[PATCH] arm64: spectre: Avoid locking on v4 mitigation enable path

[Will Deacon]

On Wed, Feb 17, 2021 at 09:02:56AM -0800, Sami Tolvanen wrote:
> On Wed, Feb 17, 2021 at 6:26 AM Will Deacon <will at kernel.org> wrote:
> >
> > The Spectre-v4 workaround is re-configured when resuming from suspend,
> > as the firmware may have re-enabled the mitigation despite the user
> > previously asking for it to be disabled.
> >
> > Enabling or disabling the workaround can result in an undefined
> > instruction exception on CPUs which implement PSTATE.SSBS but only allow
> > it to be configured by adjusting the SPSR on exception return. We handle
> > this by installing an 'undef hook' which effectively emulates the access.
> >
> > Installing this hook requires us to take a couple of spinlocks both to
> > avoid corrupting the internal list of hooks but also to ensure that we
> > don't run into an unhandled exception. Unfortunately, when resuming from
> > suspend, we haven't yet called rcu_idle_exit() and so lockdep gets angry
> > about "suspicious RCU usage". In doing so, it tries to print a warning,
> > which leads it to get even more suspicious, this time about itself:
> >
> >  |  rcu_scheduler_active = 2, debug_locks = 1
> >  |  RCU used illegally from extended quiescent state!
> >  |  1 lock held by swapper/0:
> >  |   #0: (logbuf_lock){-.-.}-{2:2}, at: vprintk_emit+0x88/0x198
> >  |
> >  |  Call trace:
> >  |   dump_backtrace+0x0/0x1d8
> >  |   show_stack+0x18/0x24
> >  |   dump_stack+0xe0/0x17c
> >  |   lockdep_rcu_suspicious+0x11c/0x134
> >  |   trace_lock_release+0xa0/0x160
> >  |   lock_release+0x3c/0x290
> >  |   _raw_spin_unlock+0x44/0x80
> >  |   vprintk_emit+0xbc/0x198
> >  |   vprintk_default+0x44/0x6c
> >  |   vprintk_func+0x1f4/0x1fc
> >  |   printk+0x54/0x7c
> >  |   lockdep_rcu_suspicious+0x30/0x134
> >  |   trace_lock_acquire+0xa0/0x188
> >  |   lock_acquire+0x50/0x2fc
> >  |   _raw_spin_lock+0x68/0x80
> >  |   spectre_v4_enable_mitigation+0xa8/0x30c
> >  |   __cpu_suspend_exit+0xd4/0x1a8
> >  |   cpu_suspend+0xa0/0x104
> >  |   psci_cpu_suspend_enter+0x3c/0x5c
> >  |   psci_enter_idle_state+0x44/0x74
> >  |   cpuidle_enter_state+0x148/0x2f8
> >  |   cpuidle_enter+0x38/0x50
> >  |   do_idle+0x1f0/0x2b4
> >
> > Prevent these splats by avoiding locking entirely if we know that a hook
> > has already been registered for the mitigation.
> >
> > Cc: Mark Rutland <mark.rutland at arm.com>
> > Cc: Lorenzo Pieralisi <lorenzo.pieralisi at arm.com>
> > Cc: Peter Zijlstra <peterz at infradead.org>
> > Cc: Boqun Feng <boqun.feng at gmail.com>
> > Cc: Marc Zyngier <maz at kernel.org>
> > Cc: Saravana Kannan <saravanak at google.com>
> > Reported-by: Sami Tolvanen <samitolvanen at google.com>
> > Fixes: c28762070ca6 ("arm64: Rewrite Spectre-v4 mitigation code")
> > Signed-off-by: Will Deacon <will at kernel.org>
> 
> Thank you for the patch. I can confirm this fixes the warning for me.
> 
> Tested-by: Sami Tolvanen <samitolvanen at google.com>

Thanks, Sami.

If nobody has any better ideas, I'll queue this as a fix for later in the
merge window.

Will