CVE-2025-38100 x86/iopl: Cure TIF_IO_BITMAP inconsistencies
A local attacker can trigger a kernel crash via NULL pointer dereference in x86 I/O bitmap handling during failed forks or kernel thread creation.
- 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-2025-38100?
CVE-2025-38100 is a medium-severity vulnerability in Debian Debian Linux, published on 3 Jul 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:
x86/iopl: Cure TIF_IO_BITMAP inconsistencies
io_bitmap_exit() is invoked from exit_thread() when a task exists or when a fork fails. In the latter case the exit_thread() cleans up resources which were allocated during fork().
io_bitmap_exit() invokes task_update_io_bitmap(), which in turn ends up in tss_update_io_bitmap(). tss_update_io_bitmap() operates on the current task. If current has TIF_IO_BITMAP set, but no bitmap installed, tss_update_io_bitmap() crashes with a NULL pointer dereference.
There are two issues, which lead to that problem:
1) io_bitmap_exit() should not invoke task_update_io_bitmap() when the task, which is cleaned up, is not the current task. That's a clear indicator for a cleanup after a failed fork().
2) A task should not have TIF_IO_BITMAP set and neither a bitmap installed nor IOPL emulation level 3 activated.
This happens when a kernel thread is created in the context of a user space thread, which has TIF_IO_BITMAP set as the thread flags are copied and the IO bitmap pointer is cleared.
Other than in the failed fork() case this has no impact because kernel threads including IO workers never return to user space and therefore never invoke tss_update_io_bitmap().
Cure this by adding the missing cleanups and checks:
1) Prevent io_bitmap_exit() to invoke task_update_io_bitmap() if the to be cleaned up task is not the current task.
2) Clear TIF_IO_BITMAP in copy_thread() unconditionally. For user space forks it is set later, when the IO bitmap is inherited in io_bitmap_share().
For paranoia sake, add a warning into tss_update_io_bitmap() to catch the case, when that code is invoked with inconsistent state.
Weakness: CWE-476
Which versions are affected by CVE-2025-38100?
It affects 2 products.
Products include Debian Debian Linux, Linux kernel.
How do I fix CVE-2025-38100?
Apply the vendor's patch.
Is CVE-2025-38100 being exploited?
There is no known exploitation of CVE-2025-38100 in the wild and no public exploit code.
Other vulnerabilities in Debian Debian Linux
Sources
Also known as AZL-64577, BELL-CVE-2025-38100, DEBIAN-CVE-2025-38100, GHSA-hhj3-j6pg-f7mv, ROOT-OS-DEBIAN-11-CVE-2025-38100, ROOT-OS-DEBIAN-12-CVE-2025-38100 and 3 more.