Re: SIGSEGV when using "perf record -g" with 3.13-rc* kernel

[Andy Lutomirski]

On Fri, Jan 10, 2014 at 9:41 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Fri, Jan 10, 2014 at 06:02:23PM +0100, Peter Zijlstra wrote:
>> On Fri, Jan 10, 2014 at 05:58:22PM +0100, Peter Zijlstra wrote:
>> > On Fri, Jan 10, 2014 at 10:29:13AM -0500, Waiman Long wrote:
>> > > Peter,
>> > >
>> > > Call Trace:
>> > > <NMI>  [<ffffffff815710af>] dump_stack+0x49/0x62
>> > >  [<ffffffff8104e3bc>] warn_slowpath_common+0x8c/0xc0
>> > >  [<ffffffff8104e40a>] warn_slowpath_null+0x1a/0x20
>> > >  [<ffffffff8105f1f1>] force_sig_info+0x131/0x140
>> > >  [<ffffffff81042a4f>] force_sig_info_fault+0x5f/0x70
>> > >  [<ffffffff8106d8da>] ? search_exception_tables+0x2a/0x50
>> > >  [<ffffffff81043b3d>] ? fixup_exception+0x1d/0x70
>> > >  [<ffffffff81042cc9>] no_context+0x159/0x1f0
>> > >  [<ffffffff81042e8d>] __bad_area_nosemaphore+0x12d/0x230
>> > >  [<ffffffff81042e8d>] ? __bad_area_nosemaphore+0x12d/0x230
>> > >  [<ffffffff81042fa3>] bad_area_nosemaphore+0x13/0x20
>> > >  [<ffffffff81578fc2>] __do_page_fault+0x362/0x480
>> > >  [<ffffffff81578fc2>] ? __do_page_fault+0x362/0x480
>> > >  [<ffffffff815791be>] do_page_fault+0xe/0x10
>> > >  [<ffffffff81575962>] page_fault+0x22/0x30
>> > >  [<ffffffff815817e4>] ? bad_to_user+0x5e/0x66b
>> > >  [<ffffffff81285316>] copy_from_user_nmi+0x76/0x90
>> > >  [<ffffffff81017a20>] perf_callchain_user+0xd0/0x360
>> > >  [<ffffffff8111f64f>] perf_callchain+0x1af/0x1f0
>> > >  [<ffffffff81117693>] perf_prepare_sample+0x2f3/0x3a0
>> > >  [<ffffffff8111a2af>] __perf_event_overflow+0x10f/0x220
>> > >  [<ffffffff8111ab14>] perf_event_overflow+0x14/0x20
>> > >  [<ffffffff8101f69e>] intel_pmu_handle_irq+0x1de/0x3c0
>> > >  [<ffffffff81008e44>] ? emulate_vsyscall+0x144/0x390
>> > >  [<ffffffff81576e64>] perf_event_nmi_handler+0x34/0x60
>> > >  [<ffffffff8157664a>] nmi_handle+0x8a/0x170
>> > >  [<ffffffff81576848>] default_do_nmi+0x68/0x210
>> > >  [<ffffffff81576a80>] do_nmi+0x90/0xe0
>> > >  [<ffffffff81575c67>] end_repeat_nmi+0x1e/0x2e
>> > >  [<ffffffff81008e44>] ? emulate_vsyscall+0x144/0x390
>> > >  [<ffffffff81008e44>] ? emulate_vsyscall+0x144/0x390
>> > >  [<ffffffff81008e44>] ? emulate_vsyscall+0x144/0x390
>> > > <<EOE>>  [<ffffffff81042f7d>] __bad_area_nosemaphore+0x21d/0x230
>> > >  [<ffffffff81042fa3>] bad_area_nosemaphore+0x13/0x20
>> > >  [<ffffffff81578fc2>] __do_page_fault+0x362/0x480
>> > >  [<ffffffff8113cfbc>] ? vm_mmap_pgoff+0xbc/0xe0
>> > >  [<ffffffff815791be>] do_page_fault+0xe/0x10
>> > >  [<ffffffff81575962>] page_fault+0x22/0x30
>> > > ---[ end trace 037bf09d279751ec ]---
>> > >
>> > > So this is a double page faults. Looking at relevant changes in
>> > > 3.13 kernel, I spotted the following one patch that modified the
>> > > perf_callchain_user() function shown up in the stack trace above:
>> > >
>> >
>> > Hurm, that's an expected double fault, not something we should take the
>> > process down for.
>> >
>> > I'll have to look at how all that works for a bit.
>
> Andy, introduced all this in 4fc3490114bb ("x86-64: Set siginfo and
> context on vsyscall emulation faults").
>
> It looks like your initial userspace fault hit the magic button and ends
> up in emulate_vsyscall. Right at that point we trigger a PMI, which
> tries to do a stack-trace. That stack-trace also stumbles into unmapped
> memory (might be the same) and faults again.
>
> Now at that point, we usually just give up on the callchain and proceed
> like normal, however because of this double fault emulate-vsyscall
> SIGSEGV magic you loose.
>
> So the below might well be a valid fix.. Anybody? Andy?

Yuck -- when I wrote that thing, I hadn't imagined that an interrupt
(there's nothing particularly special about NMIs here, I think) would
try to access user memory.  The fix below looks okay, but IMO it needs
a big fat comment explaining what's going on.

Is there a way to ask whether the previous entry into the kernel came
from user space?  The valid "sig_on_uaccess_error" case happens when
the current fault was triggered by a fault from userspace.  The
invalid case (and any invalid case from, say, an int3 that a
tracepoint stuck in there) would be a page fault triggered by a fault
handler that in turn started in kernel space (in particular, in
emulate_vsyscall).

For that matter, why does current_thread_info() work from an NMI at
all?  Does the NMI vector not have its own stack?  The call that
installs it is set_intr_gate_ist(X86_TRAP_NMI, &nmi, NMI_STACK).

In any case, this at least needs a comment.  I don't see why this same
bug couldn't be triggered by non-NMI based tracing mechanisms, though.

Sigh, corner cases of corner cases...

>
>> How easily can you reproduce this? Could you test something like the
>> below, which would allow us to take double faults from NMI context.
>>
>> ---
>>  arch/x86/mm/fault.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
>> index 9ff85bb8dd69..18c498d4274d 100644
>> --- a/arch/x86/mm/fault.c
>> +++ b/arch/x86/mm/fault.c
>> @@ -641,7 +641,7 @@ no_context(struct pt_regs *regs, unsigned long error_code,
>>
>>       /* Are we prepared to handle this kernel fault? */
>>       if (fixup_exception(regs)) {
>> -             if (current_thread_info()->sig_on_uaccess_error && signal) {
>> +             if (!in_nmi() && current_thread_info()->sig_on_uaccess_error && signal) {
>>                       tsk->thread.trap_nr = X86_TRAP_PF;
>>                       tsk->thread.error_code = error_code | PF_USER;
>>                       tsk->thread.cr2 = address;


--Andy
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