| From: Barry Song <v-songbaohua@oppo.com> |
| Subject: mm: fix kernel BUG when userfaultfd_move encounters swapcache |
| Date: Wed, 26 Feb 2025 13:14:00 +1300 |
| |
| userfaultfd_move() checks whether the PTE entry is present or a |
| swap entry. |
| |
| - If the PTE entry is present, move_present_pte() handles folio |
| migration by setting: |
| |
| src_folio->index = linear_page_index(dst_vma, dst_addr); |
| |
| - If the PTE entry is a swap entry, move_swap_pte() simply copies |
| the PTE to the new dst_addr. |
| |
| This approach is incorrect because, even if the PTE is a swap entry, |
| it can still reference a folio that remains in the swap cache. |
| |
| This creates a race window between steps 2 and 4. |
| 1. add_to_swap: The folio is added to the swapcache. |
| 2. try_to_unmap: PTEs are converted to swap entries. |
| 3. pageout: The folio is written back. |
| 4. Swapcache is cleared. |
| If userfaultfd_move() occurs in the window between steps 2 and 4, |
| after the swap PTE has been moved to the destination, accessing the |
| destination triggers do_swap_page(), which may locate the folio in |
| the swapcache. However, since the folio's index has not been updated |
| to match the destination VMA, do_swap_page() will detect a mismatch. |
| |
| This can result in two critical issues depending on the system |
| configuration. |
| |
| If KSM is disabled, both small and large folios can trigger a BUG |
| during the add_rmap operation due to: |
| |
| page_pgoff(folio, page) != linear_page_index(vma, address) |
| |
| [ 13.336953] page: refcount:6 mapcount:1 mapping:00000000f43db19c index:0xffffaf150 pfn:0x4667c |
| [ 13.337520] head: order:2 mapcount:1 entire_mapcount:0 nr_pages_mapped:1 pincount:0 |
| [ 13.337716] memcg:ffff00000405f000 |
| [ 13.337849] anon flags: 0x3fffc0000020459(locked|uptodate|dirty|owner_priv_1|head|swapbacked|node=0|zone=0|lastcpupid=0xffff) |
| [ 13.338630] raw: 03fffc0000020459 ffff80008507b538 ffff80008507b538 ffff000006260361 |
| [ 13.338831] raw: 0000000ffffaf150 0000000000004000 0000000600000000 ffff00000405f000 |
| [ 13.339031] head: 03fffc0000020459 ffff80008507b538 ffff80008507b538 ffff000006260361 |
| [ 13.339204] head: 0000000ffffaf150 0000000000004000 0000000600000000 ffff00000405f000 |
| [ 13.339375] head: 03fffc0000000202 fffffdffc0199f01 ffffffff00000000 0000000000000001 |
| [ 13.339546] head: 0000000000000004 0000000000000000 00000000ffffffff 0000000000000000 |
| [ 13.339736] page dumped because: VM_BUG_ON_PAGE(page_pgoff(folio, page) != linear_page_index(vma, address)) |
| [ 13.340190] ------------[ cut here ]------------ |
| [ 13.340316] kernel BUG at mm/rmap.c:1380! |
| [ 13.340683] Internal error: Oops - BUG: 00000000f2000800 [#1] PREEMPT SMP |
| [ 13.340969] Modules linked in: |
| [ 13.341257] CPU: 1 UID: 0 PID: 107 Comm: a.out Not tainted 6.14.0-rc3-gcf42737e247a-dirty #299 |
| [ 13.341470] Hardware name: linux,dummy-virt (DT) |
| [ 13.341671] pstate: 60000005 (nZCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--) |
| [ 13.341815] pc : __page_check_anon_rmap+0xa0/0xb0 |
| [ 13.341920] lr : __page_check_anon_rmap+0xa0/0xb0 |
| [ 13.342018] sp : ffff80008752bb20 |
| [ 13.342093] x29: ffff80008752bb20 x28: fffffdffc0199f00 x27: 0000000000000001 |
| [ 13.342404] x26: 0000000000000000 x25: 0000000000000001 x24: 0000000000000001 |
| [ 13.342575] x23: 0000ffffaf0d0000 x22: 0000ffffaf0d0000 x21: fffffdffc0199f00 |
| [ 13.342731] x20: fffffdffc0199f00 x19: ffff000006210700 x18: 00000000ffffffff |
| [ 13.342881] x17: 6c203d2120296567 x16: 6170202c6f696c6f x15: 662866666f67705f |
| [ 13.343033] x14: 6567617028454741 x13: 2929737365726464 x12: ffff800083728ab0 |
| [ 13.343183] x11: ffff800082996bf8 x10: 0000000000000fd7 x9 : ffff80008011bc40 |
| [ 13.343351] x8 : 0000000000017fe8 x7 : 00000000fffff000 x6 : ffff8000829eebf8 |
