CVE-2025-39992 mm: swap: check for stable address space before operating on the VMA
A race condition in the Linux kernel swap subsystem causes a null pointer dereference, leading to a system crash.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2025-39992?
CVE-2025-39992 is a vulnerability in Linux, published on 15 Oct 2025. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
mm: swap: check for stable address space before operating on the VMA
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.
Which versions are affected by CVE-2025-39992?
How do I fix CVE-2025-39992?
Apply the vendor's patch.
Is CVE-2025-39992 being exploited?
There is no known exploitation of CVE-2025-39992 in the wild and no public exploit code.
Sources
Also known as BELL-CVE-2025-39992, DEBIAN-CVE-2025-39992, GHSA-fcm8-3qw6-4427, ROOT-OS-DEBIAN-13-CVE-2025-39992, ROOT-OS-UBUNTU-2204-CVE-2025-39992, ROOT-OS-UBUNTU-2404-CVE-2025-39992 and 1 more.