x86/fpu: Drop fpregs lock before inheriting FPU permissions

Overview

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. ]

Affected

• Linux >= 9e798e9aa14c45fb94e47b30bf6347b369ce9df7 < c6e8a7a1780af3da65e78a615f7d0874da6aabb0
• Linux >= 9e798e9aa14c45fb94e47b30bf6347b369ce9df7 < 36b038791e1e2baea892e9276588815fd14894b4
• Linux 5.16

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.