From 5fcf7bb73f66cc1c4ad90788b0f367c4d6852b75 Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Sun, 10 Dec 2006 02:18:43 -0800 Subject: [PATCH] read_zero_pagealigned() locking fix Ramiro Voicu hits the BUG_ON(!pte_none(*pte)) in zeromap_pte_range: kernel bugzilla 7645. Right: read_zero_pagealigned uses down_read of mmap_sem, but another thread's racing read of /dev/zero, or a normal fault, can easily set that pte again, in between zap_page_range and zeromap_page_range getting there. It's been wrong ever since 2.4.3. The simple fix is to use down_write instead, but that would serialize reads of /dev/zero more than at present: perhaps some app would be badly affected. So instead let zeromap_page_range return the error instead of BUG_ON, and read_zero_pagealigned break to the slower clear_user loop in that case - there's no need to optimize for it. Use -EEXIST for when a pte is found: BUG_ON in mmap_zero (the other user of zeromap_page_range), though it really isn't interesting there. And since mmap_zero wants -EAGAIN for out-of-memory, the zeromaps better return that than -ENOMEM. Signed-off-by: Hugh Dickins Cc: Ramiro Voicu: Cc: Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/memory.c | 32 +++++++++++++++++++++----------- 1 file changed, 21 insertions(+), 11 deletions(-) (limited to 'mm') diff --git a/mm/memory.c b/mm/memory.c index 4198df0dff1c..bf6100236e62 100644 --- a/mm/memory.c +++ b/mm/memory.c @@ -1110,23 +1110,29 @@ static int zeromap_pte_range(struct mm_struct *mm, pmd_t *pmd, { pte_t *pte; spinlock_t *ptl; + int err = 0; pte = pte_alloc_map_lock(mm, pmd, addr, &ptl); if (!pte) - return -ENOMEM; + return -EAGAIN; arch_enter_lazy_mmu_mode(); do { struct page *page = ZERO_PAGE(addr); pte_t zero_pte = pte_wrprotect(mk_pte(page, prot)); + + if (unlikely(!pte_none(*pte))) { + err = -EEXIST; + pte++; + break; + } page_cache_get(page); page_add_file_rmap(page); inc_mm_counter(mm, file_rss); - BUG_ON(!pte_none(*pte)); set_pte_at(mm, addr, pte, zero_pte); } while (pte++, addr += PAGE_SIZE, addr != end); arch_leave_lazy_mmu_mode(); pte_unmap_unlock(pte - 1, ptl); - return 0; + return err; } static inline int zeromap_pmd_range(struct mm_struct *mm, pud_t *pud, @@ -1134,16 +1140,18 @@ static inline int zeromap_pmd_range(struct mm_struct *mm, pud_t *pud, { pmd_t *pmd; unsigned long next; + int err; pmd = pmd_alloc(mm, pud, addr); if (!pmd) - return -ENOMEM; + return -EAGAIN; do { next = pmd_addr_end(addr, end); - if (zeromap_pte_range(mm, pmd, addr, next, prot)) - return -ENOMEM; + err = zeromap_pte_range(mm, pmd, addr, next, prot); + if (err) + break; } while (pmd++, addr = next, addr != end); - return 0; + return err; } static inline int zeromap_pud_range(struct mm_struct *mm, pgd_t *pgd, @@ -1151,16 +1159,18 @@ static inline int zeromap_pud_range(struct mm_struct *mm, pgd_t *pgd, { pud_t *pud; unsigned long next; + int err; pud = pud_alloc(mm, pgd, addr); if (!pud) - return -ENOMEM; + return -EAGAIN; do { next = pud_addr_end(addr, end); - if (zeromap_pmd_range(mm, pud, addr, next, prot)) - return -ENOMEM; + err = zeromap_pmd_range(mm, pud, addr, next, prot); + if (err) + break; } while (pud++, addr = next, addr != end); - return 0; + return err; } int zeromap_page_range(struct vm_area_struct *vma, -- cgit v1.2.3-59-g8ed1b