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    Subject[PATCH 5.9 261/391] tracing: Fix race in trace_open and buffer resize call
    Date
    From: Gaurav Kohli <gkohli@codeaurora.org>

    commit bbeb97464eefc65f506084fd9f18f21653e01137 upstream.

    Below race can come, if trace_open and resize of
    cpu buffer is running parallely on different cpus
    CPUX CPUY
    ring_buffer_resize
    atomic_read(&buffer->resize_disabled)
    tracing_open
    tracing_reset_online_cpus
    ring_buffer_reset_cpu
    rb_reset_cpu
    rb_update_pages
    remove/insert pages
    resetting pointer

    This race can cause data abort or some times infinte loop in
    rb_remove_pages and rb_insert_pages while checking pages
    for sanity.

    Take buffer lock to fix this.

    Link: https://lkml.kernel.org/r/1601976833-24377-1-git-send-email-gkohli@codeaurora.org

    Cc: stable@vger.kernel.org
    Fixes: b23d7a5f4a07a ("ring-buffer: speed up buffer resets by avoiding synchronize_rcu for each CPU")
    Signed-off-by: Gaurav Kohli <gkohli@codeaurora.org>
    Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    kernel/trace/ring_buffer.c | 10 ++++++++++
    1 file changed, 10 insertions(+)

    --- a/kernel/trace/ring_buffer.c
    +++ b/kernel/trace/ring_buffer.c
    @@ -4866,6 +4866,9 @@ void ring_buffer_reset_cpu(struct trace_
    if (!cpumask_test_cpu(cpu, buffer->cpumask))
    return;

    + /* prevent another thread from changing buffer sizes */
    + mutex_lock(&buffer->mutex);
    +
    atomic_inc(&cpu_buffer->resize_disabled);
    atomic_inc(&cpu_buffer->record_disabled);

    @@ -4876,6 +4879,8 @@ void ring_buffer_reset_cpu(struct trace_

    atomic_dec(&cpu_buffer->record_disabled);
    atomic_dec(&cpu_buffer->resize_disabled);
    +
    + mutex_unlock(&buffer->mutex);
    }
    EXPORT_SYMBOL_GPL(ring_buffer_reset_cpu);

    @@ -4889,6 +4894,9 @@ void ring_buffer_reset_online_cpus(struc
    struct ring_buffer_per_cpu *cpu_buffer;
    int cpu;

    + /* prevent another thread from changing buffer sizes */
    + mutex_lock(&buffer->mutex);
    +
    for_each_online_buffer_cpu(buffer, cpu) {
    cpu_buffer = buffer->buffers[cpu];

    @@ -4907,6 +4915,8 @@ void ring_buffer_reset_online_cpus(struc
    atomic_dec(&cpu_buffer->record_disabled);
    atomic_dec(&cpu_buffer->resize_disabled);
    }
    +
    + mutex_unlock(&buffer->mutex);
    }

    /**

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