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    Subject[PATCH 5.17 049/158] perf: Fix sys_perf_event_open() race against self
    Date
    From: Peter Zijlstra <peterz@infradead.org>

    commit 3ac6487e584a1eb54071dbe1212e05b884136704 upstream.

    Norbert reported that it's possible to race sys_perf_event_open() such
    that the looser ends up in another context from the group leader,
    triggering many WARNs.

    The move_group case checks for races against itself, but the
    !move_group case doesn't, seemingly relying on the previous
    group_leader->ctx == ctx check. However, that check is racy due to not
    holding any locks at that time.

    Therefore, re-check the result after acquiring locks and bailing
    if they no longer match.

    Additionally, clarify the not_move_group case from the
    move_group-vs-move_group race.

    Fixes: f63a8daa5812 ("perf: Fix event->ctx locking")
    Reported-by: Norbert Slusarek <nslusarek@gmx.net>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    kernel/events/core.c | 14 ++++++++++++++
    1 file changed, 14 insertions(+)

    --- a/kernel/events/core.c
    +++ b/kernel/events/core.c
    @@ -12327,6 +12327,9 @@ SYSCALL_DEFINE5(perf_event_open,
    * Do not allow to attach to a group in a different task
    * or CPU context. If we're moving SW events, we'll fix
    * this up later, so allow that.
    + *
    + * Racy, not holding group_leader->ctx->mutex, see comment with
    + * perf_event_ctx_lock().
    */
    if (!move_group && group_leader->ctx != ctx)
    goto err_context;
    @@ -12392,6 +12395,7 @@ SYSCALL_DEFINE5(perf_event_open,
    } else {
    perf_event_ctx_unlock(group_leader, gctx);
    move_group = 0;
    + goto not_move_group;
    }
    }

    @@ -12408,7 +12412,17 @@ SYSCALL_DEFINE5(perf_event_open,
    }
    } else {
    mutex_lock(&ctx->mutex);
    +
    + /*
    + * Now that we hold ctx->lock, (re)validate group_leader->ctx == ctx,
    + * see the group_leader && !move_group test earlier.
    + */
    + if (group_leader && group_leader->ctx != ctx) {
    + err = -EINVAL;
    + goto err_locked;
    + }
    }
    +not_move_group:

    if (ctx->task == TASK_TOMBSTONE) {
    err = -ESRCH;

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