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 X-Spam-Level: X-Spam-Status: No, score=-16.6 required=3.0 tests=DKIMWL_WL_MED,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,USER_AGENT_GIT,USER_IN_DEF_DKIM_WL autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 41CA0C4360F for ; Wed, 13 Feb 2019 21:46:14 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 10136222C9 for ; Wed, 13 Feb 2019 21:46:14 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="nmSN7x4f" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2395014AbfBMVqM (ORCPT ); Wed, 13 Feb 2019 16:46:12 -0500 Received: from mail-it1-f201.google.com ([209.85.166.201]:38665 "EHLO mail-it1-f201.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732481AbfBMVqL (ORCPT ); Wed, 13 Feb 2019 16:46:11 -0500 Received: by mail-it1-f201.google.com with SMTP id m128so6365294itd.3 for ; Wed, 13 Feb 2019 13:46:11 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=date:message-id:mime-version:subject:from:to:cc; bh=FNhxP8JN/aOogF64x1RdI276bG+FvUjSYc/h3hGis10=; b=nmSN7x4f0dFsz2f8CmOy0DSZEVYELcvMbs3qFQceL8JHg4uPw7v3hsrtQFyLw9sE5X K3+ltokbjOLnlDMKXcOKKCOxKs5urZWEJmGd7HmcyAcanVs7dh0Hb+XIT6mehoPOHu1z c0KH48taiqX05DvKw2KMQDIa+P5Iky7RvtdBpMnRXl1xtahZSCjHg0ryL3QpeEfSNy7n 1Jxu74dVhDvvJF/jLy64FaZnxTo88JDD7+vqhn4Oqv214x+MzdzUXP6mwVozZrCTCvDX VNBrpaIh/tsmsJ0HLMKHyWPFXwtTsDKh39OoHk3xwKBdfASz/7QgvfdSBoToyD1iQi1H K9vQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:message-id:mime-version:subject:from:to:cc; bh=FNhxP8JN/aOogF64x1RdI276bG+FvUjSYc/h3hGis10=; b=WrXlRUzPy6X37gk7uXiuCthJtN5gVehOxnysJL5NbLOWaZ/oqy4dVmiua9BAsU7ENz DSuSjzUmBZlTph1Oh88n4DJAGvWnm1Pl0llwj3kbF40dsd7j0uK4HfKrw2h6mZE3AY8C HxeOxmkmqHajYUkryMCd6W2ogBAHwaFNQKf5iMlJMKX8H9hJnZ+A2jO3Y4Ltm3cIEWuk wzVBFWE4YM/TAfowrA8U2o/Z2VA84rGkRvbK/NRy1tdD7XIXqz2IkUJ/s/0nfTjqWBPy uPglGSvPcnAq+wxx2+KXO9cR6arqmoSmhmgH9ofnCsW9TJqD+rauimK/Dur2EMGKwvRj jmdQ== X-Gm-Message-State: AHQUAubLzMcFKkbMrEhhHH15HIF/HOg5mhcLmq+b82ba+3/YrNV/2zI7 Ehs5C/YeVE4Ahxd4VMTm2kFErt9X1Q== X-Google-Smtp-Source: AHgI3IYHqsYwFvWOqst/rKom/+dGsarTpRUx/qbL+/QdEt9tRubhgZrB/16HNXoxImLIXoRQ9XJseh71SA== X-Received: by 2002:a24:5311:: with SMTP id n17mr187606itb.39.1550094371114; Wed, 13 Feb 2019 13:46:11 -0800 (PST) Date: Wed, 13 Feb 2019 22:45:59 +0100 Message-Id: <20190213214559.125666-1-jannh@google.com> Mime-Version: 1.0 X-Mailer: git-send-email 2.20.1.791.gb4d0f1c61a-goog Subject: [RESEND PATCH net] mm: page_alloc: fix ref bias in page_frag_alloc() for 1-byte allocs From: Jann Horn To: "David S. Miller" , netdev@vger.kernel.org, jannh@google.com Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Michal Hocko , Vlastimil Babka , Pavel Tatashin , Oscar Salvador , Mel Gorman , Aaron Lu , Alexander Duyck Content-Type: text/plain; charset="UTF-8" Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The basic idea behind ->pagecnt_bias is: If we pre-allocate the maximum number of references that we might need to create in the fastpath later, the bump-allocation fastpath only has to modify the non-atomic bias value that tracks the number of extra references we hold instead of the atomic refcount. The maximum number of allocations we can serve (under the assumption that no allocation is made with size 0) is nc->size, so that's the bias used. However, even when all memory in the allocation has been given away, a reference to the page is still held; and in the `offset < 0` slowpath, the page may be reused if everyone else has dropped their references. This means that the necessary number of references is actually `nc->size+1`. Luckily, from a quick grep, it looks like the only path that can call page_frag_alloc(fragsz=1) is TAP with the IFF_NAPI_FRAGS flag, which requires CAP_NET_ADMIN in the init namespace and is only intended to be used for kernel testing and fuzzing. To test for this issue, put a `WARN_ON(page_ref_count(page) == 0)` in the `offset < 0` path, below the virt_to_page() call, and then repeatedly call writev() on a TAP device with IFF_TAP|IFF_NO_PI|IFF_NAPI_FRAGS|IFF_NAPI, with a vector consisting of 15 elements containing 1 byte each. Signed-off-by: Jann Horn --- Resending to davem at the request of akpm. mm/page_alloc.c | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 35fdde041f5c..46285d28e43b 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -4675,11 +4675,11 @@ void *page_frag_alloc(struct page_frag_cache *nc, /* Even if we own the page, we do not use atomic_set(). * This would break get_page_unless_zero() users. */ - page_ref_add(page, size - 1); + page_ref_add(page, size); /* reset page count bias and offset to start of new frag */ nc->pfmemalloc = page_is_pfmemalloc(page); - nc->pagecnt_bias = size; + nc->pagecnt_bias = size + 1; nc->offset = size; } @@ -4695,10 +4695,10 @@ void *page_frag_alloc(struct page_frag_cache *nc, size = nc->size; #endif /* OK, page count is 0, we can safely set it */ - set_page_count(page, size); + set_page_count(page, size + 1); /* reset page count bias and offset to start of new frag */ - nc->pagecnt_bias = size; + nc->pagecnt_bias = size + 1; offset = size - fragsz; } -- 2.20.1.791.gb4d0f1c61a-goog