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

[Nanyong Sun]

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)
==================================================================

Signed-off-by: Nanyong Sun <sunnanyong@xxxxxxxxxx>
---
 mm/ksm.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c
index 0662093237e4..4ce462dd31a4 100644
--- a/mm/ksm.c
+++ b/mm/ksm.c
@@ -2575,8 +2575,8 @@ 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))
-- 
2.25.1