On Fri, Jan 10, 2014 at 12:06 PM, Peter Zijlstra <[email protected]> wrote: > 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 <[email protected]> > Signed-off-by: Peter Zijlstra <[email protected]> > --- > 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; > } >
My only real nit is the same thing that confused me the first time I read your email -- when I see "double fault", I think #DF. How about something like "It's possible for an interrupt to occur while sig_on_uaccess_error is set. In that case, uaccess faults that are caused by the interrupt handler should not send signals."? --Andy -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
