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

From: Greg Kroah-Hartman <[email-masked]>

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

The Linux kernel CVE team has assigned CVE-2021-46939 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 2.6.30 with commit 14131f2f98ac and fixed in 4.4.269 with commit 91ca6f6a91f6
	Issue introduced in 2.6.30 with commit 14131f2f98ac and fixed in 4.9.269 with commit 859b47a43f5a
	Issue introduced in 2.6.30 with commit 14131f2f98ac and fixed in 4.14.233 with commit 1fca00920327
	Issue introduced in 2.6.30 with commit 14131f2f98ac and fixed in 4.19.191 with commit d43d56dbf452
	Issue introduced in 2.6.30 with commit 14131f2f98ac and fixed in 5.4.118 with commit c64da3294a7d
	Issue introduced in 2.6.30 with commit 14131f2f98ac and fixed in 5.10.36 with commit a33614d52e97
	Issue introduced in 2.6.30 with commit 14131f2f98ac and fixed in 5.11.20 with commit 6e2418576228
	Issue introduced in 2.6.30 with commit 14131f2f98ac and fixed in 5.12.3 with commit 2a1bd74b8186
	Issue introduced in 2.6.30 with commit 14131f2f98ac and fixed in 5.13 with commit aafe104aa909

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2021-46939
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	kernel/trace/trace_clock.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	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