perf/core: Fix locking for children siblings group read
We're missing ctx lock when iterating children siblings within the perf_read path for group reading. Following race and crash can happen: User space doing read syscall on event group leader: T1: perf_read lock event->ctx->mutex perf_read_group lock leader->child_mutex __perf_read_group_add(child) list_for_each_entry(sub, &leader->sibling_list, group_entry) ----> sub might be invalid at this point, because it could get removed via perf_event_exit_task_context in T2 Child exiting and cleaning up its events: T2: perf_event_exit_task_context lock ctx->mutex list_for_each_entry_safe(child_event, next, &child_ctx->event_list,... perf_event_exit_event(child) lock ctx->lock perf_group_detach(child) unlock ctx->lock ----> child is removed from sibling_list without any sync with T1 path above ... free_event(child) Before the child is removed from the leader's child_list, (and thus is omitted from perf_read_group processing), we need to ensure that perf_read_group touches child's siblings under its ctx->lock. Peter further notes: | One additional note; this bug got exposed by commit: | |ba5213ae6b
("perf/core: Correct event creation with PERF_FORMAT_GROUP") | | which made it possible to actually trigger this code-path. Tested-by: Andi Kleen <ak@linux.intel.com> Signed-off-by: Jiri Olsa <jolsa@kernel.org> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com> Cc: Arnaldo Carvalho de Melo <acme@redhat.com> Cc: Jiri Olsa <jolsa@redhat.com> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Fixes:ba5213ae6b
("perf/core: Correct event creation with PERF_FORMAT_GROUP") Link: http://lkml.kernel.org/r/20170720141455.2106-1-jolsa@kernel.org Signed-off-by: Ingo Molnar <mingo@kernel.org>
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3bda69c1c3
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2aeb188354
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@ -4372,7 +4372,9 @@ EXPORT_SYMBOL_GPL(perf_event_read_value);
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static int __perf_read_group_add(struct perf_event *leader,
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u64 read_format, u64 *values)
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{
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struct perf_event_context *ctx = leader->ctx;
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struct perf_event *sub;
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unsigned long flags;
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int n = 1; /* skip @nr */
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int ret;
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@ -4402,12 +4404,15 @@ static int __perf_read_group_add(struct perf_event *leader,
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if (read_format & PERF_FORMAT_ID)
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values[n++] = primary_event_id(leader);
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raw_spin_lock_irqsave(&ctx->lock, flags);
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list_for_each_entry(sub, &leader->sibling_list, group_entry) {
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values[n++] += perf_event_count(sub);
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if (read_format & PERF_FORMAT_ID)
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values[n++] = primary_event_id(sub);
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}
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raw_spin_unlock_irqrestore(&ctx->lock, flags);
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return 0;
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}
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