CVE-2021-46939

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

References

Affected packages

Git /

Affected ranges

Database specific

source

"https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2021-46939.json"

unresolved_ranges

[
    {
        "events": [
            {
                "introduced": "2.6.30"
            },
            {
                "fixed": "4.4.269"
            }
        ]
    },
    {
        "events": [
            {
                "introduced": "4.5.0"
            },
            {
                "fixed": "4.9.269"
            }
        ]
    },
    {
        "events": [
            {
                "introduced": "4.10.0"
            },
            {
                "fixed": "4.14.233"
            }
        ]
    },
    {
        "events": [
            {
                "introduced": "4.15.0"
            },
            {
                "fixed": "4.19.191"
            }
        ]
    },
    {
        "events": [
            {
                "introduced": "4.20.0"
            },
            {
                "fixed": "5.4.118"
            }
        ]
    },
    {
        "events": [
            {
                "introduced": "5.5.0"
            },
            {
                "fixed": "5.10.36"
            }
        ]
    },
    {
        "events": [
            {
                "introduced": "5.11.0"
            },
            {
                "fixed": "5.11.20"
            }
        ]
    },
    {
        "events": [
            {
                "introduced": "5.12.0"
            },
            {
                "fixed": "5.12.3"
            }
        ]
    }
]