From: Linus Torvalds torvalds@linux-foundation.org
commit 8fde12ca79aff9b5ba951fce1a2641901b8d8e64 upstream.
If the page refcount wraps around past zero, it will be freed while there are still four billion references to it. One of the possible avenues for an attacker to try to make this happen is by doing direct IO on a page multiple times. This patch makes get_user_pages() refuse to take a new page reference if there are already more than two billion references to the page.
Reported-by: Jann Horn jannh@google.com Acked-by: Matthew Wilcox willy@infradead.org Cc: stable@kernel.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- mm/gup.c | 48 ++++++++++++++++++++++++++++++++++++------------ mm/hugetlb.c | 13 +++++++++++++ 2 files changed, 49 insertions(+), 12 deletions(-)
--- a/mm/gup.c +++ b/mm/gup.c @@ -157,8 +157,12 @@ retry: goto retry; }
- if (flags & FOLL_GET) - get_page(page); + if (flags & FOLL_GET) { + if (unlikely(!try_get_page(page))) { + page = ERR_PTR(-ENOMEM); + goto out; + } + } if (flags & FOLL_TOUCH) { if ((flags & FOLL_WRITE) && !pte_dirty(pte) && !PageDirty(page)) @@ -295,7 +299,10 @@ retry_locked: if (pmd_trans_unstable(pmd)) ret = -EBUSY; } else { - get_page(page); + if (unlikely(!try_get_page(page))) { + spin_unlock(ptl); + return ERR_PTR(-ENOMEM); + } spin_unlock(ptl); lock_page(page); ret = split_huge_page(page); @@ -497,7 +504,10 @@ static int get_gate_page(struct mm_struc if (is_device_public_page(*page)) goto unmap; } - get_page(*page); + if (unlikely(!try_get_page(*page))) { + ret = -ENOMEM; + goto unmap; + } out: ret = 0; unmap: @@ -1393,6 +1403,20 @@ static void undo_dev_pagemap(int *nr, in } }
+/* + * Return the compund head page with ref appropriately incremented, + * or NULL if that failed. + */ +static inline struct page *try_get_compound_head(struct page *page, int refs) +{ + struct page *head = compound_head(page); + if (WARN_ON_ONCE(page_ref_count(head) < 0)) + return NULL; + if (unlikely(!page_cache_add_speculative(head, refs))) + return NULL; + return head; +} + #ifdef CONFIG_ARCH_HAS_PTE_SPECIAL static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end, int write, struct page **pages, int *nr) @@ -1427,9 +1451,9 @@ static int gup_pte_range(pmd_t pmd, unsi
VM_BUG_ON(!pfn_valid(pte_pfn(pte))); page = pte_page(pte); - head = compound_head(page);
- if (!page_cache_get_speculative(head)) + head = try_get_compound_head(page, 1); + if (!head) goto pte_unmap;
if (unlikely(pte_val(pte) != pte_val(*ptep))) { @@ -1568,8 +1592,8 @@ static int gup_huge_pmd(pmd_t orig, pmd_ refs++; } while (addr += PAGE_SIZE, addr != end);
- head = compound_head(pmd_page(orig)); - if (!page_cache_add_speculative(head, refs)) { + head = try_get_compound_head(pmd_page(orig), refs); + if (!head) { *nr -= refs; return 0; } @@ -1606,8 +1630,8 @@ static int gup_huge_pud(pud_t orig, pud_ refs++; } while (addr += PAGE_SIZE, addr != end);
- head = compound_head(pud_page(orig)); - if (!page_cache_add_speculative(head, refs)) { + head = try_get_compound_head(pud_page(orig), refs); + if (!head) { *nr -= refs; return 0; } @@ -1643,8 +1667,8 @@ static int gup_huge_pgd(pgd_t orig, pgd_ refs++; } while (addr += PAGE_SIZE, addr != end);
- head = compound_head(pgd_page(orig)); - if (!page_cache_add_speculative(head, refs)) { + head = try_get_compound_head(pgd_page(orig), refs); + if (!head) { *nr -= refs; return 0; } --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -4298,6 +4298,19 @@ long follow_hugetlb_page(struct mm_struc
pfn_offset = (vaddr & ~huge_page_mask(h)) >> PAGE_SHIFT; page = pte_page(huge_ptep_get(pte)); + + /* + * Instead of doing 'try_get_page()' below in the same_page + * loop, just check the count once here. + */ + if (unlikely(page_count(page) <= 0)) { + if (pages) { + spin_unlock(ptl); + remainder = 0; + err = -ENOMEM; + break; + } + } same_page: if (pages) { pages[i] = mem_map_offset(page, pfn_offset);