Re: [PATCH] x86/stacktrace: Do not access user space memory unnecessarily

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Tue, Jul 02, 2019 at 01:39:05PM -0400, Steven Rostedt wrote:
> On Tue, 2 Jul 2019 11:33:55 -0400
> Steven Rostedt <rostedt@goodmis.org> wrote:
> 
> > On Tue, 2 Jul 2019 16:14:05 +0200 (CEST)
> > Thomas Gleixner <tglx@linutronix.de> wrote:
> > 
> > > On Tue, 2 Jul 2019, Peter Zijlstra wrote:
> > >   
> > > > On Tue, Jul 02, 2019 at 02:31:51PM +0900, Eiichi Tsukata wrote:    
> > > > > Put the boundary check before it accesses user space to prevent unnecessary
> > > > > access which might crash the machine.
> > > > > 
> > > > > Especially, ftrace preemptirq/irq_disable event with user stack trace
> > > > > option can trigger SEGV in pid 1 which leads to panic.    
> > 
> > Note, I'm only able to trigger this crash with the irq_disable event.
> > The irq_enable and preempt_disable/enable events work just fine. This
> > leads me to believe that the TRACE_IRQS_OFF macro (which uses a thunk
> > trampoline) may have some issues and is probably the place to look at.
> 
> I figured it out.
> 
> It's another "corruption of the cr2" register issue. The following
> patch makes the issue go away. I'm not suggesting that we use this
> patch, but it shows where the bug lies.
> 
> IIRC, there was patches posted before that fixed this issue. I'll go
> look to see if I can dig them up. Was it Joel that sent them?
> 
> -- Steve
> 
> diff --git a/arch/x86/entry/thunk_64.S b/arch/x86/entry/thunk_64.S
> index be36bf4e0957..dd79256badb2 100644
> --- a/arch/x86/entry/thunk_64.S
> +++ b/arch/x86/entry/thunk_64.S
> @@ -40,7 +40,7 @@
>  
>  #ifdef CONFIG_TRACE_IRQFLAGS
>  	THUNK trace_hardirqs_on_thunk,trace_hardirqs_on_caller,1
> -	THUNK trace_hardirqs_off_thunk,trace_hardirqs_off_caller,1
> +	THUNK trace_hardirqs_off_thunk,trace_hardirqs_off_caller_cr2,1
>  #endif
>  
>  #ifdef CONFIG_DEBUG_LOCK_ALLOC
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 46df4c6aae46..b42ca3fc569d 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -1555,3 +1555,13 @@ do_page_fault(struct pt_regs *regs, unsigned long error_code)
>  	exception_exit(prev_state);
>  }
>  NOKPROBE_SYMBOL(do_page_fault);
> +
> +void trace_hardirqs_off_caller(unsigned long addr);
> +
> +void notrace trace_hardirqs_off_caller_cr2(unsigned long addr)
> +{
> +	unsigned long address = read_cr2(); /* Get the faulting address */
> +
> +	trace_hardirqs_off_caller(addr);
> +	write_cr2(address);
> +}

I'm hitting a similar panic that bisects to commit

  a0d14b8909de ("x86/mm, tracing: Fix CR2 corruption")

except I'm experiencing death immediately after starting init.

Through sheer dumb luck, I tracked (pun intended) this down to forcing
context tracking:

  CONFIG_CONTEXT_TRACKING=y
  CONFIG_CONTEXT_TRACKING_FORCE=y
  CONFIG_VIRT_CPU_ACCOUNTING_GEN=y

I haven't attempted to debug further and I'll be offline for most of the
next few days.  Hopefully this is enough to root cause the badness.

[    0.680477] Run /sbin/init as init process
[    0.682116] init[1]: segfault at 2926a7ef ip 00007f98a49d9c30 sp 00007fffd83e6af0 error 14 in ld-2.23.so[7f98a49d9000+26000]
[    0.683427] Code: Bad RIP value.
[    0.683844] Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b
[    0.684710] CPU: 0 PID: 1 Comm: init Not tainted 5.2.0+ #18
[    0.685352] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
[    0.686239] Call Trace:
[    0.686542]  dump_stack+0x46/0x5b
[    0.686915]  panic+0xf8/0x2d2
[    0.687252]  do_exit+0xb68/0xb70
[    0.687616]  do_group_exit+0x3a/0xa0
[    0.688088]  get_signal+0x184/0x880
[    0.688483]  do_signal+0x30/0x690
[    0.688857]  ? signal_wake_up_state+0x15/0x30
[    0.689433]  ? __send_signal+0x139/0x380
[    0.689908]  exit_to_usermode_loop+0x6a/0xc0
[    0.690397]  ? async_page_fault+0x8/0x40
[    0.690850]  prepare_exit_to_usermode+0x78/0xb0
[    0.691355]  retint_user+0x8/0x8
[    0.691722] RIP: 0033:0x7f98a49d9c30
[    0.692122] Code: Bad RIP value.
[    0.692484] RSP: 002b:00007fffd83e6af0 EFLAGS: 00010202
[    0.693060] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
[    0.693907] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
[    0.694739] RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
[    0.695538] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
[    0.696322] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
[    0.697195] Kernel Offset: disabled
[    0.697600] ---[ end Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b ]---