On 2020-10-22 16:44, Suzuki Poulose wrote:
On 10/22/20 12:07 PM, Sai Prakash Ranjan wrote:
On 2020-10-22 14:57, Suzuki Poulose wrote:
On 10/22/20 9:02 AM, Sai Prakash Ranjan wrote:
On 2020-10-21 15:38, Suzuki Poulose wrote:
On 10/21/20 8:29 AM, Sai Prakash Ranjan wrote:
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?
Yes. Please see perf_event_create_kernel_event(), which is an exported function allowing any kernel code (including modules) to use the PMU (just like the userspace perf tool would do). Just because your use case doesn't trigger this (because you don't run something that can trigger this) doesn't mean this can't be triggered.
Thanks for that pointer, I will add them in the next version.
And instead of redefining TASK_TOMBSTONE in the driver, you may simply use IS_ERR_OR_NULL(tsk) to cover both NULL case and kernel event.
Ugh sorry, sent out v2 exporting is_kernel_event() before seeing this comment, I will resend.
Saw that. I would say, wait until someone complains about that. If people are Ok with exporting it, it is fine. I guess it will be useful. You could fall back to this approach if there is resistance.
Sure, I will wait for some comments although I hurried to tell them to ignore it :(
Thanks, Sai