From b43a9990055958e70347c56f90ea2ae32c67334c Mon Sep 17 00:00:00 2001 From: Mike Kravetz Date: Fri, 28 Dec 2018 00:39:38 -0800 Subject: hugetlbfs: use i_mmap_rwsem for more pmd sharing synchronization While looking at BUGs associated with invalid huge page map counts, it was discovered and observed that a huge pte pointer could become 'invalid' and point to another task's page table. Consider the following: A task takes a page fault on a shared hugetlbfs file and calls huge_pte_alloc to get a ptep. Suppose the returned ptep points to a shared pmd. Now, another task truncates the hugetlbfs file. As part of truncation, it unmaps everyone who has the file mapped. If the range being truncated is covered by a shared pmd, huge_pmd_unshare will be called. For all but the last user of the shared pmd, huge_pmd_unshare will clear the pud pointing to the pmd. If the task in the middle of the page fault is not the last user, the ptep returned by huge_pte_alloc now points to another task's page table or worse. This leads to bad things such as incorrect page map/reference counts or invalid memory references. To fix, expand the use of i_mmap_rwsem as follows: - i_mmap_rwsem is held in read mode whenever huge_pmd_share is called. huge_pmd_share is only called via huge_pte_alloc, so callers of huge_pte_alloc take i_mmap_rwsem before calling. In addition, callers of huge_pte_alloc continue to hold the semaphore until finished with the ptep. - i_mmap_rwsem is held in write mode whenever huge_pmd_unshare is called. [mike.kravetz@oracle.com: add explicit check for mapping != null] Link: http://lkml.kernel.org/r/20181218223557.5202-2-mike.kravetz@oracle.com Fixes: 39dde65c9940 ("shared page table for hugetlb page") Signed-off-by: Mike Kravetz Acked-by: Kirill A. Shutemov Cc: Michal Hocko Cc: Hugh Dickins Cc: Naoya Horiguchi Cc: "Aneesh Kumar K . V" Cc: Andrea Arcangeli Cc: Davidlohr Bueso Cc: Prakash Sangappa Cc: Colin Ian King Cc: Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/rmap.c | 4 ++++ 1 file changed, 4 insertions(+) (limited to 'mm/rmap.c') diff --git a/mm/rmap.c b/mm/rmap.c index 68a1a5b869a5..21a26cf51114 100644 --- a/mm/rmap.c +++ b/mm/rmap.c @@ -25,6 +25,7 @@ * page->flags PG_locked (lock_page) * hugetlbfs_i_mmap_rwsem_key (in huge_pmd_share) * mapping->i_mmap_rwsem + * hugetlb_fault_mutex (hugetlbfs specific page fault mutex) * anon_vma->rwsem * mm->page_table_lock or pte_lock * zone_lru_lock (in mark_page_accessed, isolate_lru_page) @@ -1378,6 +1379,9 @@ static bool try_to_unmap_one(struct page *page, struct vm_area_struct *vma, /* * If sharing is possible, start and end will be adjusted * accordingly. + * + * If called for a huge page, caller must hold i_mmap_rwsem + * in write mode as it is possible to call huge_pmd_unshare. */ adjust_range_if_pmd_sharing_possible(vma, &range.start, &range.end); -- cgit v1.2.2