From: Nanyong Sun sunnanyong@huawei.com

mainline inclusion from mainline-v5.17-rc1 commit e1c63e110f977205ab9dfb38989c54e6e7b52a7b category: bugfix bugzilla: https://gitee.com/openeuler/kernel/issues/I9BWES CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?i...

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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 Signed-off-by: Jinjiang Tu tujinjiang@huawei.com --- mm/ksm.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/ksm.c b/mm/ksm.c index 62feb478a367..21bb3e7cf3bc 100644 --- a/mm/ksm.c +++ b/mm/ksm.c @@ -2727,8 +2727,8 @@ 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))