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; } -- 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/
