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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id B23D4C4332F for ; Fri, 25 Nov 2022 21:37:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230007AbiKYVhh (ORCPT ); Fri, 25 Nov 2022 16:37:37 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:40200 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229757AbiKYVhe (ORCPT ); Fri, 25 Nov 2022 16:37:34 -0500 Received: from mail-wm1-x32b.google.com (mail-wm1-x32b.google.com [IPv6:2a00:1450:4864:20::32b]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 91C25B77 for ; Fri, 25 Nov 2022 13:37:29 -0800 (PST) Received: by mail-wm1-x32b.google.com with SMTP id t4so4347504wmj.5 for ; Fri, 25 Nov 2022 13:37:29 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20210112; h=content-transfer-encoding:mime-version:message-id:date:subject:cc :to:from:from:to:cc:subject:date:message-id:reply-to; bh=PMWbLEOm6/iOpsMSbMIlf3wKmoaEQMgHyilsDSmEvuU=; b=BOwP2WulGHimVFJJg7+ebwT37QuzaJXKKAxDMC8zTNWnHnzJf2ND6hQMz/nOtsfhwN SVLCkwkcI/KBIkCvoJkKYM4ANqNF6LaD5FDk0AmSrqCMfutBwoARiJE/okhrLW8H0bGB eXccaf9QfTyuqkGMGCaf+dkyO0PFK80zE3qZKfJmmh2FWIlmF4SROyUtCJakLCbmKrP7 2SJ3MOj13n5eiM6BKCkmagmWdUZop6J/JTsNRAwsJbeplXifx9bjzPiTC5iPkspldCfy XYXSlgBDapyGf1nLp1uUgVYchH23PG2dyO3eSI59trh0Votmjm9Dvvp9q0sXiON5vRfK zv5A== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=content-transfer-encoding:mime-version:message-id:date:subject:cc :to:from:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=PMWbLEOm6/iOpsMSbMIlf3wKmoaEQMgHyilsDSmEvuU=; b=Pm+sPkIL3lGAF01dWibJGmx2qxi7zKYvjL5B/Lk8EiEnlnNIABQ6MCIPXwQX/jQnUr 6S8QBSZH6Y/BfBW0t8kpkVjF6Pl3LlM66NfkCkrnroARfSgc7mS/2koHRh1so66moWkl vB6V8v4ezYVZ52aS1TS3FTqUSsyrecnITowbEbUMdVUjxDmWwtO0zrTQFPR9Mzlgjdnb HvGUG39uX9lrr9QsbITXH8uJXSvIFiqmtXzhSovKWIphXh5he1cOuFQBtj2Vv9FzA+Nk jdIDZlgOXOzR6TWAi1kXHX6YMRwPbC7rKeel1t02wizHszgMTD95mLhifBB3SpKml5EH yYsg== X-Gm-Message-State: ANoB5pn/P2mM6/Hnawc2FuR4qW0KavHewA9ltUL9PFb+E4J1sT5/rBNQ /aRu39IgN9ocIjdqnllysVgiFg== X-Google-Smtp-Source: AA0mqf65PDuMPIHNpEGh/67IuFkprKjIYgRmJbhk3IcafIKD6SNwc5u2aHez6E1eZJ1Q5EeWchAW9w== X-Received: by 2002:a7b:c859:0:b0:3cf:dcee:c349 with SMTP id c25-20020a7bc859000000b003cfdceec349mr16949063wml.82.1669412247939; Fri, 25 Nov 2022 13:37:27 -0800 (PST) Received: from localhost ([2a00:79e0:9d:4:9104:e316:5846:858e]) by smtp.gmail.com with ESMTPSA id p11-20020a05600c358b00b003cf71b1f66csm7025668wmq.0.2022.11.25.13.37.26 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 25 Nov 2022 13:37:27 -0800 (PST) From: Jann Horn To: security@kernel.org, Andrew Morton Cc: Yang Shi , David Hildenbrand , Peter Xu , John Hubbard , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: [PATCH v3 1/3] mm/khugepaged: Take the right locks for page table retraction Date: Fri, 25 Nov 2022 22:37:12 +0100 Message-Id: <20221125213714.4115729-1-jannh@google.com> X-Mailer: git-send-email 2.38.1.584.g0f3c55d4c2-goog MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org pagetable walks on address ranges mapped by VMAs can be done under the mmap lock, the lock of an anon_vma attached to the VMA, or the lock of the VMA's address_space. Only one of these needs to be held, and it does not need to be held in exclusive mode. Under those circumstances, the rules for concurrent access to page table entries are: - Terminal page table entries (entries that don't point to another page table) can be arbitrarily changed under the page table lock, with the exception that they always need to be consistent for hardware page table walks and lockless_pages_from_mm(). This includes that they can be changed into non-terminal entries. - Non-terminal page table entries (which point to another page table) can not be modified; readers are allowed to READ_ONCE() an entry, verify that it is non-terminal, and then assume that its value will stay as-is. Retracting a page table involves modifying a non-terminal entry, so page-table-level locks are insufficient to protect against concurrent page table traversal; it requires taking all the higher-level locks under which it is possible to start a page walk in the relevant range in exclusive mode. The collapse_huge_page() path