[RFC PATCH] x86: Drop fpregs lock before inheriting FPU permissions during clone

[Mel Gorman <mgorman@techsingularity.net>]

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

   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+0xfa/0x480
   CPU: 6 PID: 1 Comm: systemd Tainted: G            E       (unreleased)
   Call Trace:
    <TASK>
    dump_stack_lvl+0x45/0x5b
    ? fpu_clone+0xfa/0x480
    __might_resched+0x165/0x200
    rt_spin_lock+0x2d/0x70
    fpu_clone+0x32a/0x480
    ? copy_thread+0xef/0x270
    ? copy_process+0xd2c/0x1c00
    ? shmem_alloc_inode+0x16/0x30
    ? kmem_cache_alloc+0x120/0x2a0
    ? kernel_clone+0x9b/0x460
    ? __do_sys_clone+0x72/0xa0
    ? do_syscall_64+0x58/0x80
    ? __x64_sys_rt_sigprocmask+0x93/0xd0
    ? syscall_exit_to_user_mode+0x18/0x40
    ? do_syscall_64+0x67/0x80
    ? syscall_exit_to_user_mode+0x18/0x40
    ? do_syscall_64+0x67/0x80
    ? syscall_exit_to_user_mode+0x18/0x40
    ? do_syscall_64+0x67/0x80
    ? exc_page_fault+0x6a/0x190
    ? entry_SYSCALL_64_after_hwframe+0x61/0xcb
    </TASK>

  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 PREEMPT_RT disables
preemption only so the spin_lock_irq() in fpu_inherit_perms is unsafe
and converting siglock to raw spinlock would be an unwelcome change.
This problem exists since commit 9e798e9aa14c ("x86/fpu: Prepare fpu_clone()
for dynamically enabled features"). While the bug triggering is probably a
mistake for the affected machine and due to a bug that is not in mainline,
spin_lock_irq within a preempt_disable section on PREEMPT_RT is problematic.

In this specific context, it may not be necessary to hold fpregs_lock at
all. The lock is necessary when editing the FPU registers or a tasks fpstate
but in this case, the only write of any FP state in fpu_inherit_perms is
for the new child which is not running yet so it 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. The siglock still needs
to be acquired by fpu_inherit_perms as the read of the parents permissions
has to be serialised.

This is not tested as I did not access to a machine with Intel's
eXtended Feature Disable (XFD) feature that enables the relevant path
in fpu_inherit_perms and the bug is against a non-mainline kernel.

Reported-by: Mike Galbraith <efault@gmx.de>
Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
---
 arch/x86/kernel/fpu/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/fpu/core.c b/arch/x86/kernel/fpu/core.c
index 3b28c5b25e12..d00db56a8868 100644
--- a/arch/x86/kernel/fpu/core.c
+++ b/arch/x86/kernel/fpu/core.c
@@ -605,9 +605,9 @@ int fpu_clone(struct task_struct *dst, unsigned long clone_flags, bool minimal)
 	if (test_thread_flag(TIF_NEED_FPU_LOAD))
 		fpregs_restore_userregs();
 	save_fpregs_to_fpstate(dst_fpu);
+	fpregs_unlock();
 	if (!(clone_flags & CLONE_THREAD))
 		fpu_inherit_perms(dst_fpu);
-	fpregs_unlock();
 
 	/*
 	 * Children never inherit PASID state.