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 Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 2121CCD6E56 for ; Sun, 31 May 2026 18:59:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:Content-Transfer-Encoding: Content-Type:In-Reply-To:From:References:Cc:To:Subject:MIME-Version:Date: Message-ID:Reply-To:Content-ID:Content-Description:Resent-Date:Resent-From: Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=GjVt0UsISSTwZDvffigjt8YBiFpMw+nOdlcd3vzCHGc=; b=1xyMjUSNODDxzcNSCZ2d0ZS1PE tFC2qe46Uv9TstHeNnu8IJBk5kdE/5hHLeVLyhkkZDmZYd+nPs1x6sT3k4eWk7FLQcb9NRt67Lz4d wGUkbscvW63i/Y2xVjwRwajqJj6yd0T8knWCOkAN+rEtbfXrhRatKNxLq7a2emkT6mjc0rIGEJaO4 zWpaS2whu3utfQABdqPy/D2gfz03aljnsLe8qMsg14Mf2pld+WiNm/0EQFbeMQDXkGSFCEV2Xraek UW+eQPMAXturBbd1zjLejZi72cc74CY1He7Yfqa8zkH6x67ILJ7enD2sDOyQrpAMvv93htoozWIp5 Ob2gplvw==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1wTlNH-00000009pm9-1I6K; Sun, 31 May 2026 18:59:03 +0000 Received: from sea.source.kernel.org ([2600:3c0a:e001:78e:0:1991:8:25]) by bombadil.infradead.org with esmtps (Exim 4.99.1 #2 (Red Hat Linux)) id 1wTlNE-00000009plk-03kU for linux-arm-kernel@lists.infradead.org; Sun, 31 May 2026 18:59:01 +0000 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id 62899441CB; Sun, 31 May 2026 18:58:58 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id B3D161F00893; Sun, 31 May 2026 18:58:52 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1780253938; bh=GjVt0UsISSTwZDvffigjt8YBiFpMw+nOdlcd3vzCHGc=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=YNh/LumMIZ723DQ/0VP1GsxHxDjZrdxhVNhzemW/jiRb5w5/80kYkSqAPFZGJBQEw +U2f7XtX8p48n1O6HbjMPXAfpgWnxCaoO2NbStZkaiRGY+/urn0fAUopy2gsspP47m HXcV9+JVpjHDRaGp4ZYSIZTH/8Kw0TUWjTtfNGkM0v6rUMA2eetPYF33K4zwyNwjJ3 9pVf6drwd/aAxwQDBUqiMjmffm/SvVRkspQEAYPgs59JoR4gQHwPECpfA/PNSm6tHR emsS5rbPW8TwzN+gl4QTRujSNvPF7Cc3z8OFL3e2itKVZHK58fQbVAGDl9vnE0Zllw xKt9ksAra4hZQ== Message-ID: <1ed6751d-e286-4c6f-864a-800f8baf54c3@kernel.org> Date: Sun, 31 May 2026 20:58:51 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 2/8] bpf: Recover arena kernel faults with scratch page To: Tejun Heo , Alexei Starovoitov Cc: David Vernet , Andrea Righi , Changwoo Min , Andrii Nakryiko , Daniel Borkmann , Martin KaFai Lau , Kumar Kartikeya Dwivedi , Peter Zijlstra , Catalin Marinas , Will Deacon , Thomas Gleixner , Ingo Molnar , Borislav Petkov , Dave Hansen , Andrew Morton , Mike Rapoport , Emil Tsalapatis , sched-ext@lists.linux.dev, bpf@vger.kernel.org, x86@kernel.org, linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260522172219.1423324-1-tj@kernel.org> <20260522172219.1423324-3-tj@kernel.org> <7fd673df-22f3-4d70-a779-ea0b878188b3@kernel.org> <3901fe0537edee9d7acdfd91695ead28@kernel.org> <8cc56c7a4aa29628b7d17d85be7eadb9@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260531_115900_101207_41339044 X-CRM114-Status: GOOD ( 15.67 ) 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: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 5/31/26 19:47, Tejun Heo wrote: > Hello, > > I posted the check removal [1], and Sashiko's review flagged a > break-before-make problem with it [2] that I think is real. Yeah, and as I raised previously, this is very questionable locking design :) Either everybody works with atomics or nobody. > > The scratch page is a present PAGE_KERNEL mapping, so having > apply_range_set_cb() overwrite it via set_pte_at() during > bpf_arena_alloc_pages() is a valid->valid PFN change. I'm not familiar with > arm at all. David, my understanding is that's a break-before-make violation > on arm64, and that on any arch the stale TLB entry keeps resolving to the > shared scratch page until it's flushed, so a later access can hit scratch > instead of the new page. Is that what you were worried about? > > So instead of just dropping the check, the install should route through an > invalid entry rather than overwrite in place: > > while (!ptep_try_set(pte, mk_pte(page, PAGE_KERNEL))) { > old = ptep_get(pte); > if (pte_none(old)) > continue; > if (WARN_ON_ONCE(pte_page(old) != arena->scratch_page)) > return -EBUSY; > ptep_get_and_clear(&init_mm, addr, pte); > broke_scratch = true; > } We have to handle architectures where ptep_try_set() is not implemented (as I tried with my variant). -- Cheers, David