mm: ksm: fix use-after-free kasan report in ksm_might_need_to_copy

When under the stress of swapping in/out with KSM enabled, there is a
low probability that kasan reports the BUG of use-after-free in
ksm_might_need_to_copy() when do swap in.  The freed object is the
anon_vma got from page_anon_vma(page).

It is because a swapcache page associated with one anon_vma now needed
for another anon_vma, but the page's original vma was unmapped and the
anon_vma was freed.  In this case the if condition below always return
false and then alloc a new page to copy.  Swapin process then use the
new page and can continue to run well, so this is harmless actually.

      } else if (anon_vma->root == vma->anon_vma->root &&
                 page->index == linear_page_index(vma, address)) {

This patch exchange the order of above two judgment statement to avoid
the kasan warning.  Let cpu run "page->index == linear_page_index(vma,
address)" firstly and return false basically to skip the read of
anon_vma->root which may trigger the kasan use-after-free warning:

    ==================================================================
    BUG: KASAN: use-after-free in ksm_might_need_to_copy+0x12e/0x5b0
    Read of size 8 at addr ffff88be9977dbd0 by task khugepaged/694

     CPU: 8 PID: 694 Comm: khugepaged Kdump: loaded Tainted: G OE - 4.18.0.x86_64
     Hardware name: 1288H V5/BC11SPSC0, BIOS 7.93 01/14/2021
    Call Trace:
     dump_stack+0xf1/0x19b
     print_address_description+0x70/0x360
     kasan_report+0x1b2/0x330
     ksm_might_need_to_copy+0x12e/0x5b0
     do_swap_page+0x452/0xe70
     __collapse_huge_page_swapin+0x24b/0x720
     khugepaged_scan_pmd+0xcae/0x1ff0
     khugepaged+0x8ee/0xd70
     kthread+0x1a2/0x1d0
     ret_from_fork+0x1f/0x40

    Allocated by task 2306153:
     kasan_kmalloc+0xa0/0xd0
     kmem_cache_alloc+0xc0/0x1c0
     anon_vma_clone+0xf7/0x380
     anon_vma_fork+0xc0/0x390
     copy_process+0x447b/0x4810
     _do_fork+0x118/0x620
     do_syscall_64+0x112/0x360
     entry_SYSCALL_64_after_hwframe+0x65/0xca

    Freed by task 2306242:
     __kasan_slab_free+0x130/0x180
     kmem_cache_free+0x78/0x1d0
     unlink_anon_vmas+0x19c/0x4a0
     free_pgtables+0x137/0x1b0
     exit_mmap+0x133/0x320
     mmput+0x15e/0x390
     do_exit+0x8c5/0x1210
     do_group_exit+0xb5/0x1b0
     __x64_sys_exit_group+0x21/0x30
     do_syscall_64+0x112/0x360
     entry_SYSCALL_64_after_hwframe+0x65/0xca

    The buggy address belongs to the object at ffff88be9977dba0
     which belongs to the cache anon_vma_chain of size 64
    The buggy address is located 48 bytes inside of
     64-byte region [ffff88be9977dba0, ffff88be9977dbe0)
    The buggy address belongs to the page:
    page:ffffea00fa65df40 count:1 mapcount:0 mapping:ffff888107717800 index:0x0
    flags: 0x17ffffc0000100(slab)
    ==================================================================

Link: https://lkml.kernel.org/r/20211202102940.1069634-1-sunnanyong@huawei.com
Signed-off-by: Nanyong Sun <sunnanyong@huawei.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/ksm.c

@@ -2576,8 +2576,8 @@ struct page *ksm_might_need_to_copy(struct page *page,
 			return page;	/* no need to copy it */
 	} else if (!anon_vma) {
 		return page;		/* no need to copy it */
-	} else if (anon_vma->root == vma->anon_vma->root &&
-		 page->index == linear_page_index(vma, address)) {
+	} else if (page->index == linear_page_index(vma, address) &&
+			anon_vma->root == vma->anon_vma->root) {
 		return page;		/* still no need to copy it */
 	}
 	if (!PageUptodate(page))