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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-Stat-Signature: z4kqdpjgchoityn6w4wr85zswtoqfmq4 X-Rspamd-Queue-Id: 5694A80005 X-Rspam-User: X-Rspamd-Server: rspam10 X-HE-Tag: 1780253939-303528 X-HE-Meta: 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 ryjV3g8G J912Opl0PdYYMkzjQJggY6Zeg3Sf4tOPkMwXukfQwEu3LzqkzbL5WlK1pRJPzH+c/mcEX3radIx5FJabKVywq8nXsqCrz7edYD6ak0w5DdtMDdGleB095+EUaqkgPM826XpfPl1Mx/L0YUlXxh6iygxTckn4avXUEfJ7lsrIVCgRnNyeTAmhxMmYq5Q6xpiD/N357W2AGlsbcRBkcHhxZIMeaYNGO0fS5r9PCpS5311KbSCDf1ZOfEFWP/7nXPgN5mkfk Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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