On Sat, 21 Jun 2025, Gavin Guo wrote:
[ Upstream commit be6e843fc51a584672dfd9c4a6a24c8cb81d5fb7 ]
When migrating a THP, concurrent access to the PMD migration entry during a deferred split scan can lead to an invalid address access, as illustrated below. To prevent this invalid access, it is necessary to check the PMD migration entry and return early. In this context, there is no need to use pmd_to_swp_entry and pfn_swap_entry_to_page to verify the equality of the target folio. Since the PMD migration entry is locked, it cannot be served as the target.
Mailing list discussion and explanation from Hugh Dickins: "An anon_vma lookup points to a location which may contain the folio of interest, but might instead contain another folio: and weeding out those other folios is precisely what the "folio != pmd_folio((*pmd)" check (and the "risk of replacing the wrong folio" comment a few lines above it) is for."
BUG: unable to handle page fault for address: ffffea60001db008 CPU: 0 UID: 0 PID: 2199114 Comm: tee Not tainted 6.14.0+ #4 NONE Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2 04/01/2014 RIP: 0010:split_huge_pmd_locked+0x3b5/0x2b60 Call Trace:
<TASK> try_to_migrate_one+0x28c/0x3730 rmap_walk_anon+0x4f6/0x770 unmap_folio+0x196/0x1f0 split_huge_page_to_list_to_order+0x9f6/0x1560 deferred_split_scan+0xac5/0x12a0 shrinker_debugfs_scan_write+0x376/0x470 full_proxy_write+0x15c/0x220 vfs_write+0x2fc/0xcb0 ksys_write+0x146/0x250 do_syscall_64+0x6a/0x120 entry_SYSCALL_64_after_hwframe+0x76/0x7e
The bug is found by syzkaller on an internal kernel, then confirmed on upstream.
Link: https://lkml.kernel.org/r/20250421113536.3682201-1-gavinguo@igalia.com Link: https://lore.kernel.org/all/20250414072737.1698513-1-gavinguo@igalia.com/ Link: https://lore.kernel.org/all/20250418085802.2973519-1-gavinguo@igalia.com/ Fixes: 84c3fc4e9c56 ("mm: thp: check pmd migration entry in common path") Signed-off-by: Gavin Guo gavinguo@igalia.com Acked-by: David Hildenbrand david@redhat.com Acked-by: Hugh Dickins hughd@google.com Acked-by: Zi Yan ziy@nvidia.com Reviewed-by: Gavin Shan gshan@redhat.com Cc: Florent Revest revest@google.com Cc: Matthew Wilcox (Oracle) willy@infradead.org Cc: Miaohe Lin linmiaohe@huawei.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org [gavin: backport the migration checking logic to __split_huge_pmd] Signed-off-by: Gavin Guo gavinguo@igalia.com
Thanks, yes, this new 5.4 version Acked-by: Hugh Dickins hughd@google.com
mm/huge_memory.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 03b57323c53b..ceb5b6d720f0 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -2334,7 +2334,7 @@ void __split_huge_pmd(struct vm_area_struct *vma, pmd_t *pmd, VM_BUG_ON(freeze && !page); if (page) { VM_WARN_ON_ONCE(!PageLocked(page));
if (page != pmd_page(*pmd))
if (is_pmd_migration_entry(*pmd) || page != pmd_page(*pmd)) goto out;base-commit: 44613a259decccddd2bd4520f73cc4d5107546c6
2.43.0