From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-oi0-f69.google.com (mail-oi0-f69.google.com [209.85.218.69]) by kanga.kvack.org (Postfix) with ESMTP id 871486B0253 for ; Thu, 5 Oct 2017 12:57:12 -0400 (EDT) Received: by mail-oi0-f69.google.com with SMTP id p187so13149207oif.1 for ; Thu, 05 Oct 2017 09:57:12 -0700 (PDT) Received: from foss.arm.com (foss.arm.com. [217.140.101.70]) by mx.google.com with ESMTP id z65si5131715oig.73.2017.10.05.09.57.10 for ; Thu, 05 Oct 2017 09:57:11 -0700 (PDT) From: Will Deacon Subject: [PATCH] mm: page_vma_mapped: Ensure pmd is loaded with READ_ONCE outside of lock Date: Thu, 5 Oct 2017 17:57:10 +0100 Message-Id: <1507222630-5839-1-git-send-email-will.deacon@arm.com> Sender: owner-linux-mm@kvack.org List-ID: To: linux-mm@kvack.org Cc: linux-kernel@vger.kernel.org, akpm@linux-foundation.org, Will Deacon , "Kirill A. Shutemov" , "Paul E. McKenney" , Peter Zijlstra , stable@vger.kernel.org Loading the pmd without holding the pmd_lock exposes us to races with concurrent updaters of the page tables but, worse still, it also allows the compiler to cache the pmd value in a register and reuse it later on, even if we've performed a READ_ONCE in between and seen a more recent value. In the case of page_vma_mapped_walk, this leads to the following crash when the pmd loaded for the initial pmd_trans_huge check is all zeroes and a subsequent valid table entry is loaded by check_pmd. We then proceed into map_pte, but the compiler re-uses the zero entry inside pte_offset_map, resulting in a junk pointer being installed in pvmw->pte: [ 254.032812] PC is at check_pte+0x20/0x170 [ 254.032948] LR is at page_vma_mapped_walk+0x2e0/0x540 [...] [ 254.036114] Process doio (pid: 2463, stack limit = 0xffff00000f2e8000) [ 254.036361] Call trace: [ 254.038977] [] check_pte+0x20/0x170 [ 254.039137] [] page_vma_mapped_walk+0x2e0/0x540 [ 254.039332] [] page_mkclean_one+0xac/0x278 [ 254.039489] [] rmap_walk_file+0xf0/0x238 [ 254.039642] [] rmap_walk+0x64/0xa0 [ 254.039784] [] page_mkclean+0x90/0xa8 [ 254.040029] [] clear_page_dirty_for_io+0x84/0x2a8 [ 254.040311] [] mpage_submit_page+0x34/0x98 [ 254.040518] [] mpage_process_page_bufs+0x164/0x170 [ 254.040743] [] mpage_prepare_extent_to_map+0x134/0x2b8 [ 254.040969] [] ext4_writepages+0x484/0xe30 [ 254.041175] [] do_writepages+0x44/0xe8 [ 254.041372] [] __filemap_fdatawrite_range+0xbc/0x110 [ 254.041568] [] file_write_and_wait_range+0x48/0xd8 [ 254.041739] [] ext4_sync_file+0x80/0x4b8 [ 254.041907] [] vfs_fsync_range+0x64/0xc0 [ 254.042106] [] SyS_msync+0x194/0x1e8 This patch fixes the problem by ensuring that READ_ONCE is used before the initial checks on the pmd, and this value is subsequently used when checking whether or not the pmd is present. pmd_check is removed and the pmd_present check is inlined directly. Cc: "Kirill A. Shutemov" Cc: "Paul E. McKenney" Cc: Peter Zijlstra Cc: Fixes: f27176cfc363 ("mm: convert page_mkclean_one() to use page_vma_mapped_walk()") Tested-by: Yury Norov Tested-by: Richard Ruigrok Signed-off-by: Will Deacon --- mm/page_vma_mapped.c | 25 ++++++++++--------------- 1 file changed, 10 insertions(+), 15 deletions(-) diff --git a/mm/page_vma_mapped.c b/mm/page_vma_mapped.c index 6a03946469a9..6b85f5464246 100644 --- a/mm/page_vma_mapped.c +++ b/mm/page_vma_mapped.c @@ -6,17 +6,6 @@ #include "internal.h" -static inline bool check_pmd(struct page_vma_mapped_walk *pvmw) -{ - pmd_t pmde; - /* - * Make sure we don't re-load pmd between present and !trans_huge check. - * We need a consistent view. - */ - pmde = READ_ONCE(*pvmw->pmd); - return pmd_present(pmde) && !pmd_trans_huge(pmde); -} - static inline bool not_found(struct page_vma_mapped_walk *pvmw) { page_vma_mapped_walk_done(pvmw); @@ -116,6 +105,7 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) pgd_t *pgd; p4d_t *p4d; pud_t *pud; + pmd_t pmde; /* The only possible pmd mapping has been handled on last iteration */ if (pvmw->pmd && !pvmw->pte) @@ -148,7 +138,13 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) if (!pud_present(*pud)) return false; pvmw->pmd = pmd_offset(pud, pvmw->address); - if (pmd_trans_huge(*pvmw->pmd) || is_pmd_migration_entry(*pvmw->pmd)) { + /* + * Make sure the pmd value isn't cached in a register by the + * compiler and used as a stale value after we've observed a + * subsequent update. + */ + pmde = READ_ONCE(*pvmw->pmd); + if (pmd_trans_huge(pmde) || is_pmd_migration_entry(pmde)) { pvmw->ptl = pmd_lock(mm, pvmw->pmd); if (likely(pmd_trans_huge(*pvmw->pmd))) { if (pvmw->flags & PVMW_MIGRATION) @@ -175,9 +171,8 @@ bool page_vma_mapped_walk(struct page_vma_mapped_walk *pvmw) spin_unlock(pvmw->ptl); pvmw->ptl = NULL; } - } else { - if (!check_pmd(pvmw)) - return false; + } else if (!pmd_present(pmde)) { + return false; } if (!map_pte(pvmw)) goto next_pte; -- 2.1.4 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. 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