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:
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().
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:
Prevent io_bitmap_exit() to invoke task_update_io_bitmap() if the to be cleaned up task is not the current task.
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.
| Software | From | Fixed in |
|---|---|---|
| linux / linux_kernel | 5.5 | 5.10.239 |
| linux / linux_kernel | 5.11 | 5.15.186 |
| linux / linux_kernel | 5.16 | 6.1.142 |
| linux / linux_kernel | 6.2 | 6.6.94 |
| linux / linux_kernel | 6.7 | 6.12.34 |
| linux / linux_kernel | 6.13 | 6.15.3 |
| debian / debian_linux | 11.0 | 11.0.x |
A security vulnerability is a weakness in software, hardware, or configuration that can be exploited to compromise confidentiality, integrity, or availability. Many vulnerabilities are tracked as CVEs (Common Vulnerabilities and Exposures), which provide a standardized identifier so teams can coordinate patching, mitigation, and risk assessment across tools and vendors.
CVSS (Common Vulnerability Scoring System) estimates technical severity, but it doesn't automatically equal business risk. Prioritize using context like internet exposure, affected asset criticality, known exploitation (proof-of-concept or in-the-wild), and whether compensating controls exist. A "Medium" CVSS on an exposed, production system can be more urgent than a "Critical" on an isolated, non-production host.
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