From patchwork Fri Jun 11 08:00:27 2021 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Jann Horn X-Patchwork-Id: 459124 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=-26.3 required=3.0 tests=BAYES_00,DKIMWL_WL_MED, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER, INCLUDES_PATCH, MAILING_LIST_MULTI, SPF_HELO_NONE, SPF_PASS, USER_AGENT_GIT, USER_IN_DEF_DKIM_WL autolearn=unavailable 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 43794C4743D for ; Fri, 11 Jun 2021 08:02:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 28770613BA for ; Fri, 11 Jun 2021 08:02:16 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230332AbhFKIEM (ORCPT ); Fri, 11 Jun 2021 04:04:12 -0400 Received: from mail-wr1-f45.google.com ([209.85.221.45]:41864 "EHLO mail-wr1-f45.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231688AbhFKIEK (ORCPT ); Fri, 11 Jun 2021 04:04:10 -0400 Received: by mail-wr1-f45.google.com with SMTP id o3so5002211wri.8 for ; Fri, 11 Jun 2021 01:02:01 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=from:to:cc:subject:date:message-id:mime-version :content-transfer-encoding; bh=bE/KUZ9QKkdW8cpXSSyU518CP3tPDoswPEAOU9fuFew=; b=r1ucxI47ris1Yyujr58rPBjyNMwf0oO3T7TBdzTpWtzhzrkvdA+rkJdqLe6+CDPPzA 2E3mwuTrlr0mW2bfVEk51AOjX+QvUVU+5euXfZJqdWVqJLOKswJDc4TErtFr0DOzkft6 Of1o+nShLFfiUO3iUFIHe8XRT0QsJY2RYTg4H53d8uOjSH8UW1/S3GrpBF1WF7IXWMlH ufNmZoNkQ/Qp3R3QfTDCNXPUvFs7lH3Z5jjFNfmpM5EzVkkbWluQz3d5AQ3GtznVeHbN yAhSXXeEtdSXrKFNc5aZXG+1pJRV6DJOPbIwaMKFkJhckP38iuWjUYt2/MFd6msdfHBJ OpXw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:cc:subject:date:message-id:mime-version :content-transfer-encoding; bh=bE/KUZ9QKkdW8cpXSSyU518CP3tPDoswPEAOU9fuFew=; b=twj9IDhtblL+jFLBN1Wg8FPK1QlzFDH8KdSFyPyRG6DhoWch1Dil9dHLnexju2sDl7 Ga2B24W9gJEgjmk/TFr95sq+H4+dVwpaYpPs8x4vqM3C9Sw9IyKIjze3KM15M3J3eT/B qOxdRqLdS3z6iQ6C2Dv/xXcB8ROofCR+phIFw/bBlCF3Y+Z20hng26MmkbgX4bKhBDJM sN/5e7vSf20ZOOYMCik6rzo7V1/QLsvyX+bDnxf+JTnCb5Bc9Sas7DQRIkoMTOw7jMMZ XgPIXGQddAjqvu4062eEECCgsvtVsaB2QzdXtUVSvJg09YS9ArMQmc8qdXZC8XQj5S5h Buvw== X-Gm-Message-State: AOAM531UD+MYO48fXY67crfeYlv/mW5enXOBySgYD5AFFr2y9+iEii+y C1L1IvrOEosRofTcRl0d3U3XzA== X-Google-Smtp-Source: ABdhPJwrDUmeg8YRTI7WihhIRANvJqSROo7Yhgyb7Hwx9jgOH3Lv/9V/uXxDzj6jwe46tOrjayFKzQ== X-Received: by 2002:a5d:6b0e:: with SMTP id v14mr2502091wrw.297.1623398460461; Fri, 11 Jun 2021 01:01:00 -0700 (PDT) Received: from localhost ([2a02:168:96c5:1:55ed:514f:6ad7:5bcc]) by smtp.gmail.com with ESMTPSA id x18sm6079898wrw.19.2021.06.11.01.00.59 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 11 Jun 2021 01:00:59 -0700 (PDT) From: Jann Horn To: Andrew Morton Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Jann Horn , Matthew Wilcox , "Kirill A . Shutemov" , John Hubbard , Jan Kara , stable@vger.kernel.org Subject: [PATCH] mm/gup: fix try_grab_compound_head() race with split_huge_page() Date: Fri, 11 Jun 2021 10:00:27 +0200 Message-Id: <20210611080027.984937-1-jannh@google.com> X-Mailer: git-send-email 2.32.0.272.g935e593368-goog MIME-Version: 1.0 Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org try_grab_compound_head() is used to grab a reference to a page from get_user_pages_fast(), which is only protected against concurrent freeing of page tables (via local_irq_save()), but not against concurrent TLB flushes, freeing of data pages, or splitting of compound pages. Because no reference is held to the page when try_grab_compound_head() is called, the page may have been freed and reallocated by the time its refcount has been elevated; therefore, once we're holding a stable reference to the page, the caller re-checks whether the PTE still points to the same page (with the same access rights). The problem is that try_grab_compound_head() has to grab a reference on the head page; but between the time we look up what the head page is and the time we actually grab a reference on the head page, the compound page may have been split up (either explicitly through split_huge_page() or by freeing the compound page to the buddy allocator and then allocating its individual order-0 pages). If that happens, get_user_pages_fast() may end up returning