Re: [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
--
QUALCOMM INDIA, on behalf of Qualcomm Innovation Center, Inc. is a 
member
of Code Aurora Forum, hosted by The Linux Foundation