Re: perf: bug, kernel ignores the buffer size on large read

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Sep 02, 2015 at 12:34:41PM -0400, Vince Weaver wrote:
> 
> OK, this time I found the actual bug.
> 
> event->read_size is declared as a u16 in include/linux/perf_event.h
> 
> but it is very easy to get event->read_size larger than 64k
> (in my case, create 10000 events in a group).
> 
> Because we wrap around the u16, the 
> 	 if (count < event->read_size) return -ENOSPC;
> in perf_read_hw() doesn't trigger reliably and so if you do a read
> on a large group event the kernel will quite happily copy_to_user()
> beyond the bounds of the value set in the read syscall.  
> 
> In my case it completely smashed the stack and caused the program to 
> segfault.
> 
> I'm not sure what the solution is here.  Change read_size to be larger?
> Ban events whose read size would be larger than 64k?  Although that gets 
> tricky because the related header_size is also only a u16.

I think we should try and just ban events with a read_size > 64k; that's
one _large_ group -- there's other issues with that as well I imagine.

If we really want to allow something larger, we _could_ fudge something
for cases where we do not have PERF_SAMPLE_READ set, but that's not
particularly nice either.