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=-10.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 04F23C49EA6 for ; Thu, 24 Jun 2021 18:56:02 +0000 (UTC) Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by mail.kernel.org (Postfix) with ESMTP id 9491F613D3 for ; Thu, 24 Jun 2021 18:56:01 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 9491F613D3 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=owner-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix) id 75B126B0036; Thu, 24 Jun 2021 14:56:00 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 6E32B6B005D; Thu, 24 Jun 2021 14:56:00 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 55DD16B006C; Thu, 24 Jun 2021 14:56:00 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from forelay.hostedemail.com (smtprelay0014.hostedemail.com [216.40.44.14]) by kanga.kvack.org (Postfix) with ESMTP id 1F2D86B0036 for ; Thu, 24 Jun 2021 14:56:00 -0400 (EDT) Received: from smtpin32.hostedemail.com (10.5.19.251.rfc1918.com [10.5.19.251]) by forelay04.hostedemail.com (Postfix) with ESMTP id 4FED81F87C for ; Thu, 24 Jun 2021 18:56:00 +0000 (UTC) X-FDA: 78289522080.32.BB59487 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by imf11.hostedemail.com (Postfix) with ESMTP id D882B20010A2 for ; Thu, 24 Jun 2021 18:55:59 +0000 (UTC) Received: by mail.kernel.org (Postfix) with ESMTPSA id EACB1613CC; Thu, 24 Jun 2021 18:55:56 +0000 (UTC) Date: Thu, 24 Jun 2021 19:55:54 +0100 From: Catalin Marinas To: Al Viro Cc: Robin Murphy , Matthew Wilcox , Christoph Hellwig , Chen Huang , Mark Rutland , 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: <20210624185554.GC25097@arm.com> References: <20210623132223.GA96264@C02TD0UTHF1T.local> <1c635945-fb25-8871-7b34-f475f75b2caf@huawei.com> <27fbb8c1-2a65-738f-6bec-13f450395ab7@arm.com> 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) Authentication-Results: imf11.hostedemail.com; dkim=none; spf=pass (imf11.hostedemail.com: domain of cmarinas@kernel.org designates 198.145.29.99 as permitted sender) smtp.mailfrom=cmarinas@kernel.org; dmarc=fail reason="SPF not aligned (relaxed), No valid DKIM" header.from=arm.com (policy=none) X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: D882B20010A2 X-Stat-Signature: 3uyf7hu1kkzu771adsf673dt4h77ajwr X-HE-Tag: 1624560959-657243 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, Jun 24, 2021 at 04:27:17PM +0000, Al Viro wrote: > On Thu, Jun 24, 2021 at 02:22:27PM +0100, Robin Murphy wrote: > > FWIW I think the only way to make the kernel behaviour any more robust here > > would be to make the whole uaccess API more expressive, such that rather > > than simply saying "I only got this far" it could actually differentiate > > between stopping due to a fault which may be recoverable and worth retrying, > > and one which definitely isn't. > > ... and propagate that "more expressive" information through what, 3 or 4 > levels in the call chain? > > From include/linux/uaccess.h: > > * If raw_copy_{to,from}_user(to, from, size) returns N, size - N bytes starting > * at to must become equal to the bytes fetched from the corresponding area > * starting at from. All data past to + size - N must be left unmodified. > * > * If copying succeeds, the return value must be 0. If some data cannot be > * fetched, it is permitted to copy less than had been fetched; the only > * hard requirement is that not storing anything at all (i.e. returning size) > * should happen only when nothing could be copied. In other words, you don't > * have to squeeze as much as possible - it is allowed, but not necessary. > > arm64 instances violate the aforementioned hard requirement. After reading the above a few more times, I think I get it. The key sentence is: not storing anything at all should happen only when nothing could be copied. In the MTE case, something can still be copied. > Please, fix > it there; it's not hard. All you need is an exception handler in .Ltiny15 > that would fall back to (short) byte-by-byte copy if the faulting address > happened to be unaligned. Or just do one-byte copy, not that it had been > considerably cheaper than a loop. Will be cheaper than propagating that extra > information up the call chain, let alone paying for extra ->write_begin() > and ->write_end() for single byte in generic_perform_write(). Yeah, it's definitely fixable in the arch code. I misread the above requirements and thought it could be fixed in the core code. Quick hack, though I think in the actual exception handling path in .S more sense (and it needs the copy_to_user for symmetry): diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h index b5f08621fa29..903f8a2a457b 100644 --- a/arch/arm64/include/asm/uaccess.h +++ b/arch/arm64/include/asm/uaccess.h @@ -415,6 +415,15 @@ extern unsigned long __must_check __arch_copy_from_user(void *to, const void __u uaccess_ttbr0_enable(); \ __acfu_ret = __arch_copy_from_user((to), \ __uaccess_mask_ptr(from), (n)); \ + if (__acfu_ret == n) { \ + int __cfu_err = 0; \ + char __cfu_val; \ + __raw_get_mem("ldtr", __cfu_val, (char *)from, __cfu_err);\ + if (!__cfu_err) { \ + *(char *)to = __cfu_val; \ + __acfu_ret--; \ + } \ + } \ uaccess_ttbr0_disable(); \ __acfu_ret; \ }) Of course, it only fixes the MTE problem, I'll ignore the MMIO case (though it may work in certain configurations like synchronous faults). -- Catalin