[PATCH 1/2] coresight: tmc-etf: Fix NULL ptr dereference in tmc_enable_etf_sink_perf()

[Sai Prakash Ranjan]

Hi Suzuki,

On 2020-10-13 22:05, Suzuki K Poulose wrote:
> On 10/07/2020 02:00 PM, Sai Prakash Ranjan wrote:
>> There was a report of NULL pointer dereference in ETF enable
>> path for perf CS mode with PID monitoring. It is almost 100%
>> reproducible when the process to monitor is something very
>> active such as chrome and with ETF as the sink and not ETR.
>> Currently in a bid to find the pid, the owner is dereferenced
>> via task_pid_nr() call in tmc_enable_etf_sink_perf() and with
>> owner being NULL, we get a NULL pointer dereference.
>> 
>> Looking at the ETR and other places in the kernel, ETF and the
>> ETB are the only places trying to dereference the task(owner)
>> in tmc_enable_etf_sink_perf() which is also called from the
>> sched_in path as in the call trace. Owner(task) is NULL even
>> in the case of ETR in tmc_enable_etr_sink_perf(), but since we
>> cache the PID in alloc_buffer() callback and it is done as part
>> of etm_setup_aux() when allocating buffer for ETR sink, we never
>> dereference this NULL pointer and we are safe. So lets do the
> 
> The patch is necessary to fix some of the issues. But I feel it is
> not complete. Why is it safe earlier and not later ? I believe we are
> simply reducing the chances of hitting the issue, by doing this earlier 
> than
> later.

I did stress it for a long time with this patch and did not face
any issues but I guess it doesn't hurt to have the check as you
suggested.

> I would say we better fix all instances to make sure that the
> event->owner is valid. (e.g, I can see that the for kernel events
> event->owner == -1 ?)
> 
> struct task_struct *tsk = READ_ONCE(event->owner);
> 
> if (!tsk || is_kernel_event(event))
>    /* skip ? */
> 

So to confirm my understanding, I will add the above checks on top
of this patch for ETR, ETB and ETF something like below?

diff --git a/drivers/hwtracing/coresight/coresight-tmc-etf.c 
b/drivers/hwtracing/coresight/coresight-tmc-etf.c
index 989d965f3d90..86ff0dda0444 100644
--- a/drivers/hwtracing/coresight/coresight-tmc-etf.c
+++ b/drivers/hwtracing/coresight/coresight-tmc-etf.c
@@ -392,6 +392,10 @@ static void *tmc_alloc_etf_buffer(struct 
coresight_device *csdev,
  {
         int node;
         struct cs_buffers *buf;
+       struct task_struct *task = READ_ONCE(event->owner);
+
+       if (!task || is_kernel_event(event))
+               return NULL;

         node = (event->cpu == -1) ? NUMA_NO_NODE : 
cpu_to_node(event->cpu);

@@ -400,7 +404,7 @@ static void *tmc_alloc_etf_buffer(struct 
coresight_device *csdev,
         if (!buf)
                 return NULL;

-       buf->pid = task_pid_nr(event->owner);
+       buf->pid = task_pid_nr(task);
         buf->snapshot = overwrite;
         buf->nr_pages = nr_pages;
         buf->data_pages = pages;


Thanks,
Sai
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