for anonymous THP already follows this rule, but the shmem/file THP path was getting it wrong, making it possible for concurrent rmap-based operations to cause corruption. Cc: stable@kernel.org Fixes: 27e1f8273113 ("khugepaged: enable collapse pmd for pte-mapped THP") Signed-off-by: Jann Horn --- mm/khugepaged.c | 55 +++++++++++++++++++++++++++++++++++++++++++++---- 1 file changed, 51 insertions(+), 4 deletions(-) diff --git a/mm/khugepaged.c b/mm/khugepaged.c index 4734315f79407..674b111a24fa7 100644 --- a/mm/khugepaged.c +++ b/mm/khugepaged.c @@ -1384,16 +1384,37 @@ static int set_huge_pmd(struct vm_area_struct *vma, unsigned long addr, return SCAN_SUCCEED; } +/* + * A note about locking: + * Trying to take the page table spinlocks would be useless here because those + * are only used to synchronize: + * + * - modifying terminal entries (ones that point to a data page, not to another + * page table) + * - installing *new* non-terminal entries + * + * Instead, we need roughly the same kind of protection as free_pgtables() or + * mm_take_all_locks() (but only for a single VMA): + * The mmap lock together with this VMA's rmap locks covers all paths towards + * the page table entries we're messing with here, except for hardware page + * table walks and lockless_pages_from_mm(). + */ static void collapse_and_free_pmd(struct mm_struct *mm, struct vm_area_struct *vma, unsigned long addr, pmd_t *pmdp) { - spinlock_t *ptl; pmd_t pmd; mmap_assert_write_locked(mm); - ptl = pmd_lock(vma->vm_mm, pmdp); + if (vma->vm_file) + lockdep_assert_held_write(&vma->vm_file->f_mapping->i_mmap_rwsem); + /* + * All anon_vmas attached to the VMA have the same root and are + * therefore locked by the same lock. + */ + if (vma->anon_vma) + lockdep_assert_held_write(&vma->anon_vma->root->rwsem); + pmd = pmdp_collapse_flush(vma, addr, pmdp); - spin_unlock(ptl); mm_dec_nr_ptes(mm); page_table_check_pte_clear_range(mm, addr, pmd); pte_free(mm, pmd_pgtable(pmd)); @@ -1444,6 +1465,14 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr, if (!hugepage_vma_check(vma, vma->vm_flags, false, false, false)) return SCAN_VMA_CHECK; + /* + * Symmetry with retract_page_tables(): Exclude MAP_PRIVATE mappings + * that got written to. Without this, we'd have to also lock the + * anon_vma if one exists. + */ + if (vma->anon_vma) + return SCAN_VMA_CHECK; + /* Keep pmd pgtable for uffd-wp; see comment in retract_page_tables() */ if (userfaultfd_wp(vma)) return SCAN_PTE_UFFD_WP; @@ -1477,6 +1506,20 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr, goto drop_hpage; } + /* + * We need to lock the mapping so that from here on, only GUP-fast and + * hardware page walks can access the parts of the page tables that + * we're operating on. + * See collapse_and_free_pmd(). + */ + i_mmap_lock_write(vma->vm_file->f_mapping); + + /* + * This spinlock should be unnecessary: Nobody else should be accessing + * the page tables under spinlock protection here, only + * lockless_pages_from_mm() and the hardware page walker can access page + * tables while all the high-level locks are held in write mode. + */ start_pte = pte_offset_map_lock(mm, pmd, haddr, &ptl); result = SCAN_FAIL; @@ -1531,6 +1574,8 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr, /* step 4: remove pte entries */ collapse_and_free_pmd(mm, vma, haddr, pmd); + i_mmap_unlock_write(vma->vm_file->f_mapping); + maybe_install_pmd: /* step 5: install pmd entry */ result = install_pmd @@ -1544,6 +1589,7 @@ int collapse_pte_mapped_thp(struct mm_struct *mm, unsigned long addr, abort: pte_unmap_unlock(start_pte, ptl); + i_mmap_unlock_write(vma->vm_file->f_mapping); goto drop_hpage; } @@ -1600,7 +1646,8 @@ static int retract_page_tables(struct address_space *mapping, pgoff_t pgoff, * An alternative would be drop the check, but check that page * table is clear before calling pmdp_collapse_flush() under * ptl. It has higher chance to recover THP for the VMA, but - * has higher cost too. + * has higher cost too. It would also probably require locking + * the anon_vma. */ if (vma->anon_vma) { result = SCAN_PAGE_ANON; base-commit: eb7081409f94a9a8608593d0fb63a1aa3d6f95d8 -- 2.38.1.584.g0f3c55d4c2-goog