the right page but lifting the refcount on a now-unrelated page, leading to use-after-free of pages. To fix it: Re-check whether the pages still belong together after lifting the refcount on the head page. Move anything else that checks compound_head(page) below the refcount increment. This can't actually happen on bare-metal x86 (because there, disabling IRQs locks out remote TLB flushes), but it can happen on virtualized x86 (e.g. under KVM) and probably also on arm64. The race window is pretty narrow, and constantly allocating and shattering hugepages isn't exactly fast; for now I've only managed to reproduce this in an x86 KVM guest with an artificially widened timing window (by adding a loop that repeatedly calls `inl(0x3f8 + 5)` in `try_get_compound_head()` to force VM exits, so that PV TLB flushes are used instead of IPIs). Cc: Matthew Wilcox Cc: Kirill A. Shutemov Cc: John Hubbard Cc: Jan Kara Cc: stable@vger.kernel.org Fixes: 7aef4172c795 ("mm: handle PTE-mapped tail pages in gerneric fast gup implementaiton") Signed-off-by: Jann Horn --- mm/gup.c | 54 +++++++++++++++++++++++++++++++++++++++--------------- 1 file changed, 39 insertions(+), 15 deletions(-) base-commit: 614124bea77e452aa6df7a8714e8bc820b489922 diff --git a/mm/gup.c b/mm/gup.c index 3ded6a5f26b2..1f9c0ac15073 100644 --- a/mm/gup.c +++ b/mm/gup.c @@ -43,8 +43,21 @@ static void hpage_pincount_sub(struct page *page, int refs) atomic_sub(refs, compound_pincount_ptr(page)); } +/* Equivalent to calling put_page() @refs times. */ +static void put_page_refs(struct page *page, int refs) +{ + VM_BUG_ON_PAGE(page_ref_count(page) < refs, page); + /* + * Calling put_page() for each ref is unnecessarily slow. Only the last + * ref needs a put_page(). + */ + if (refs > 1) + page_ref_sub(page, refs - 1); + put_page(page); +} + /* * Return the compound head page with ref appropriately incremented, * or NULL if that failed. */ @@ -55,8 +68,23 @@ static inline struct page *try_get_compound_head(struct page *page, int refs) if (WARN_ON_ONCE(page_ref_count(head) < 0)) return NULL; if (unlikely(!page_cache_add_speculative(head, refs))) return NULL; + + /* + * At this point we have a stable reference to the head page; but it + * could be that between the compound_head() lookup and the refcount + * increment, the compound page was split, in which case we'd end up + * holding a reference on a page that has nothing to do with the page + * we were given anymore. + * So now that the head page is stable, recheck that the pages still + * belong together. + */ + if (unlikely(compound_head(page) != head)) { + put_page_refs(head, refs); + return NULL; + } + return head; } /* @@ -94,25 +122,28 @@ __maybe_unused struct page *try_grab_compound_head(struct page *page, if (unlikely((flags & FOLL_LONGTERM) && !is_pinnable_page(page))) return NULL; + /* + * CAUTION: Don't use compound_head() on the page before this + * point, the result won't be stable. + */ + page = try_get_compound_head(page, refs); + if (!page) + return NULL; + /* * When pinning a compound page of order > 1 (which is what * hpage_pincount_available() checks for), use an exact count to * track it, via hpage_pincount_add/_sub(). * * However, be sure to *also* increment the normal page refcount * field at least once, so that the page really is pinned. */ - if (!hpage_pincount_available(page)) - refs *= GUP_PIN_COUNTING_BIAS; - - page = try_get_compound_head(page, refs); - if (!page) - return NULL; - if (hpage_pincount_available(page)) hpage_pincount_add(page, refs); + else + page_ref_add(page, refs * (GUP_PIN_COUNTING_BIAS - 1)); mod_node_page_state(page_pgdat(page), NR_FOLL_PIN_ACQUIRED, orig_refs); @@ -134,16 +165,9 @@ static void put_compound_head(struct page *page, int refs, unsigned int flags) else refs *= GUP_PIN_COUNTING_BIAS; } - VM_BUG_ON_PAGE(page_ref_count(page) < refs, page); - /* - * Calling put_page() for each ref is unnecessarily slow. Only the last - * ref needs a put_page(). - */ - if (refs > 1) - page_ref_sub(page, refs - 1); - put_page(page); + put_page_refs(page, refs); } /** * try_grab_page() - elevate a page's refcount by a flag-dependent amount