| [ 13.343498] x5 : c0000000fffff000 x4 : 0000000000000000 x3 : 0000000000000000 |
| [ 13.343645] x2 : 0000000000000000 x1 : ffff0000062db980 x0 : 000000000000005f |
| [ 13.343876] Call trace: |
| [ 13.344045] __page_check_anon_rmap+0xa0/0xb0 (P) |
| [ 13.344234] folio_add_anon_rmap_ptes+0x22c/0x320 |
| [ 13.344333] do_swap_page+0x1060/0x1400 |
| [ 13.344417] __handle_mm_fault+0x61c/0xbc8 |
| [ 13.344504] handle_mm_fault+0xd8/0x2e8 |
| [ 13.344586] do_page_fault+0x20c/0x770 |
| [ 13.344673] do_translation_fault+0xb4/0xf0 |
| [ 13.344759] do_mem_abort+0x48/0xa0 |
| [ 13.344842] el0_da+0x58/0x130 |
| [ 13.344914] el0t_64_sync_handler+0xc4/0x138 |
| [ 13.345002] el0t_64_sync+0x1ac/0x1b0 |
| [ 13.345208] Code: aa1503e0 f000f801 910f6021 97ff5779 (d4210000) |
| [ 13.345504] ---[ end trace 0000000000000000 ]--- |
| [ 13.345715] note: a.out[107] exited with irqs disabled |
| [ 13.345954] note: a.out[107] exited with preempt_count 2 |
| |
| If KSM is enabled, Peter Xu also discovered that do_swap_page() may |
| trigger an unexpected CoW operation for small folios because |
| ksm_might_need_to_copy() allocates a new folio when the folio index |
| does not match linear_page_index(vma, addr). |
| |
| This patch also checks the swapcache when handling swap entries. If a |
| match is found in the swapcache, it processes it similarly to a present |
| PTE. |
| However, there are some differences. For example, the folio is no longer |
| exclusive because folio_try_share_anon_rmap_pte() is performed during |
| unmapping. |
| Furthermore, in the case of swapcache, the folio has already been |
| unmapped, eliminating the risk of concurrent rmap walks and removing the |
| need to acquire src_folio's anon_vma or lock. |
| |
| Note that for large folios, in the swapcache handling path, we directly |
| return -EBUSY since split_folio() will return -EBUSY regardless if |
| the folio is under writeback or unmapped. This is not an urgent issue, |
| so a follow-up patch may address it separately. |
| |
| [v-songbaohua@oppo.com: minor cleanup according to Peter Xu] |
| Link: https://lkml.kernel.org/r/20250226024411.47092-1-21cnbao@gmail.com |
| Link: https://lkml.kernel.org/r/20250226001400.9129-1-21cnbao@gmail.com |
| Fixes: adef440691ba ("userfaultfd: UFFDIO_MOVE uABI") |
| Signed-off-by: Barry Song <v-songbaohua@oppo.com> |
| Acked-by: Peter Xu <peterx@redhat.com> |
| Reviewed-by: Suren Baghdasaryan <surenb@google.com> |
| Cc: Andrea Arcangeli <aarcange@redhat.com> |
| Cc: Al Viro <viro@zeniv.linux.org.uk> |
| Cc: Axel Rasmussen <axelrasmussen@google.com> |
| Cc: Brian Geffon <bgeffon@google.com> |
| Cc: Christian Brauner <brauner@kernel.org> |
| Cc: David Hildenbrand <david@redhat.com> |
| Cc: Hugh Dickins <hughd@google.com> |
| Cc: Jann Horn <jannh@google.com> |
| Cc: Kalesh Singh <kaleshsingh@google.com> |
| Cc: Liam R. Howlett <Liam.Howlett@oracle.com> |
| Cc: Lokesh Gidra <lokeshgidra@google.com> |
| Cc: Matthew Wilcox (Oracle) <willy@infradead.org> |
| Cc: Michal Hocko <mhocko@suse.com> |
| Cc: Mike Rapoport (IBM) <rppt@kernel.org> |
| Cc: Nicolas Geoffray <ngeoffray@google.com> |
| Cc: Ryan Roberts <ryan.roberts@arm.com> |
| Cc: Shuah Khan <shuah@kernel.org> |
| Cc: ZhangPeng <zhangpeng362@huawei.com> |
| Cc: Tangquan Zheng <zhengtangquan@oppo.com> |
| Cc: <stable@vger.kernel.org> |
| Signed-off-by: Andrew Morton <akpm@linux-foundation.org> |
| --- |
| |
| mm/userfaultfd.c | 74 ++++++++++++++++++++++++++++++++++++++++----- |
| 1 file changed, 66 insertions(+), 8 deletions(-) |
| |
| --- a/mm/userfaultfd.c~mm-fix-kernel-bug-when-userfaultfd_move-encounters-swapcache |
| +++ a/mm/userfaultfd.c |
| @@ -18,6 +18,7 @@ |
| #include <asm/tlbflush.h> |
| #include <asm/tlb.h> |
| #include "internal.h" |
| +#include "swap.h" |
| |
| static __always_inline |
| bool validate_dst_vma(struct vm_area_struct *dst_vma, unsigned long dst_end) |
