From: Will Deacon <will.deacon@xxxxxxx> Subject: mm: page_vma_mapped: ensure pmd is loaded with READ_ONCE outside of lock 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] [<ffff000008233328>] check_pte+0x20/0x170 [ 254.039137] [<ffff000008233758>] page_vma_mapped_walk+0x2e0/0x540 [ 254.039332] [<ffff000008234adc>] page_mkclean_one+0xac/0x278 [ 254.039489] [<ffff000008234d98>] rmap_walk_file+0xf0/0x238 [ 254.039642] [<ffff000008236e74>] rmap_walk+0x64/0xa0 [ 254.039784] [<ffff0000082370c8>] page_mkclean+0x90/0xa8 [ 254.040029] [<ffff0000081f3c64>] clear_page_dirty_for_io+0x84/0x2a8 [ 254.040311] [<ffff00000832f984>] mpage_submit_page+0x34/0x98 [ 254.040518] [<ffff00000832fb4c>] mpage_process_page_bufs+0x164/0x170 [ 254.040743] [<ffff00000832fc8c>] mpage_prepare_extent_to_map+0x134/0x2b8 [ 254.040969] [<ffff00000833530c>] ext4_writepages+0x484/0xe30 [ 254.041175] [<ffff0000081f6ab4>] do_writepages+0x44/0xe8 [ 254.041372] [<ffff0000081e5bd4>] __filemap_fdatawrite_range+0xbc/0x110 [ 254.041568] [<ffff0000081e5e68>] file_write_and_wait_range+0x48/0xd8 [ 254.041739] [<ffff000008324310>] ext4_sync_file+0x80/0x4b8 [ 254.041907] [<ffff0000082bd434>] vfs_fsync_range+0x64/0xc0 [ 254.042106] [<ffff0000082332b4>] 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. Link: http://lkml.kernel.org/r/1507222630-5839-1-git-send-email-will.deacon@xxxxxxx Fixes: f27176cfc363 ("mm: convert page_mkclean_one() to use page_vma_mapped_walk()") Signed-off-by: Will Deacon <will.deacon@xxxxxxx> Tested-by: Yury Norov <ynorov@xxxxxxxxxxxxxxxxxx> Tested-by: Richard Ruigrok <rruigrok@xxxxxxxxxxxxxx> Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Cc: <stable@xxxxxxxxxxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- mm/page_vma_mapped.c | 25 ++++++++++--------------- 1 file changed, 10 insertions(+), 15 deletions(-) diff -puN mm/page_vma_mapped.c~mm-page_vma_mapped-ensure-pmd-is-loaded-with-read_once-outside-of-lock mm/page_vma_mapped.c --- a/mm/page_vma_mapped.c~mm-page_vma_mapped-ensure-pmd-is-loaded-with-read_once-outside-of-lock +++ a/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_vm 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 @@ restart: 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) @@ -174,9 +170,8 @@ restart: 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; _ -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
