Re: perf_fuzzer compiled for x32 causes reboot

[Vince Weaver <vincent.weaver@maine.edu>]

I spent some more time on this.
I managed to get a trace that exhibited the bug practically right 
away, but still unable to generate a reproducible trace :(  

So instead I'm adding WARN's and trace_printks to see what I can find out.

Here's a summary of what I think is happneing.  Please let me know if I'm 
wildly wrong in analyzing this.



Userspace accesses the perf 
ring-buffer-user-page of an event for the first time (in this trace
it's a SW_CPU_CLOCK event).

This triggers a page_fault to bring in the page.

This triggers a TRACEPOINT event (task/task_newtask) which
has PERF_SAMPLE_IP|PERF_SAMPLE_TID|PERF_SAMPLE_TIME|PERF_SAMPLE_CALLCHAIN|PERF_SAMPLE_ID|PERF_SAMPLE_CPU|PERF_SAMPLE_PERIOD|PERF_SAMPLE_STREAM_ID|PERF_SAMPLE_WEIGHT|PERF_SAMPLE_DATA_SRC|PERF_SAMPLE_TRANSACTION;
and exclude_callchain_kernel=1, triggering a perf_callchain().

Here is a dump of the stack right after we enter perf_callchain()

vince@core2:~$ [  202.320444] ------------[ cut here ]------------
[  202.324001] WARNING: CPU: 1 PID: 2873 at kernel/events/callchain.c:168 perf_callchain+0x67/0x211()
hermal_sys ehci_pci ehci_hcd sg sd_mod usbcore usb_common
[  202.324001] CPU: 1 PID: 2873 Comm: perf_fuzzer Not tainted 3.14.0-rc3+ #24
[  202.324001] Hardware name: AOpen   DE7000/nMCP7ALPx-DE R1.06 Oct.19.2012, BIOS 080015  10/19/2012
[  202.324001]  00000000000000a8 ffff880119ec1b48 ffffffff81531dc1 00000000000000a8
[  202.324001]  0000000000000000 ffff880119ec1b88 ffffffff81040ce4 ffff880119ec1b88
[  202.324001]  ffffffff810d416f ffff880119ec1e38 ffff8800cba67800 0000000000000000
[  202.324001] Call Trace:
[  202.324001]  [<ffffffff81531dc1>] dump_stack+0x49/0x60
[  202.324001]  [<ffffffff81040ce4>] warn_slowpath_common+0x81/0x9b
[  202.324001]  [<ffffffff810d416f>] ? perf_callchain+0x67/0x211
[  202.324001]  [<ffffffff81040d18>] warn_slowpath_null+0x1a/0x1c
[  202.324001]  [<ffffffff810d416f>] perf_callchain+0x67/0x211
[  202.324001]  [<ffffffff8106b0ab>] ? local_clock+0x1b/0x24
[  202.324001]  [<ffffffff810d0872>] perf_prepare_sample+0x7b/0x304
[  202.324001]  [<ffffffff810d1079>] __perf_event_overflow+0x156/0x1c1
[  202.324001]  [<ffffffff810f4613>] ? free_pgtables+0xa7/0xc9
[  202.324001]  [<ffffffff810d125e>] perf_swevent_overflow+0x41/0x5b
[  202.324001]  [<ffffffff810d12ea>] perf_swevent_event+0x72/0x74
[  202.324001]  [<ffffffff810d1478>] perf_tp_event+0xea/0x1ef
[  202.324001]  [<ffffffff8103515a>] ? perf_trace_x86_exceptions+0x4c/0xba
[  202.324001]  [<ffffffff810351b6>] perf_trace_x86_exceptions+0xa8/0xba
[  202.324001]  [<ffffffff8103515a>] ? perf_trace_x86_exceptions+0x4c/0xba
[  202.324001]  [<ffffffff81538439>] trace_do_page_fault+0x48/0x99
[  202.324001]  [<ffffffff815354b2>] trace_page_fault+0x22/0x30
[  202.324001] ---[ end trace 31ccd31b4e82cb42 ]---

This triggers a kernel pagefault because the BP register is not valid and
copy_from_user_nmi() tries to copy the user stack area from there.

This *should* be OK?  And it maybe looks like it works, but then

	copy_user_handle_tail()

causes another page fault again at the invaid BP register.

But then the code continues to
	perf_output_begin()


237.265689: page_fault_kernel:    address=0x17a0 ip=copy_user_handle_tail error_code=0x0
237.265689: function:                __do_page_fault
237.265689: function:                   bad_area_nosemaphore
237.265690: function:                      __bad_area_nosemaphore
237.265690: function:                         no_context
237.265690: function:                            fixup_exception
237.265690: function:                               search_exception_tables
237.265690: function:                                  search_extable
237.265691: function:             perf_output_begin
237.265692: bprint:               perf_output_begin: VMW: event type 2 config 2a st: 2c3e7

how are we back in __do_page_fault again here?

237.265692: function:             __do_page_fault
237.265692: function:                down_read_trylock
237.265692: function:                _cond_resched
237.265693: function:                find_vma
237.265693: function:                bad_area
237.265693: function:                   up_read
237.265693: function:                   __bad_area_nosemaphore
237.265694: function:                      is_prefetch
237.265694: function:                         convert_ip_to_linear
237.265695: function:                      unhandled_signal
237.265695: function:                      __printk_ratelimit
237.265695: function:             _raw_spin_trylock
237.265695: function:             _raw_spin_unlock_irqrestore
237.265696: function:                      printk
237.265696: function:                         vprintk_emit

[  202.877004] perf_fuzzer[2873]: segfault at 17a0 ip 00000000004017fd sp 00000000ffd19d10 error 6 in perf_fuzzer[400000+d1000]


Vince