| @@ -1076,16 +1077,14 @@ out: |
| return err; |
| } |
| |
| -static int move_swap_pte(struct mm_struct *mm, |
| +static int move_swap_pte(struct mm_struct *mm, struct vm_area_struct *dst_vma, |
| unsigned long dst_addr, unsigned long src_addr, |
| pte_t *dst_pte, pte_t *src_pte, |
| pte_t orig_dst_pte, pte_t orig_src_pte, |
| pmd_t *dst_pmd, pmd_t dst_pmdval, |
| - spinlock_t *dst_ptl, spinlock_t *src_ptl) |
| + spinlock_t *dst_ptl, spinlock_t *src_ptl, |
| + struct folio *src_folio) |
| { |
| - if (!pte_swp_exclusive(orig_src_pte)) |
| - return -EBUSY; |
| - |
| double_pt_lock(dst_ptl, src_ptl); |
| |
| if (!is_pte_pages_stable(dst_pte, src_pte, orig_dst_pte, orig_src_pte, |
| @@ -1094,6 +1093,16 @@ static int move_swap_pte(struct mm_struc |
| return -EAGAIN; |
| } |
| |
| + /* |
| + * The src_folio resides in the swapcache, requiring an update to its |
| + * index and mapping to align with the dst_vma, where a swap-in may |
| + * occur and hit the swapcache after moving the PTE. |
| + */ |
| + if (src_folio) { |
| + folio_move_anon_rmap(src_folio, dst_vma); |
| + src_folio->index = linear_page_index(dst_vma, dst_addr); |
| + } |
| + |
| orig_src_pte = ptep_get_and_clear(mm, src_addr, src_pte); |
| set_pte_at(mm, dst_addr, dst_pte, orig_src_pte); |
| double_pt_unlock(dst_ptl, src_ptl); |
| @@ -1141,6 +1150,7 @@ static int move_pages_pte(struct mm_stru |
| __u64 mode) |
| { |
| swp_entry_t entry; |
| + struct swap_info_struct *si = NULL; |
| pte_t orig_src_pte, orig_dst_pte; |
| pte_t src_folio_pte; |
| spinlock_t *src_ptl, *dst_ptl; |
| @@ -1322,6 +1332,8 @@ retry: |
| orig_dst_pte, orig_src_pte, dst_pmd, |
| dst_pmdval, dst_ptl, src_ptl, src_folio); |
| } else { |
| + struct folio *folio = NULL; |
| + |
| entry = pte_to_swp_entry(orig_src_pte); |
| if (non_swap_entry(entry)) { |
| if (is_migration_entry(entry)) { |
| @@ -1335,9 +1347,53 @@ retry: |
| goto out; |
| } |
| |
| - err = move_swap_pte(mm, dst_addr, src_addr, dst_pte, src_pte, |
| - orig_dst_pte, orig_src_pte, dst_pmd, |
| - dst_pmdval, dst_ptl, src_ptl); |
| + if (!pte_swp_exclusive(orig_src_pte)) { |
| + err = -EBUSY; |
| + goto out; |
| + } |
| + |
| + si = get_swap_device(entry); |
| + if (unlikely(!si)) { |
| + err = -EAGAIN; |
| + goto out; |
| + } |
| + /* |
| + * Verify the existence of the swapcache. If present, the folio's |
| + * index and mapping must be updated even when the PTE is a swap |
| + * entry. The anon_vma lock is not taken during this process since |
| + * the folio has already been unmapped, and the swap entry is |
| + * exclusive, preventing rmap walks. |
| + * |
| + * For large folios, return -EBUSY immediately, as split_folio() |
| + * also returns -EBUSY when attempting to split unmapped large |
| + * folios in the swapcache. This issue needs to be resolved |
| + * separately to allow proper handling. |
| + */ |
| + if (!src_folio) |
| + folio = filemap_get_folio(swap_address_space(entry), |
| + swap_cache_index(entry)); |
| + if (!IS_ERR_OR_NULL(folio)) { |
| + if (folio_test_large(folio)) { |
| + err = -EBUSY; |
| + folio_put(folio); |
| + goto out; |
| + } |
| + src_folio = folio; |
| + src_folio_pte = orig_src_pte; |
| + if (!folio_trylock(src_folio)) { |
| + pte_unmap(&orig_src_pte); |
| + pte_unmap(&orig_dst_pte); |
| + src_pte = dst_pte = NULL; |
| + put_swap_device(si); |
| + si = NULL; |
| + /* now we can block and wait */ |
| + folio_lock(src_folio); |
| + goto retry; |
| + } |
| + } |
| + err = move_swap_pte(mm, dst_vma, dst_addr, src_addr, dst_pte, src_pte, |
| + orig_dst_pte, orig_src_pte, dst_pmd, dst_pmdval, |
| + dst_ptl, src_ptl, src_folio); |
| } |
| |
| out: |
| @@ -1354,6 +1410,8 @@ out: |
| if (src_pte) |
| pte_unmap(src_pte); |
| mmu_notifier_invalidate_range_end(&range); |
| + if (si) |
| + put_swap_device(si); |
| |
| return err; |
| } |
| _ |
