# CVE-2022-49783: x86/fpu: Drop fpregs lock before inheriting FPU permissions

Status: published. Published: 2025-05-01. Updated: 2026-11-05.

In the Linux kernel, the following vulnerability has been resolved:

x86/fpu: Drop fpregs lock before inheriting FPU permissions

Mike Galbraith reported the following against an old fork of preempt-rt
but the same issue also applies to the current preempt-rt tree.

   BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:46
   in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 1, name: systemd
   preempt_count: 1, expected: 0
   RCU nest depth: 0, expected: 0
   Preemption disabled at:
   fpu_clone
   CPU: 6 PID: 1 Comm: systemd Tainted: G            E       (unreleased)
   Call Trace:
    <TASK>
    dump_stack_lvl
    ? fpu_clone
    __might_resched
    rt_spin_lock
    fpu_clone
    ? copy_thread
    ? copy_process
    ? shmem_alloc_inode
    ? kmem_cache_alloc
    ? kernel_clone
    ? __do_sys_clone
    ? do_syscall_64
    ? __x64_sys_rt_sigprocmask
    ? syscall_exit_to_user_mode
    ? do_syscall_64
    ? syscall_exit_to_user_mode
    ? do_syscall_64
    ? syscall_exit_to_user_mode
    ? do_syscall_64
    ? exc_page_fault
    ? entry_SYSCALL_64_after_hwframe
    </TASK>

Mike says:

  The splat comes from fpu_inherit_perms() being called under fpregs_lock(),
  and us reaching the spin_lock_irq() therein due to fpu_state_size_dynamic()
  returning true despite static key __fpu_state_size_dynamic having never
  been enabled.

Mike's assessment looks correct. fpregs_lock on a PREEMPT_RT kernel disables
preemption so calling spin_lock_irq() in fpu_inherit_perms() is unsafe. This
problem exists since commit

  9e798e9aa14c ("x86/fpu: Prepare fpu_clone() for dynamically enabled features").

Even though the original bug report should not have enabled the paths at
all, the bug still exists.

fpregs_lock is necessary when editing the FPU registers or a task's FP
state but it is not necessary for fpu_inherit_perms(). The only write
of any FP state in fpu_inherit_perms() is for the new child which is
not running yet and cannot context switch or be borrowed by a kernel
thread yet. Hence, fpregs_lock is not protecting anything in the new
child until clone() completes and can be dropped earlier. The siglock
still needs to be acquired by fpu_inherit_perms() as the read of the
parent's permissions has to be serialised.

  [ bp: Cleanup splat. ]

## Remediation

No remediation reported in collected sources

## Affected products and versions

- Linux / Linux: ≥ 9e798e9aa14c45fb94e47b30bf6347b369ce9df7; < c6e8a7a1780af3da65e78a615f7d0874da6aabb0 (affected, git); ≥ 9e798e9aa14c45fb94e47b30bf6347b369ce9df7; < 36b038791e1e2baea892e9276588815fd14894b4 (affected, git). Source: CNA: Linux
- Linux / Linux: 5.16 (affected); ≥ 0; < 5.16 (unaffected, semver); ≥ 6.0.10; ≤ 6.0.* (unaffected, semver); ≥ 6.1; ≤ * (unaffected, original_commit_for_fix). Source: CNA: Linux

## Severity and prioritization

- CISA KEV: No listing in collected sources

- EPSS: 0.17% as of 2026-11-05; percentile 6.47%

- SSVC: exploitation none; automatable no; technical impact partial; source ADP: CISA-ADP

- Weaknesses: Not reported

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://git.kernel.org/stable/c/c6e8a7a1780af3da65e78a615f7d0874da6aabb0 (CNA: Linux)

- https://git.kernel.org/stable/c/36b038791e1e2baea892e9276588815fd14894b4 (CNA: Linux)

## Provenance

- CNA: Linux: https://www.cve.org/CVERecord?id=CVE-2022-49783; retrieved 2026-11-05; source updated 2026-11-05

- FIRST EPSS: https://api.first.org/data/v1/epss?cve=CVE-2022-49783; retrieved 2026-11-05; source updated Not reported

Revision: 6e49e02c24ae9cf3c33782cdddd6f5f47ce8f8b7d43852e9564d5073b2ef0024

Canonical: https://y2.dev/cve/CVE-2022-49783

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