CVE-2021-46939tracing: Restructure trace_clock_global() to never block

Official description

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

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/91ca6f6a91f679c8645d7f3307e03ce86ad518c4
• https://git.kernel.org/stable/c/859b47a43f5a0e5b9a92b621dc6ceaad39fb5c8b
• https://git.kernel.org/stable/c/1fca00920327be96f3318224f502e4d5460f9545
• https://git.kernel.org/stable/c/d43d56dbf452ccecc1ec735cd4b6840118005d7c
• https://git.kernel.org/stable/c/c64da3294a7d59a4bf6874c664c13be892f15f44
• https://git.kernel.org/stable/c/a33614d52e97fc8077eb0b292189ca7d964cc534
• https://git.kernel.org/stable/c/6e2418576228eeb12e7ba82edb8f9500623942ff
• https://git.kernel.org/stable/c/2a1bd74b8186d7938bf004f5603f25b84785f63e
• https://git.kernel.org/stable/c/aafe104aa9096827a429bc1358f8260ee565b7cc