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[209.85.167.50]) by smtp.gmail.com with ESMTPSA id j4sm2278205lfg.197.2020.12.21.16.11.18 for (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 21 Dec 2020 16:11:18 -0800 (PST) Received: by mail-lf1-f50.google.com with SMTP id o13so27892042lfr.3 for ; Mon, 21 Dec 2020 16:11:18 -0800 (PST) X-Received: by 2002:ac2:41d9:: with SMTP id d25mr7001569lfi.377.1608595877900; Mon, 21 Dec 2020 16:11:17 -0800 (PST) MIME-Version: 1.0 References: <76B4F49B-ED61-47EA-9BE4-7F17A26B610D@gmail.com> <9E301C7C-882A-4E0F-8D6D-1170E792065A@gmail.com> <1FCC8F93-FF29-44D3-A73A-DF943D056680@gmail.com> <20201221223041.GL6640@xz-x1> In-Reply-To: From: Linus Torvalds Date: Mon, 21 Dec 2020 16:11:01 -0800 X-Gmail-Original-Message-ID: Message-ID: Subject: Re: [PATCH] mm/userfaultfd: fix memory corruption due to writeprotect To: Yu Zhao Cc: Peter Xu , Nadav Amit , Andrea Arcangeli , linux-mm , lkml , Pavel Emelyanov , Mike Kravetz , Mike Rapoport , stable , Minchan Kim , Andy Lutomirski , Will Deacon , Peter Zijlstra Content-Type: text/plain; charset="UTF-8" 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 Mon, Dec 21, 2020 at 4:00 PM Yu Zhao wrote: > > My first instinct is to be conservative and revert 09854ba94c6a ("mm: > do_wp_page() simplification") so people are less likely to come back > and complain about performance issues from holding mmap lock for > write when clearing pte_write. Well, the thing is, that simplificaiton was actually part of fixing a real regression wrt GUP. Reverting that would break a308c71bf1e6 ("mm/gup: Remove enfornced COW mechanism"). And that one was the (better) fix for commit 17839856fd58 that fixed a real security issue, but did it with a big hammer that then caused problems for uffd-wp (and some other loads). There's a bit more context in the merge message in commit b25d1dc9474e Merge branch 'simplify-do_wp_page'. So while that commit 09854ba94c6a on its own is "just" a simplification, it's actually part of a bigger series that fixes serious problems. Linus