From: Steven Rostedt Date: Thu, 7 Jun 2012 14:21:21 +0000 (-0400) Subject: x86: Save cr2 in NMI in case NMIs take a page fault X-Git-Url: http://drtracing.org/?a=commitdiff_plain;h=7fbb98c5cb07563d3ee08714073a8e5452a96be2;p=deliverable%2Flinux.git x86: Save cr2 in NMI in case NMIs take a page fault Avi Kivity reported that page faults in NMIs could cause havic if the NMI preempted another page fault handler: The recent changes to NMI allow exceptions to take place in NMI handlers, but I think that a #PF (say, due to access to vmalloc space) is still problematic. Consider the sequence #PF (cr2 set by processor) NMI ... #PF (cr2 clobbered) do_page_fault() IRET ... IRET do_page_fault() address = read_cr2() The last line reads the overwritten cr2 value. Originally I wrote a patch to solve this by saving the cr2 on the stack. Brian Gerst suggested to save it in the r12 register as both r12 and rbx are saved by the do_nmi handler as required by the C standard. But rbx is already used for saving if swapgs needs to be run on exit of the NMI handler. Link: http://lkml.kernel.org/r/4FBB8C40.6080304@redhat.com Link: http://lkml.kernel.org/r/1337763411.13348.140.camel@gandalf.stny.rr.com Reported-by: Avi Kivity Cc: Linus Torvalds Cc: H. Peter Anvin Cc: Thomas Gleixner Suggested-by: Brian Gerst Signed-off-by: Steven Rostedt --- diff --git a/arch/x86/kernel/entry_64.S b/arch/x86/kernel/entry_64.S index 7d65133b51be..111f6bbd8b38 100644 --- a/arch/x86/kernel/entry_64.S +++ b/arch/x86/kernel/entry_64.S @@ -1758,10 +1758,30 @@ end_repeat_nmi: */ call save_paranoid DEFAULT_FRAME 0 + + /* + * Save off the CR2 register. If we take a page fault in the NMI then + * it could corrupt the CR2 value. If the NMI preempts a page fault + * handler before it was able to read the CR2 register, and then the + * NMI itself takes a page fault, the page fault that was preempted + * will read the information from the NMI page fault and not the + * origin fault. Save it off and restore it if it changes. + * Use the r12 callee-saved register. + */ + movq %cr2, %r12 + /* paranoidentry do_nmi, 0; without TRACE_IRQS_OFF */ movq %rsp,%rdi movq $-1,%rsi call do_nmi + + /* Did the NMI take a page fault? Restore cr2 if it did */ + movq %cr2, %rcx + cmpq %rcx, %r12 + je 1f + movq %r12, %cr2 +1: + testl %ebx,%ebx /* swapgs needed? */ jnz nmi_restore nmi_swapgs: