[PATCH v2] mm: Fix kernel BUG when userfaultfd_move encounters swapcache
From: Barry Song
Date: Tue Feb 25 2025 - 19:15:16 EST
From: Barry Song <v-songbaohua@xxxxxxxx>
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.
Fixes: adef440691bab ("userfaultfd: UFFDIO_MOVE uABI")
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
Cc: Brian Geffon <bgeffon@xxxxxxxxxx>
Cc: Christian Brauner <brauner@xxxxxxxxxx>
Cc: David Hildenbrand <david@xxxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Jann Horn <jannh@xxxxxxxxxx>
Cc: Kalesh Singh <kaleshsingh@xxxxxxxxxx>
Cc: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
Cc: Lokesh Gidra <lokeshgidra@xxxxxxxxxx>
Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Mike Rapoport (IBM) <rppt@xxxxxxxxxx>
Cc: Nicolas Geoffray <ngeoffray@xxxxxxxxxx>
Cc: Peter Xu <peterx@xxxxxxxxxx>
Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
Cc: Shuah Khan <shuah@xxxxxxxxxx>
Cc: ZhangPeng <zhangpeng362@xxxxxxxxxx>
Cc: Tangquan Zheng <zhengtangquan@xxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Barry Song <v-songbaohua@xxxxxxxx>
---
mm/userfaultfd.c | 76 ++++++++++++++++++++++++++++++++++++++++++------
1 file changed, 67 insertions(+), 9 deletions(-)
diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 867898c4e30b..2df5d100e76d 100644
--- a/mm/userfaultfd.c
+++ b/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)
@@ -1072,16 +1073,14 @@ static int move_present_pte(struct mm_struct *mm,
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,
@@ -1090,10 +1089,20 @@ static int move_swap_pte(struct mm_struct *mm,
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);
+ double_pt_unlock(dst_ptl, src_ptl);
return 0;
}
@@ -1137,6 +1146,7 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
__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;
@@ -1318,6 +1328,8 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
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)) {
@@ -1331,9 +1343,53 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
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 && 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;
+ /* now we can block and wait */
+ folio_lock(src_folio);
+ put_swap_device(si);
+ si = NULL;
+ 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:
@@ -1350,6 +1406,8 @@ static int move_pages_pte(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd,
if (src_pte)
pte_unmap(src_pte);
mmu_notifier_invalidate_range_end(&range);
+ if (si)
+ put_swap_device(si);
return err;
}
--
2.39.3 (Apple Git-146)