From: Nanyong Sun <sunnanyong@xxxxxxxxxx>
Subject: 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@xxxxxxxxxx
Signed-off-by: Nanyong Sun <sunnanyong@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Kefeng Wang <wangkefeng.wang@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/ksm.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
--- a/mm/ksm.c~mm-ksm-fix-use-after-free-kasan-report-in-ksm_might_need_to_copy
+++ a/mm/ksm.c
@@ -2576,8 +2576,8 @@ struct page *ksm_might_need_to_copy(stru
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))
_
