🛡️ CVE-2025-38100 — debian-linux

x86/iopl: Cure TIF_IO_BITMAP inconsistencies

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.