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tracing: Fix trace_marker copy link list updates

When the "copy_trace_marker" option is enabled for an instance, anything
written into /sys/kernel/tracing/trace_marker is also copied into that
instances buffer. When the option is set, that instance's trace_array
descriptor is added to the marker_copies link list. This list is protected
by RCU, as all iterations uses an RCU protected list traversal.

When the instance is deleted, all the flags that were enabled are cleared.
This also clears the copy_trace_marker flag and removes the trace_array
descriptor from the list.

The issue is after the flags are called, a direct call to
update_marker_trace() is performed to clear the flag. This function
returns true if the state of the flag changed and false otherwise. If it
returns true here, synchronize_rcu() is called to make sure all readers
see that its removed from the list.

But since the flag was already cleared, the state does not change and the
synchronization is never called, leaving a possible UAF bug.

Move the clearing of all flags below the updating of the copy_trace_marker
option which then makes sure the synchronization is performed.

Also use the flag for checking the state in update_marker_trace() instead
of looking at if the list is empty.

Cc: stable@vger.kernel.org
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://patch.msgid.link/20260318185512.1b6c7db4@gandalf.local.home
Fixes: 7b382efd5e8a ("tracing: Allow the top level trace_marker to write into another instances")
Reported-by: Sasha Levin <sashal@kernel.org>
Closes: https://lore.kernel.org/all/20260225133122.237275-1-sashal@kernel.org/
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Steven Rostedt 07183aac edca33a5

+10 -9
+10 -9
kernel/trace/trace.c
··· 555 555 lockdep_assert_held(&event_mutex); 556 556 557 557 if (enabled) { 558 - if (!list_empty(&tr->marker_list)) 558 + if (tr->trace_flags & TRACE_ITER(COPY_MARKER)) 559 559 return false; 560 560 561 561 list_add_rcu(&tr->marker_list, &marker_copies); ··· 563 563 return true; 564 564 } 565 565 566 - if (list_empty(&tr->marker_list)) 566 + if (!(tr->trace_flags & TRACE_ITER(COPY_MARKER))) 567 567 return false; 568 568 569 - list_del_init(&tr->marker_list); 569 + list_del_rcu(&tr->marker_list); 570 570 tr->trace_flags &= ~TRACE_ITER(COPY_MARKER); 571 571 return true; 572 572 } ··· 9761 9761 9762 9762 list_del(&tr->list); 9763 9763 9764 + if (printk_trace == tr) 9765 + update_printk_trace(&global_trace); 9766 + 9767 + /* Must be done before disabling all the flags */ 9768 + if (update_marker_trace(tr, 0)) 9769 + synchronize_rcu(); 9770 + 9764 9771 /* Disable all the flags that were enabled coming in */ 9765 9772 for (i = 0; i < TRACE_FLAGS_MAX_SIZE; i++) { 9766 9773 if ((1ULL << i) & ZEROED_TRACE_FLAGS) 9767 9774 set_tracer_flag(tr, 1ULL << i, 0); 9768 9775 } 9769 - 9770 - if (printk_trace == tr) 9771 - update_printk_trace(&global_trace); 9772 - 9773 - if (update_marker_trace(tr, 0)) 9774 - synchronize_rcu(); 9775 9776 9776 9777 tracing_set_nop(tr); 9777 9778 clear_ftrace_function_probes(tr);