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dmarc=fail (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:Content-Type: Content-Transfer-Encoding:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:Date:Message-ID:From: References:Cc:To:Subject:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=rsaQUHpW/lYuEqu8qag/ehUtOFeoviIDCWtWyuT9htM=; b=LMuNerdZuxZL5dqd8LybD0CVb/ DQnYuFZtqNOiyBbPb/lNvl7N5Vmz6HGx+u8WpMMswCqUAA8EO/PaLLoYMYHHgOt/iyVMj5ytsf0O2 zJ+799Qexbvz3m3c4g99QDjKUU6NWJjMAUN3m80qzRnewoUICzAWdWQ3uHE4APKQUddir/QeqSA59 am2Kl7Ql57wFv31yfQ/NEQYgEq350sD1n0f0jvWdwXxGlI53RFfqXIoYz2Pq5/sQ/ZRbrQMdiKIxs KIT+8ZuFtbwP3P65vtmNQ4CiAKM6vjXMZ5XevBOlx9MjFiU14LMaR1lLz5LqW8S8yN3gNuTVpbtIn LbbYSTSg==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1m0qY9-00Cf86-7h; Tue, 06 Jul 2021 19:16:05 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.94.2 #2 (Red Hat Linux)) id 1m0qY4-00Cf6d-26 for linux-arm-kernel@lists.infradead.org; Tue, 06 Jul 2021 19:16:01 +0000 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 4132F1042; Tue, 6 Jul 2021 12:15:54 -0700 (PDT) Received: from [10.57.40.45] (unknown [10.57.40.45]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 4FABB3F5A1; Tue, 6 Jul 2021 12:15:52 -0700 (PDT) Subject: Re: [BUG] arm64: an infinite loop in generic_perform_write() To: Catalin Marinas Cc: Chen Huang , Al Viro , Matthew Wilcox , Christoph Hellwig , Mark Rutland , Andrew Morton , Stephen Rothwell , Randy Dunlap , Will Deacon , Linux ARM , linux-mm , open list References: <1c635945-fb25-8871-7b34-f475f75b2caf@huawei.com> <27fbb8c1-2a65-738f-6bec-13f450395ab7@arm.com> <20210624185554.GC25097@arm.com> <20210625103905.GA20835@arm.com> <7f14271a-9b2f-1afc-3caf-c4e5b36efa73@arm.com> <20210706175052.GD15218@arm.com> From: Robin Murphy Message-ID: Date: Tue, 6 Jul 2021 20:15:47 +0100 User-Agent: Mozilla/5.0 (Windows NT 10.0; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: <20210706175052.GD15218@arm.com> Content-Language: en-GB X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210706_121600_241483_D402D6FB X-CRM114-Status: GOOD ( 19.14 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 2021-07-06 18:50, Catalin Marinas wrote: > On Mon, Jun 28, 2021 at 05:22:30PM +0100, Robin Murphy wrote: >> diff --git a/arch/arm64/lib/copy_to_user.S b/arch/arm64/lib/copy_to_user.S >> index 043da90f5dd7..cfb598ae4812 100644 >> --- a/arch/arm64/lib/copy_to_user.S >> +++ b/arch/arm64/lib/copy_to_user.S >> @@ -32,7 +32,7 @@ >> .endm >> .macro strh1 reg, ptr, val >> - user_ldst 9998f, sttrh, \reg, \ptr, \val >> + user_ldst 9997f, sttrh, \reg, \ptr, \val >> .endm >> .macro ldr1 reg, ptr, val >> @@ -40,7 +40,7 @@ >> .endm >> .macro str1 reg, ptr, val >> - user_ldst 9998f, sttr, \reg, \ptr, \val >> + user_ldst 9997f, sttr, \reg, \ptr, \val >> .endm >> .macro ldp1 reg1, reg2, ptr, val >> @@ -48,12 +48,14 @@ >> .endm >> .macro stp1 reg1, reg2, ptr, val >> - user_stp 9998f, \reg1, \reg2, \ptr, \val >> + user_stp 9997f, \reg1, \reg2, \ptr, \val >> .endm >> end .req x5 >> +srcin .req x15 >> SYM_FUNC_START(__arch_copy_to_user) >> add end, x0, x2 >> + mov srcin, x1 >> #include "copy_template.S" >> mov x0, #0 >> ret >> @@ -62,6 +64,12 @@ EXPORT_SYMBOL(__arch_copy_to_user) >> .section .fixup,"ax" >> .align 2 >> +9997: cmp dst, dstin >> + b.ne 9998f >> + // Before being absolutely sure we couldn't copy anything, try harder >> + ldrb tmp1w, [srcin] >> +USER(9998f, sttrb tmp1w, [dstin]) >> + add dst, dstin, #1 >> 9998: sub x0, end, dst // bytes not copied >> ret >> .previous > > I think it's worth doing the copy_to_user() fallback in a loop until it > faults or hits the end of the buffer. This would solve the problem we > currently have with writing more bytes than actually reported. The > copy_from_user() is not necessary, a byte would suffice. The thing is, we don't really have that problem since the set_fs cleanup removed IMP-DEF STP behaviour from the picture - even with the current mess we could perfectly well know which of the two STTRs faulted if we just put a little more effort in. Even, at worst, simply this: diff --git a/arch/arm64/include/asm/asm-uaccess.h b/arch/arm64/include/asm/asm-uaccess.h index ccedf548dac9..7513758bab3a 100644 --- a/arch/arm64/include/asm/asm-uaccess.h +++ b/arch/arm64/include/asm/asm-uaccess.h @@ -74,8 +74,9 @@ alternative_else_nop_endif .macro user_stp l, reg1, reg2, addr, post_inc 8888: sttr \reg1, [\addr]; -8889: sttr \reg2, [\addr, #8]; - add \addr, \addr, \post_inc; + add \addr, \addr, \post_inc / 2; +8889: sttr \reg2, [\addr]; + add \addr, \addr, \post_inc / 2; _asm_extable 8888b,\l; _asm_extable 8889b,\l; But yuck... If you think the potential under-reporting is worth fixing right now, rather than just letting it disappear in a future rewrite, then I'd still rather do it by passing the actual fault address to the current copy_to_user fixup. A retry loop could still technically under-report if the page disappears (or tag changes) between faulting on the second word of a pair and retrying from the first, so we'd want to pin the initial fault down to a single access anyway. All the loop would achieve after that is potentially fill in an extra 1-7 bytes right up to the offending page/tag boundary for the sake of being nice, which I remain unconvinced is worth the bother :) Cheers, Robin. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel