khugepaged: fix null-pointer dereference due to race

kiryl · gregkh · commit 40c5836b4a48 · 2020-07-29T10:18:44.000+02:00
commit 594cced upstream. 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. Fixes: 99cb0db ("mm,thp: add read-only THP support for (non-shmem) FS") Reported-by: syzbot+ed318e8b790ca72c5ad0@syzkaller.appspotmail.com Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Reviewed-by: David Hildenbrand <david@redhat.com> Acked-by: Yang Shi <yang.shi@linux.alibaba.com> Cc: <stable@vger.kernel.org> Link: http://lkml.kernel.org/r/20200722121439.44328-1-kirill.shutemov@linux.intel.com Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
@@ -876,6 +876,9 @@ static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,
 		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;
 }
 

Original file line number	Diff line number	Diff line change
`@@ -876,6 +876,9 @@ static int hugepage_vma_revalidate(struct mm_struct *mm, unsigned long address,`
`876`	`876`	`return SCAN_ADDRESS_RANGE;`
`877`	`877`	`if (!hugepage_vma_check(vma, vma->vm_flags))`
`878`	`878`	`return SCAN_VMA_CHECK;`
	`879`	`+ /* Anon VMA expected */`
	`880`	`+ if (!vma->anon_vma \|\| vma->vm_ops)`
	`881`	`+ return SCAN_VMA_CHECK;`
`879`	`882`	`return 0;`
`880`	`883`	`}`
`881`	`884`