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=-5.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS 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 C0ABAC4743C for ; Wed, 23 Jun 2021 11:52:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id A4A9660FEE for ; Wed, 23 Jun 2021 11:52:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230268AbhFWLyz (ORCPT ); Wed, 23 Jun 2021 07:54:55 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:51974 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230135AbhFWLyz (ORCPT ); Wed, 23 Jun 2021 07:54:55 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8D614C061574 for ; Wed, 23 Jun 2021 04:52:37 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=/GACbFUQBsL3k4dufWqeympRiym3hWqjCaP9FTsmnJ4=; b=U9X4VQnXQPkXPJ39J6DWS3cOey d2rYg1uiuzUSKa2STWp7meESuGlq97IGwyOpE3fq9roWMn8HAmCHcE9Tki8Bnmn5yBw8z86WX4Osm DGW9oFDxxAKNwb6xcjLoKMFkSksdyCUhhM9v9huiN7qL3Gm2gx+EguwtsIjvGChIHzrJS10YYW+mm VsDukwoGemLtc6Ow1A4va747/uqarC5Pb5Fi+KPQ8uUugl2djIan4bZoXh6uHMArR5e6WjVljIAay D2ynNMwyscuEqSe8mZC5SaboEUwGzdkLHrW9ZylVI+Ipg1GjOELfFAqTwBxR3x1TxmKHyWLAdqeob 7Z/YtKmg==; Received: from willy by casper.infradead.org with local (Exim 4.94.2 #2 (Red Hat Linux)) id 1lw1Po-00FNnk-3j; Wed, 23 Jun 2021 11:51:38 +0000 Date: Wed, 23 Jun 2021 12:51:32 +0100 From: Matthew Wilcox To: Catalin Marinas Cc: Al Viro , Xiaoming Ni , Chen Huang , Andrew Morton , Stephen Rothwell , Randy Dunlap , Will Deacon , Linux ARM , linux-mm , open list Subject: Re: [BUG] arm64: an infinite loop in generic_perform_write() Message-ID: References: <92fa298d-9d88-0ca4-40d9-13690dcd42f9@huawei.com> <20210623093220.GA3718@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210623093220.GA3718@arm.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jun 23, 2021 at 10:32:21AM +0100, Catalin Marinas wrote: > On Wed, Jun 23, 2021 at 04:27:37AM +0000, Al Viro wrote: > > On Wed, Jun 23, 2021 at 11:24:54AM +0800, Xiaoming Ni wrote: > > > On 2021/6/23 10:50, Al Viro wrote: > > > > On Wed, Jun 23, 2021 at 10:39:31AM +0800, Chen Huang wrote: > > > > > > > > > Then when kernel handles the alignment_fault, it will not panic. As the > > > > > arm64 memory model spec said, when the address is not a multiple of the > > > > > element size, the access is unaligned. Unaligned accesses are allowed to > > > > > addresses marked as Normal, but not to Device regions. An unaligned access > > > > > to a Device region will trigger an exception (alignment fault). > > > > > > > > > > do_alignment_fault > > > > > do_bad_area > > > > > __do_kernel_fault > > > > > fixup_exception > > > > > > > > > > But that fixup cann't handle the unaligned copy, so the > > > > > copy_page_from_iter_atomic returns 0 and traps in loop. > > > > > > > > Looks like you need to fix your raw_copy_from_user(), then... > > > > > > Exit loop when iov_iter_copy_from_user_atomic() returns 0. > > > This should solve the problem, too, and it's easier. > > > > It might be easier, but it's not going to work correctly. > > If the page gets evicted by memory pressure, you are going > > to get spurious short write. > > > > Besides, it's simply wrong - write(2) does *NOT* require an > > aligned source. It (and raw_copy_from_user()) should act the > > same way memcpy(3) does. > > On arm64, neither memcpy() nor raw_copy_from_user() are expected to work > on Device mappings, we have memcpy_fromio() for this but only for > ioremap(). There's no (easy) way to distinguish in the write() syscall > how the source buffer is mapped. generic_perform_write() does an > iov_iter_fault_in_readable() check but that's not sufficient and it also > breaks the cases where you can get intra-page faults (arm64 MTE or SPARC > ADI). I think in the general case it's racy anyway (another thread doing > an mprotect(PROT_NONE) after the readable check passed). > > So I think generic_perform_write() returning -EFAULT if copied == 0 > would make sense (well, unless it breaks other cases I'm not aware of). It does break other cases -- that's what happens if the page has gone missing after being faulted in. You need to fix your copy_from_user().