CVE-2021-46939

In the Linux kernel, the following vulnerability has been resolved:
tracing: Restructure trace_clock_global() to never block It was reported
that a fix to the ring buffer recursion detection would cause a hung
machine when performing suspend / resume testing. The following backtrace
was extracted from debugging that case: Call Trace:
trace_clock_global+0x91/0xa0 __rb_reserve_next+0x237/0x460
ring_buffer_lock_reserve+0x12a/0x3f0 trace_buffer_lock_reserve+0x10/0x50
__trace_graph_return+0x1f/0x80 trace_graph_return+0xb7/0xf0 ?
trace_clock_global+0x91/0xa0 ftrace_return_to_handler+0x8b/0xf0 ?
pv_hash+0xa0/0xa0 return_to_handler+0x15/0x30 ?
ftrace_graph_caller+0xa0/0xa0 ? trace_clock_global+0x91/0xa0 ?
__rb_reserve_next+0x237/0x460 ? ring_buffer_lock_reserve+0x12a/0x3f0 ?
trace_event_buffer_lock_reserve+0x3c/0x120 ?
trace_event_buffer_reserve+0x6b/0xc0 ?
trace_event_raw_event_device_pm_callback_start+0x125/0x2d0 ?
dpm_run_callback+0x3b/0xc0 ? pm_ops_is_empty+0x50/0x50 ?
platform_get_irq_byname_optional+0x90/0x90 ?
trace_device_pm_callback_start+0x82/0xd0 ? dpm_run_callback+0x49/0xc0 With
the following RIP: RIP: 0010:native_queued_spin_lock_slowpath+0x69/0x200
Since the fix to the recursion detection would allow a single recursion to
happen while tracing, this lead to the trace_clock_global() taking a spin
lock and then trying to take it again: ring_buffer_lock_reserve() {
trace_clock_global() { arch_spin_lock() { queued_spin_lock_slowpath() { /*
lock taken / (something else gets traced by function graph tracer)
ring_buffer_lock_reserve() { trace_clock_global() { arch_spin_lock() {
queued_spin_lock_slowpath() { / DEAD LOCK! */ Tracing should never
block, as it can lead to strange lockups like the above. Restructure the
trace_clock_global() code to instead of simply taking a lock to update the
recorded “prev_time” simply use it, as two events happening on two
different CPUs that calls this at the same time, really doesn’t matter
which one goes first. Use a trylock to grab the lock for updating the
prev_time, and if it fails, simply try again the next time. If it failed to
be taken, that means something else is already updating it. Bugzilla:
https://bugzilla.kernel.org/show_bug.cgi?id=212761