer-Encoding: 8bit
In-Reply-To: <20170510031254.GC390@ZenIV.linux.org.uk>
User-Agent: Mutt/1.8.0 (2017-02-23)
X-SRS-Rewrite: SMTP reverse-path rewritten from <hch@infradead.org> by bombadil.infradead.org. See http://www.infradead.org/rpr.html
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Wed, May 10, 2017 at 04:12:54AM +0100, Al Viro wrote:
> What's the point?  What's wrong with having kernel_read()/kernel_readv()/etc.?
> You still have set_fs() in there; doing that one level up in call chain would
> be just fine...  IDGI.

The problem is that they modify the address limit, which the whole
subthread here wants to get rid of.

> Broken commit: "net: don't play with address limits in kernel_recvmsg".
> It would be OK if it was only about data.  Unfortunately, that's not
> true in one case: svc_udp_recvfrom() wants ->msg_control.

Dropped, but we'll need to fix that eventually.

> Another delicate place: you can't assume that write() always advances
> file position by its (positive) return value.  btrfs stuff is sensitive
> to that.

If we don't want to assume that we need to pass pointer to pos to
kernel_read/write.  Which might be a good idea in general.

> ashmem probably _is_ OK with demanding ->read_iter(), but I'm not sure
> about blind asma->file->f_pos += ret.  That's begging for races.  Actually,
> scratch that - it *is* racy.

I think the proper fix is to not even bother to maintain f_pos of the
backing file, as we don't ever use it - all reads from it pass in
an explicit position anyway.