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

On Wed, Feb 17, 2021 at 09:02:56AM -0800, Sami Tolvanen wrote:

On Wed, Feb 17, 2021 at 6:26 AM Will Deacon [off-list ref] wrote:

quoted

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@arm.com>
Cc: Lorenzo Pieralisi <redacted>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Boqun Feng <redacted>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Saravana Kannan <redacted>
Reported-by: Sami Tolvanen <samitolvanen@google.com>
Fixes: c28762070ca6 ("arm64: Rewrite Spectre-v4 mitigation code")
Signed-off-by: Will Deacon <will@kernel.org>

Thank you for the patch. I can confirm this fixes the warning for me.

Tested-by: Sami Tolvanen <samitolvanen@google.com>

Thanks, Sami.

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

Will

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