Revision 2aeb1883547626d82c597cce2c99f0b9c62e2425 authored by Jiri Olsa on 20 July 2017, 14:14:55 UTC, committed by Ingo Molnar on 21 July 2017, 07:54:23 UTC
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: | | ba5213ae6b88 ("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: ba5213ae6b88 ("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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