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Date: Sat, 9 Jan 2021 21:54:05 -0500 From: Andrea Arcangeli <aarcange@...hat.com> To: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org Cc: linux-kernel@...r.kernel.org, Yu Zhao <yuzhao@...gle.com>, Andy Lutomirski <luto@...nel.org>, Peter Xu <peterx@...hat.com>, Pavel Emelyanov <xemul@...nvz.org>, Mike Kravetz <mike.kravetz@...cle.com>, Mike Rapoport <rppt@...ux.vnet.ibm.com>, Minchan Kim <minchan@...nel.org>, Will Deacon <will@...nel.org>, Peter Zijlstra <peterz@...radead.org>, Linus Torvalds <torvalds@...ux-foundation.org>, Hugh Dickins <hughd@...gle.com>, "Kirill A. Shutemov" <kirill@...temov.name>, Matthew Wilcox <willy@...radead.org>, Oleg Nesterov <oleg@...hat.com>, Jann Horn <jannh@...gle.com>, Kees Cook <keescook@...omium.org>, John Hubbard <jhubbard@...dia.com>, Leon Romanovsky <leonro@...dia.com>, Jason Gunthorpe <jgg@...pe.ca>, Jan Kara <jack@...e.cz>, Kirill Tkhai <ktkhai@...tuozzo.com>, Nadav Amit <nadav.amit@...il.com>, Jens Axboe <axboe@...nel.dk> Subject: Re: [PATCH 1/1] mm: restore full accuracy in COW page reuse Hello, On Sat, Jan 09, 2021 at 07:44:35PM -0500, Andrea Arcangeli wrote: > allowing a child to corrupt memory in the parent. That's a problem > that could happen not-maliciously too. So the scenario described I updated the above partly quoted sentence since in the previous version it didn't have full accuracy: https://git.kernel.org/pub/scm/linux/kernel/git/andrea/aa.git/commit/?id=fc5a76b1c14e5e6cdc64ece306fc03773662d98a "However since a single transient GUP pin on a tail page, would elevate the page_count for all other tail pages (unlike the mapcount which is subpage granular), the COW page reuse inaccuracy would then cross different vmas and the effect would happen at a distance in vma of different processes. A single GUP pin taken on a subpage mapped in a different process could trigger 511 false positive COWs copies in the local process, after a fork()." This a best effort to try to document all side effects, but it'd be great to hear from Kirill too on the above detail to have confirmation. Thanks and have a great weekend everyone, Andrea
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