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. [1] https://lore.kernel.org/all/20250815191031.3769540-1-Liam.Howlett@oracle.com/ Fixes: d24062914837 ("fork: use __mt_dup() to duplicate maple tree in dup_mmap()") Suggested-by: David Hildenbrand Signed-off-by: Charan Teja Kalla --- V1: -- Checking for xa_zero_entry() instead of most cleaner way of checking for MMF_UNSTABLE -- https://lore.kernel.org/linux-mm/20250808092156.1918973-1-quic_charante@quicinc.com/ mm/swapfile.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/mm/swapfile.c b/mm/swapfile.c index 890b410d77b6..10760240a3a2 100644 --- a/mm/swapfile.c +++ b/mm/swapfile.c @@ -2389,6 +2389,8 @@ static int unuse_mm(struct mm_struct *mm, unsigned int type) 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, unsigned int type) cond_resched(); } +unlock: mmap_read_unlock(mm); return ret; } -- 2.34.1