Commit 0ba3a4ab authored by Gabriel Shahrouzi's avatar Gabriel Shahrouzi Committed by Ingo Molnar
Browse files

perf/core: Fix WARN_ON(!ctx) in __free_event() for partial init 



Move the get_ctx(child_ctx) call and the child_event->ctx assignment to
occur immediately after the child event is allocated. Ensure that
child_event->ctx is non-NULL before any subsequent error path within
inherit_event calls free_event(), satisfying the assumptions of the
cleanup code.

Details:

There's no clear Fixes tag, because this bug is a side-effect of
multiple interacting commits over time (up to 15 years old), not
a single regression.

The code initially incremented refcount then assigned context
immediately after the child_event was created. Later, an early
validity check for child_event was added before the
refcount/assignment. Even later, a WARN_ON_ONCE() cleanup check was
added, assuming event->ctx is valid if the pmu_ctx is valid.
The problem is that the WARN_ON_ONCE() could trigger after the initial
check passed but before child_event->ctx was assigned, violating its
precondition. The solution is to assign child_event->ctx right after
its initial validation. This ensures the context exists for any
subsequent checks or cleanup routines, resolving the WARN_ON_ONCE().

To resolve it, defer the refcount update and child_event->ctx assignment
directly after child_event->pmu_ctx is set but before checking if the
parent event is orphaned. The cleanup routine depends on
event->pmu_ctx being non-NULL before it verifies event->ctx is
non-NULL. This also maintains the author's original intent of passing
in child_ctx to find_get_pmu_context before its refcount/assignment.

[ mingo: Expanded the changelog from another email by Gabriel Shahrouzi. ]

Reported-by: default avatar <syzbot+ff3aa851d46ab82953a3@syzkaller.appspotmail.com>
Signed-off-by: default avatarGabriel Shahrouzi <gshahrouzi@gmail.com>
Signed-off-by: default avatarIngo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ravi Bangoria <ravi.bangoria@amd.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Link: https://lore.kernel.org/r/20250405203036.582721-1-gshahrouzi@gmail.com
Closes: https://syzkaller.appspot.com/bug?extid=ff3aa851d46ab82953a3
parent dda88878

kernel/events/core.c

+3 −3
Original line number Diff line number Diff line
@@ -14016,6 +14016,9 @@ inherit_event(struct perf_event *parent_event, 
	if (IS_ERR(child_event))

		return child_event;



	get_ctx(child_ctx);

	child_event->ctx = child_ctx;



	pmu_ctx = find_get_pmu_context(child_event->pmu, child_ctx, child_event);

	if (IS_ERR(pmu_ctx)) {

		free_event(child_event);
@@ -14037,8 +14040,6 @@ inherit_event(struct perf_event *parent_event, 
		return NULL;

	}



	get_ctx(child_ctx);



	/*

	 * Make the child state follow the state of the parent event,

	 * not its attr.disabled bit.  We hold the parent's mutex,
@@ -14059,7 +14060,6 @@ inherit_event(struct perf_event *parent_event, 
		local64_set(&hwc->period_left, sample_period);

	}



	child_event->ctx = child_ctx;

	child_event->overflow_handler = parent_event->overflow_handler;

	child_event->overflow_handler_context

		= parent_event->overflow_handler_context;