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

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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: syzbot+ff3aa851d46ab82953a3@syzkaller.appspotmail.com
Signed-off-by: Gabriel Shahrouzi <gshahrouzi@gmail.com>
Signed-off-by: Ingo 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
This commit is contained in:
Gabriel Shahrouzi 2025-04-05 16:30:36 -04:00 committed by Ingo Molnar
parent dda8887894
commit 0ba3a4ab76
1 changed files with 3 additions and 3 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

6
kernel/events/core.c
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@ -14016,6 +14016,9 @@ inherit_event(struct perf_event *parent_event,
if (IS_ERR(child_event)) if (IS_ERR(child_event))
return 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); pmu_ctx = find_get_pmu_context(child_event->pmu, child_ctx, child_event);
if (IS_ERR(pmu_ctx)) { if (IS_ERR(pmu_ctx)) {
free_event(child_event); free_event(child_event);
@ -14037,8 +14040,6 @@ inherit_event(struct perf_event *parent_event,
return NULL; return NULL;
} }
get_ctx(child_ctx);
/* /*
* Make the child state follow the state of the parent event, * Make the child state follow the state of the parent event,
* not its attr.disabled bit. We hold the parent's mutex, * 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); local64_set(&hwc->period_left, sample_period);
} }
child_event->ctx = child_ctx;
child_event->overflow_handler = parent_event->overflow_handler; child_event->overflow_handler = parent_event->overflow_handler;
child_event->overflow_handler_context child_event->overflow_handler_context
= parent_event->overflow_handler_context; = parent_event->overflow_handler_context;