From d03df42ddae196f20e9cfb775704e60269eb6b44 Mon Sep 17 00:00:00 2001 From: Chris Mason Date: Sat, 31 Oct 2026 10:40:07 +0100 Subject: [PATCH] mm: fix deadlock of mremap() against UFFDIO_MOVE With per-VMA locks, UFFDIO_MOVE can run concurrently with mremap() on different VMAs in the same mm. Those VMAs can share page tables, so both operations can acquire the same pair of page-table locks. UFFDIO_MOVE orders the locks by address, while mremap() takes the source lock first. These conflicting orders can deadlock; mremap() holding mmap_lock for writing no longer helps. Move double_pt_lock() and double_pt_unlock() to mm/internal.h and use them for all page-table moves in mremap(). Preserve the source entry checks after acquiring both locks. Remove the comments claiming that exclusive mmap_lock makes lock ordering irrelevant. In move_ptes(), make sure to just recheck source and destination PMD after taking the locks and drop the overly-long comment. Fix generated by an LLM and sent by Chris Mason in private. [ david: use new helpers consistently; simplify description and comments; move_ptes() fixup ] Fixes: 867a43a34ff8 ("userfaultfd: use per-vma locks in userfaultfd operations") Cc: stable@vger.kernel.org Signed-off-by: Chris Mason Reviewed-by: Lorenzo Stoakes (ARM) Acked-by: Muchun Song Tested-by: Muhammad Usama Anjum Link: https://patch.msgid.link/20261031-fixes_core-v3-38-d6437171ecc3@kernel.org Reviewed-by: Lance Yang Signed-off-by: David Hildenbrand (Arm) --- include/linux/userfaultfd_k.h | 2 - mm/huge_memory.c | 19 +++----- mm/hugetlb.c | 14 ++---- mm/internal.h | 28 +++++++++++ mm/mremap.c | 87 ++++++++++++++--------------------- mm/userfaultfd.c | 27 ----------- 6 files changed, 72 insertions(+), 105 deletions(-) diff --git a/include/linux/userfaultfd_k.h b/include/linux/userfaultfd_k.h index a14b8a9ffb7b..565e98f342dc 100644 --- a/include/linux/userfaultfd_k.h +++ b/include/linux/userfaultfd_k.h @@ -153,8 +153,6 @@ extern int mrwprotect_range(struct userfaultfd_ctx *ctx, unsigned long start, unsigned long len, bool enable_rwp); /* move_pages */ -void double_pt_lock(spinlock_t *ptl1, spinlock_t *ptl2); -void double_pt_unlock(spinlock_t *ptl1, spinlock_t *ptl2); int move_pages_huge_pmd(struct mm_struct *mm, pmd_t *dst_pmd, pmd_t *src_pmd, pmd_t dst_pmdval, struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma, diff --git a/mm/huge_memory.c b/mm/huge_memory.c index 75b09a06edb2..f10a199548d8 100644 --- a/mm/huge_memory.c +++ b/mm/huge_memory.c @@ -2646,15 +2646,11 @@ bool move_huge_pmd(struct vm_area_struct *vma, unsigned long old_addr, return false; } - /* - * We don't have to worry about the ordering of src and dst - * ptlocks because exclusive mmap_lock prevents deadlock. - */ - old_ptl = __pmd_trans_huge_lock(old_pmd, vma); - if (old_ptl) { - new_ptl = pmd_lockptr(mm, new_pmd); - if (new_ptl != old_ptl) - spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING); + old_ptl = pmd_lockptr(mm, old_pmd); + new_ptl = pmd_lockptr(mm, new_pmd); + + double_pt_lock(old_ptl, new_ptl); + if (likely(pmd_is_huge(*old_pmd))) { pmd = pmdp_huge_get_and_clear(mm, old_addr, old_pmd); if (pmd_present(pmd)) force_flush = true; @@ -2683,11 +2679,10 @@ bool move_huge_pmd(struct vm_area_struct *vma, unsigned long old_addr, set_pmd_at(mm, new_addr, new_pmd, pmd); if (force_flush) flush_pmd_tlb_range(vma, old_addr, old_addr + PMD_SIZE); - if (new_ptl != old_ptl) - spin_unlock(new_ptl); - spin_unlock(old_ptl); + double_pt_unlock(old_ptl, new_ptl); return true; } + double_pt_unlock(old_ptl, new_ptl); return false; } diff --git a/mm/hugetlb.c b/mm/hugetlb.c index da5d04edf7c1..84141527fe73 100644 --- a/mm/hugetlb.c +++ b/mm/hugetlb.c @@ -5153,15 +5153,9 @@ static void move_huge_pte(struct vm_area_struct *vma, unsigned long old_addr, spinlock_t *src_ptl, *dst_ptl; pte_t pte; - dst_ptl = huge_pte_lock(h, mm, dst_pte); + dst_ptl = huge_pte_lockptr(h, mm, dst_pte); src_ptl = huge_pte_lockptr(h, mm, src_pte); - - /* - * We don't have to worry about the ordering of src and dst ptlocks - * because exclusive mmap_lock (or the i_mmap_lock) prevents deadlock. - */ - if (src_ptl != dst_ptl) - spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING); + double_pt_lock(dst_ptl, src_ptl); pte = huge_ptep_get_and_clear(mm, old_addr, src_pte, sz); @@ -5190,9 +5184,7 @@ static void move_huge_pte(struct vm_area_struct *vma, unsigned long old_addr, set_huge_pte_at(mm, new_addr, dst_pte, pte, sz); } - if (src_ptl != dst_ptl) - spin_unlock(src_ptl); - spin_unlock(dst_ptl); + double_pt_unlock(dst_ptl, src_ptl); } int move_hugetlb_page_tables(struct vm_area_struct *vma, diff --git a/mm/internal.h b/mm/internal.h index 566cbd2dce48..824c2919a37f 100644 --- a/mm/internal.h +++ b/mm/internal.h @@ -624,6 +624,34 @@ static inline void force_page_cache_readahead(struct address_space *mapping, force_page_cache_ra(&ractl, nr_to_read); } +/* + * Take two page table locks at the same level in address order such that + * we cannot deadlock. + */ +static inline void double_pt_lock(spinlock_t *ptl1, spinlock_t *ptl2) + __acquires(ptl1) + __acquires(ptl2) +{ + if (ptl1 > ptl2) + swap(ptl1, ptl2); + spin_lock(ptl1); + if (ptl1 != ptl2) + spin_lock_nested(ptl2, SINGLE_DEPTH_NESTING); + else + __acquire(ptl2); +} + +static inline void double_pt_unlock(spinlock_t *ptl1, spinlock_t *ptl2) + __releases(ptl1) + __releases(ptl2) +{ + spin_unlock(ptl1); + if (ptl1 != ptl2) + spin_unlock(ptl2); + else + __release(ptl2); +} + unsigned find_lock_entries(struct address_space *mapping, pgoff_t *start, pgoff_t end, struct folio_batch *fbatch, pgoff_t *indices); unsigned find_get_entries(struct address_space *mapping, pgoff_t *start, diff --git a/mm/mremap.c b/mm/mremap.c index 1237718848f5..70b7264aa6bf 100644 --- a/mm/mremap.c +++ b/mm/mremap.c @@ -202,7 +202,7 @@ static int move_ptes(struct pagetable_move_control *pmc, struct mm_struct *mm = vma->vm_mm; pte_t *old_ptep, *new_ptep; pte_t old_pte, pte; - pmd_t dummy_pmdval; + pmd_t old_pmdval, new_pmdval; spinlock_t *old_ptl, *new_ptl; bool force_flush = false; unsigned long old_addr = pmc->old_addr; @@ -234,31 +234,30 @@ static int move_ptes(struct pagetable_move_control *pmc, if (pmc->need_rmap_locks) take_rmap_locks(vma); - /* - * We don't have to worry about the ordering of src and dst - * pte locks because exclusive mmap_lock prevents deadlock. - */ - old_ptep = pte_offset_map_lock(mm, old_pmd, old_addr, &old_ptl); + old_ptep = pte_offset_map_rw_nolock(mm, old_pmd, old_addr, &old_pmdval, + &old_ptl); if (!old_ptep) { err = -EAGAIN; goto out; } - /* - * Now new_pte is none, so collapse_scan_file() path can not find - * this by traversing file->f_mapping, so there is no concurrency with - * retract_page_tables(). In addition, we already hold the exclusive - * mmap_lock, so this new_pte page is stable, so there is no need to get - * pmdval and do pmd_same() check. - */ - new_ptep = pte_offset_map_rw_nolock(mm, new_pmd, new_addr, &dummy_pmdval, - &new_ptl); + new_ptep = pte_offset_map_rw_nolock(mm, new_pmd, new_addr, &new_pmdval, + &new_ptl); if (!new_ptep) { - pte_unmap_unlock(old_ptep, old_ptl); + pte_unmap(old_ptep); err = -EAGAIN; goto out; } - if (new_ptl != old_ptl) - spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING); + + double_pt_lock(old_ptl, new_ptl); + if (unlikely(!pmd_same(old_pmdval, pmdp_get_lockless(old_pmd)) || + !pmd_same(new_pmdval, pmdp_get_lockless(new_pmd)))) { + double_pt_unlock(old_ptl, new_ptl); + pte_unmap(new_ptep); + pte_unmap(old_ptep); + err = -EAGAIN; + goto out; + } + flush_tlb_batched_pending(vma->vm_mm); lazy_mmu_mode_enable(); @@ -317,10 +316,9 @@ static int move_ptes(struct pagetable_move_control *pmc, lazy_mmu_mode_disable(); if (force_flush) flush_tlb_range(vma, old_end - len, old_end); - if (new_ptl != old_ptl) - spin_unlock(new_ptl); + double_pt_unlock(old_ptl, new_ptl); pte_unmap(new_ptep - 1); - pte_unmap_unlock(old_ptep - 1, old_ptl); + pte_unmap(old_ptep - 1); out: if (pmc->need_rmap_locks) drop_rmap_locks(vma); @@ -395,14 +393,9 @@ static bool move_normal_pmd(struct pagetable_move_control *pmc, if (WARN_ON_ONCE(!pmd_none(*new_pmd))) return false; - /* - * We don't have to worry about the ordering of src and dst - * ptlocks because exclusive mmap_lock prevents deadlock. - */ - old_ptl = pmd_lock(mm, old_pmd); + old_ptl = pmd_lockptr(mm, old_pmd); new_ptl = pmd_lockptr(mm, new_pmd); - if (new_ptl != old_ptl) - spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING); + double_pt_lock(old_ptl, new_ptl); pmd = *old_pmd; @@ -418,9 +411,7 @@ static bool move_normal_pmd(struct pagetable_move_control *pmc, pmd_populate(mm, new_pmd, pmd_pgtable(pmd)); flush_tlb_range(vma, pmc->old_addr, pmc->old_addr + PMD_SIZE); out_unlock: - if (new_ptl != old_ptl) - spin_unlock(new_ptl); - spin_unlock(old_ptl); + double_pt_unlock(old_ptl, new_ptl); return res; } @@ -452,14 +443,10 @@ static bool move_normal_pud(struct pagetable_move_control *pmc, if (WARN_ON_ONCE(!pud_none(*new_pud))) return false; - /* - * We don't have to worry about the ordering of src and dst - * ptlocks because exclusive mmap_lock prevents deadlock. - */ - old_ptl = pud_lock(mm, old_pud); + old_ptl = pud_lockptr(mm, old_pud); new_ptl = pud_lockptr(mm, new_pud); - if (new_ptl != old_ptl) - spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING); + + double_pt_lock(old_ptl, new_ptl); pud = *old_pud; if (unlikely(pud_none(pud))) @@ -473,9 +460,7 @@ static bool move_normal_pud(struct pagetable_move_control *pmc, pud_populate(mm, new_pud, pud_pgtable(pud)); flush_tlb_range(vma, pmc->old_addr, pmc->old_addr + PUD_SIZE); out_unlock: - if (new_ptl != old_ptl) - spin_unlock(new_ptl); - spin_unlock(old_ptl); + double_pt_unlock(old_ptl, new_ptl); return true; } @@ -503,16 +488,14 @@ static bool move_huge_pud(struct pagetable_move_control *pmc, if (WARN_ON_ONCE(!pud_none(*new_pud))) return false; - /* - * We don't have to worry about the ordering of src and dst - * ptlocks because exclusive mmap_lock prevents deadlock. - */ - old_ptl = __pud_trans_huge_lock(old_pud, vma); - if (unlikely(!old_ptl)) - return false; + old_ptl = pud_lockptr(mm, old_pud); new_ptl = pud_lockptr(mm, new_pud); - if (new_ptl != old_ptl) - spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING); + + double_pt_lock(old_ptl, new_ptl); + if (unlikely(!pud_trans_huge(*old_pud))) { + double_pt_unlock(old_ptl, new_ptl); + return false; + } /* Clear the pud */ pud = pudp_huge_get_and_clear(mm, pmc->old_addr, old_pud); @@ -523,9 +506,7 @@ static bool move_huge_pud(struct pagetable_move_control *pmc, /* mark soft_ditry when we add pud level soft dirty support */ set_pud_at(mm, pmc->new_addr, new_pud, pud); flush_pud_tlb_range(vma, pmc->old_addr, pmc->old_addr + HPAGE_PUD_SIZE); - if (new_ptl != old_ptl) - spin_unlock(new_ptl); - spin_unlock(old_ptl); + double_pt_unlock(old_ptl, new_ptl); return true; } diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index dac696bda19f..169f681f9849 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -1206,33 +1206,6 @@ int mrwprotect_range(struct userfaultfd_ctx *ctx, unsigned long start, return 0; } -void double_pt_lock(spinlock_t *ptl1, - spinlock_t *ptl2) - __acquires(ptl1) - __acquires(ptl2) -{ - if (ptl1 > ptl2) - swap(ptl1, ptl2); - /* lock in virtual address order to avoid lock inversion */ - spin_lock(ptl1); - if (ptl1 != ptl2) - spin_lock_nested(ptl2, SINGLE_DEPTH_NESTING); - else - __acquire(ptl2); -} - -void double_pt_unlock(spinlock_t *ptl1, - spinlock_t *ptl2) - __releases(ptl1) - __releases(ptl2) -{ - spin_unlock(ptl1); - if (ptl1 != ptl2) - spin_unlock(ptl2); - else - __release(ptl2); -} - static inline bool is_pte_pages_stable(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)