From: "Kirill A. Shutemov" kirill.shutemov@linux.intel.com Subject: khugepaged: fix null-pointer dereference due to race

khugepaged has to drop mmap lock several times while collapsing a page. The situation can change while the lock is dropped and we need to re-validate that the VMA is still in place and the PMD is still subject for collapse.

But we miss one corner case: while collapsing an anonymous pages the VMA could be replaced with file VMA. If the file VMA doesn't have any private pages we get NULL pointer dereference:

general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] PREEMPT SMP KASAN KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007] anon_vma_lock_write include/linux/rmap.h:120 [inline] collapse_huge_page mm/khugepaged.c:1110 [inline] khugepaged_scan_pmd mm/khugepaged.c:1349 [inline] khugepaged_scan_mm_slot mm/khugepaged.c:2110 [inline] khugepaged_do_scan mm/khugepaged.c:2193 [inline] khugepaged+0x3bba/0x5a10 mm/khugepaged.c:2238

The fix is to make sure that the VMA is anonymous in hugepage_vma_revalidate(). The helper is only used for collapsing anonymous pages.

Link: http://lkml.kernel.org/r/20200722121439.44328-1-kirill.shutemov@linux.intel.... Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS") Signed-off-by: Kirill A. Shutemov kirill.shutemov@linux.intel.com Reported-by: syzbot+ed318e8b790ca72c5ad0@syzkaller.appspotmail.com Reviewed-by: David Hildenbrand david@redhat.com Acked-by: Yang Shi yang.shi@linux.alibaba.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/khugepaged.c | 3 +++ 1 file changed, 3 insertions(+)

--- a/mm/khugepaged.c~khugepaged-fix-null-pointer-dereference-due-to-race +++ a/mm/khugepaged.c @@ -958,6 +958,9 @@ static int hugepage_vma_revalidate(struc return SCAN_ADDRESS_RANGE; if (!hugepage_vma_check(vma, vma->vm_flags)) return SCAN_VMA_CHECK; + /* Anon VMA expected */ + if (!vma->anon_vma || vma->vm_ops) + return SCAN_VMA_CHECK; return 0; }

_