The patch titled Subject: mm: swap: check for stable address space before operating on the VMA has been added to the -mm mm-new branch. Its filename is mm-swap-check-for-stable-address-space-before-operating-on-the-vma.patch
This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches...
This patch will later appear in the mm-new branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------ From: Charan Teja Kalla charan.kalla@oss.qualcomm.com Subject: mm: swap: check for stable address space before operating on the VMA Date: Wed, 24 Sep 2025 23:41:38 +0530
It is possible to hit a zero entry while traversing the vmas in unuse_mm() called from swapoff path and accessing it causes the OOPS:
Unable to handle kernel NULL pointer dereference at virtual address 0000000000000446--> Loading the memory from offset 0x40 on the XA_ZERO_ENTRY as address. Mem abort info: ESR = 0x0000000096000005 EC = 0x25: DABT (current EL), IL = 32 bits SET = 0, FnV = 0 EA = 0, S1PTW = 0 FSC = 0x05: level 1 translation fault
The issue is manifested from the below race between the fork() on a process and swapoff: fork(dup_mmap()) swapoff(unuse_mm) --------------- ----------------- 1) Identical mtree is built using __mt_dup().
2) copy_pte_range()--> copy_nonpresent_pte(): The dst mm is added into the mmlist to be visible to the swapoff operation.
3) Fatal signal is sent to the parent process(which is the current during the fork) thus skip the duplication of the vmas and mark the vma range with XA_ZERO_ENTRY as a marker for this process that helps during exit_mmap().
4) swapoff is tried on the 'mm' added to the 'mmlist' as part of the 2.
5) unuse_mm(), that iterates through the vma's of this 'mm' will hit the non-NULL zero entry and operating on this zero entry as a vma is resulting into the oops.
The proper fix would be around not exposing this partially-valid tree to others when droping the mmap lock, which is being solved with [1]. A simpler solution would be checking for MMF_UNSTABLE, as it is set if mm_struct is not fully initialized in dup_mmap().
Thanks to Liam/Lorenzo/David for all the suggestions in fixing this issue.
Link: https://lkml.kernel.org/r/20250924181138.1762750-1-charan.kalla@oss.qualcomm... Link: https://lore.kernel.org/all/20250815191031.3769540-1-Liam.Howlett@oracle.com... [1] Fixes: d24062914837 ("fork: use __mt_dup() to duplicate maple tree in dup_mmap()") Signed-off-by: Charan Teja Kalla charan.kalla@oss.qualcomm.com Suggested-by: David Hildenbrand david@redhat.com Cc: Baoquan He bhe@redhat.com Cc: Barry Song baohua@kernel.org Cc: Chris Li chrisl@kernel.org Cc: Kairui Song kasong@tencent.com Cc: Kemeng Shi shikemeng@huaweicloud.com Cc: Liam Howlett liam.howlett@oracle.com Cc: Lorenzo Stoakes lorenzo.stoakes@oracle.com Cc: Nhat Pham nphamcs@gmail.com Cc: Peng Zhang zhangpeng.00@bytedance.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---
mm/swapfile.c | 3 +++ 1 file changed, 3 insertions(+)
--- a/mm/swapfile.c~mm-swap-check-for-stable-address-space-before-operating-on-the-vma +++ a/mm/swapfile.c @@ -2389,6 +2389,8 @@ static int unuse_mm(struct mm_struct *mm VMA_ITERATOR(vmi, mm, 0);
mmap_read_lock(mm); + if (check_stable_address_space(mm)) + goto unlock; for_each_vma(vmi, vma) { if (vma->anon_vma && !is_vm_hugetlb_page(vma)) { ret = unuse_vma(vma, type); @@ -2398,6 +2400,7 @@ static int unuse_mm(struct mm_struct *mm
cond_resched(); } +unlock: mmap_read_unlock(mm); return ret; } _
Patches currently in -mm which might be from charan.kalla@oss.qualcomm.com are
mm-swap-check-for-stable-address-space-before-operating-on-the-vma.patch