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From: Peter Zijlstra <peterz@infradead.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: Waiman Long <waiman.long@hp.com>, Ingo Molnar <mingo@redhat.com>,
	Arnaldo Carvalho de Melo <acme@ghostprotocols.net>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Aswin Chandramouleeswaran <aswin@hp.com>,
	Scott J Norton <scott.norton@hp.com>,
	Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: SIGSEGV when using "perf record -g" with 3.13-rc* kernel
Date: Fri, 10 Jan 2014 21:06:03 +0100
Message-ID: <20140110200603.GJ7572@laptop.programming.kicks-ass.net> (raw)
In-Reply-To: <CALCETrW2r=_Vt7+4aNKkKGeCAi8=WPULvPX3eyUEKH5nsu_CKQ@mail.gmail.com>

On Fri, Jan 10, 2014 at 10:54:52AM -0800, Andy Lutomirski wrote:
> 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.

Agreed on both points, we can equally trigger this using software
timers, so any interrupt must be exempt. And yes a comment!

> Is there a way to ask whether the previous entry into the kernel came
> from user space?

Not afaik, but in_interrupt() gets us any interrupt context, whatever
remains must be task context. Still not quite the same, but close enough
I think.

> 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).

NMIs do have their own stack, however x86_64 grabs kernel_stack from a
per-cpu variable, not rsp.

> 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...

:-)

Something like this perhaps?

---
Subject: x86, mm: Allow double faults from interrupts

Waiman managed to trigger a PMI while in a emulate_vsyscall() fault, the
PMI in turn managed to trigger a fault while obtaining a stack trace.
This triggered the double fault logic and killed the process dead.

Fix this by explicitly excluding interrupts from the double fault logic.

Reported-by: Waiman Long <waiman.long@hp.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
---
 arch/x86/mm/fault.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 9ff85bb8dd69..4c8e32986aad 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -641,6 +641,20 @@ no_context(struct pt_regs *regs, unsigned long error_code,
 
 	/* Are we prepared to handle this kernel fault? */
 	if (fixup_exception(regs)) {
+		/*
+		 * Any interrupt that takes a fault gets the fixup. This
+		 * makes the below double fault logic only apply to a
+		 * task double faulting from task context.
+		 */
+		if (in_interrupt())
+			return;
+
+		/*
+		 * Per the above we're !in_interrupt(), aka. task context.
+		 *
+		 * In this case we need to make sure we're not double faulting
+		 * through the emulate_vsyscall() logic.
+		 */
 		if (current_thread_info()->sig_on_uaccess_error && signal) {
 			tsk->thread.trap_nr = X86_TRAP_PF;
 			tsk->thread.error_code = error_code | PF_USER;
@@ -649,6 +663,10 @@ no_context(struct pt_regs *regs, unsigned long error_code,
 			/* XXX: hwpoison faults will set the wrong code. */
 			force_sig_info_fault(signal, si_code, address, tsk, 0);
 		}
+
+		/*
+		 * Barring that, we can do the fixup and be happy.
+		 */
 		return;
 	}
 

  parent reply index

Thread overview: 18+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-01-10 15:29 Waiman Long
2014-01-10 16:58 ` Peter Zijlstra
2014-01-10 17:02   ` Peter Zijlstra
2014-01-10 17:41     ` Peter Zijlstra
2014-01-10 18:54       ` Andy Lutomirski
2014-01-10 19:43         ` Waiman Long
2014-01-10 19:56           ` Andy Lutomirski
2014-01-10 20:12             ` Peter Zijlstra
2014-01-10 20:06         ` Peter Zijlstra [this message]
2014-01-10 20:28           ` Andy Lutomirski
2014-01-15 15:33           ` Waiman Long
2014-01-16 13:39           ` [tip:perf/core] x86, mm, perf: Allow recursive faults from interrupts tip-bot for Peter Zijlstra
2014-01-17 18:10             ` Waiman Long
2014-01-17 19:17               ` Andy Lutomirski
2014-01-17 20:08                 ` Waiman Long
2014-01-17 21:07                   ` Andy Lutomirski
2014-01-10 19:37     ` SIGSEGV when using "perf record -g" with 3.13-rc* kernel Waiman Long
2014-01-10 20:10       ` Peter Zijlstra

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