From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 44FBFC2D0CD for ; Tue, 17 Dec 2019 15:40:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 16E0A24655 for ; Tue, 17 Dec 2019 15:40:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727825AbfLQPki (ORCPT ); Tue, 17 Dec 2019 10:40:38 -0500 Received: from foss.arm.com ([217.140.110.172]:40798 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727415AbfLQPkh (ORCPT ); Tue, 17 Dec 2019 10:40:37 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id C29281FB; Tue, 17 Dec 2019 07:40:36 -0800 (PST) Received: from arrakis.emea.arm.com (arrakis.cambridge.arm.com [10.1.197.42]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 0D4A73F67D; Tue, 17 Dec 2019 07:40:33 -0800 (PST) Date: Tue, 17 Dec 2019 15:40:31 +0000 From: Catalin Marinas To: Magnus Karlsson Cc: Daniel Borkmann , kirill.shutemov@linux.intel.com, justin.he@arm.com, linux-mm@kvack.org, syzbot , Andrii Nakryiko , Alexei Starovoitov , =?iso-8859-1?Q?Bj=F6rn_T=F6pel?= , bpf , "David S. Miller" , hawk@kernel.org, Jakub Kicinski , John Fastabend , Jonathan Lemon , Martin KaFai Lau , linux-kernel@vger.kernel.org, "Karlsson, Magnus" , Network Development , Song Liu , syzkaller-bugs@googlegroups.com, Yonghong Song Subject: Re: WARNING in wp_page_copy Message-ID: <20191217154031.GI5624@arrakis.emea.arm.com> References: <000000000000a6f2030598bbe38c@google.com> <0000000000000e32950599ac5a96@google.com> <20191216150017.GA27202@linux.fritz.box> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Magnus, Thanks for investigating this. I have more questions below rather than a solution. On Tue, Dec 17, 2019 at 02:27:22PM +0100, Magnus Karlsson wrote: > On Mon, Dec 16, 2019 at 4:10 PM Magnus Karlsson > wrote: > > On Mon, Dec 16, 2019 at 4:00 PM Daniel Borkmann wrote: > > > > > > On Sat, Dec 14, 2019 at 08:20:07AM -0800, syzbot wrote: > > > > syzbot has found a reproducer for the following crash on: > > > > > > > > HEAD commit: 1d1997db Revert "nfp: abm: fix memory leak in nfp_abm_u32_.. > > > > git tree: net-next > > > > console output: https://syzkaller.appspot.com/x/log.txt?x=1029f851e00000 > > > > kernel config: https://syzkaller.appspot.com/x/.config?x=cef1fd5032faee91 > > > > dashboard link: https://syzkaller.appspot.com/bug?extid=9301f2f33873407d5b33 > > > > compiler: gcc (GCC) 9.0.0 20181231 (experimental) > > > > syz repro: https://syzkaller.appspot.com/x/repro.syz?x=119d9fb1e00000 > > > > > > > > IMPORTANT: if you fix the bug, please add the following tag to the commit: > > > > Reported-by: syzbot+9301f2f33873407d5b33@syzkaller.appspotmail.com > > > > > > Bjorn / Magnus, given xsk below, PTAL, thanks! > > > > Thanks. I will take a look at it right away. > > > > /Magnus > > After looking through the syzcaller report, I have the following > hypothesis that would dearly need some comments from MM-savy people > out there. Syzcaller creates, using mmap, a memory area that is I guess that's not an anonymous mmap() since we don't seem to have a struct page for src in cow_user_page() (the WARN_ON_ONCE path). Do you have more information on the mmap() call? > write-only and supplies this to a getsockopt call (in this case > XDP_STATISTICS, but probably does not matter really) as the area where > it wants the values to be stored. When the getsockopt implementation > gets to copy_to_user() to write out the values to user space, it > encounters a page fault when accessing this write-only page. When > servicing this, it gets to the following piece of code that triggers > the warning that syzcaller reports: > > static inline bool cow_user_page(struct page *dst, struct page *src, > struct vm_fault *vmf) > { > .... > snip > .... > /* > * This really shouldn't fail, because the page is there > * in the page tables. But it might just be unreadable, > * in which case we just give up and fill the result with > * zeroes. > */ > if (__copy_from_user_inatomic(kaddr, uaddr, PAGE_SIZE)) { > /* > * Give a warn in case there can be some obscure > * use-case > */ > WARN_ON_ONCE(1); > clear_page(kaddr); > } So on x86, a PROT_WRITE-only private page is mapped as non-readable? I had the impression that write-only still allows reading by looking at the __P010 definition. Anyway, if it's not an anonymous mmap(), whoever handled the mapping may have changed the permissions (e.g. some device). > So without a warning. My hypothesis is that if we create a page in the > same way as syzcaller then any getsockopt that does a copy_to_user() > (pretty much all of them I guess) will get this warning. The copy_to_user() only triggers the do_wp_page() fault handling. If this is a CoW page (private read-only presumably, or at least not writeable), the kernel tries to copy the original page given to getsockopt into a new page and restart the copy_to_user(). Since the kernel doesn't have a struct page for this (e.g. PFN mapping), it uses __copy_from_user_inatomic() which fails because of the read permission. > I have not tried this, so I might be wrong. If this is true, then the > question is what to do about it. One possible fix would be just to > remove the warning to get the same behavior as before. But it was > probably put there for a reason. It was there for some obscure cases, as the comment says ;). If the above is a valid scenario that the user can trigger, we should probably remove the WARN_ON. -- Catalin