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

[Mel Gorman <mgorman@techsingularity.net>]

On Wed, Nov 09, 2022 at 05:25:47PM +0100, Thomas Gleixner wrote:
> On Wed, Nov 09 2022 at 11:30, Mel Gorman wrote:
> >    BUG: sleeping function called from invalid context at kernel/locking/spinlock_rt.c:46
> ...
> >   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.
> 
> That's correct and siglock is the real protection for the permissions.
> 
> > 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.
> 
> It's still entirely correct on mainline as there is no requirement to
> hold fpregs_lock in this case
> 
> > Reported-by: Mike Galbraith <efault@gmx.de>
> > Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> 
> Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

Perfect, I'll rephase the changelog slightly and resend without RFC and
all the x86 maintainers cc'd. Thanks Thomas!


-- 
Mel Gorman
SUSE Labs