Messages in this thread Patch in this message | | | Date | Wed, 21 Oct 2020 12:59:35 +0530 | From | Sai Prakash Ranjan <> | Subject | Re: [PATCH 1/2] coresight: tmc-etf: Fix NULL ptr dereference in tmc_enable_etf_sink_perf() |
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On 2020-10-20 21:40, Sai Prakash Ranjan wrote: > On 2020-10-14 21:29, Sai Prakash Ranjan wrote: >> On 2020-10-14 18:46, Suzuki K Poulose wrote: >>> On 10/14/2020 10:36 AM, Sai Prakash Ranjan wrote: >>>> 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 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 ? */ >>>>> >>>> >>>> Looking at it some more, is_kernel_event() is not exposed >>>> outside events core and probably for good reason. Why do >>>> we need to check for this and not just tsk? >>> >>> Because the event->owner could be : >>> >>> = NULL >>> = -1UL // kernel event >>> = valid. >>> >> >> Yes I understood that part, but here we were trying to >> fix the NULL pointer dereference right and hence the >> question as to why we need to check for kernel events? >> I am no expert in perf but I don't see anywhere in the >> kernel checking for is_kernel_event(), so I am a bit >> skeptical if exporting that is actually right or not. >> > > I have stress tested with the original patch many times > now, i.e., without a check for event->owner and is_kernel_event() > and didn't observe any crash. Plus on ETR where this was already > done, no crashes were reported till date and with ETF, the issue > was quickly reproducible, so I am fairly confident that this > doesn't just delay the original issue but actually fixes > it. I will run an overnight test again to confirm this. >
I ran the overnight test which collected aroung 4G data(see below), with the following small change to see if the two cases (event->owner=NULL and is_kernel_event()) are triggered with suggested changes and it didn't trigger at all. Do we still need those additional checks?
[ perf record: Captured and wrote 4677.989 MB perf.data ]
diff --git a/drivers/hwtracing/coresight/coresight-tmc-etf.c b/drivers/hwtracing/coresight/coresight-tmc-etf.c index 989d965f3d90..123c446ce585 100644 --- a/drivers/hwtracing/coresight/coresight-tmc-etf.c +++ b/drivers/hwtracing/coresight/coresight-tmc-etf.c @@ -13,6 +13,13 @@ #include "coresight-tmc.h" #include "coresight-etm-perf.h"
+#define TASK_TOMBSTONE ((void *)-1L) + +static bool is_kernel_event2(struct perf_event *event) +{ + return READ_ONCE(event->owner) == TASK_TOMBSTONE; +} + static int tmc_set_etf_buffer(struct coresight_device *csdev, struct perf_output_handle *handle);
@@ -392,6 +399,15 @@ 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) { + pr_info("**sai in task=NULL**\n"); + return NULL; + } + + if (is_kernel_event2(event)) + pr_info("**sai in is_kernel_event**\n");
node = (event->cpu == -1) ? NUMA_NO_NODE : cpu_to_node(event->cpu);
Thanks, Sai
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