[tip:x86/debug] x86: Save cr2 in NMI in case NMIs take a page fault (for i386)

[tip-bot for Steven Rostedt]

Commit-ID:  70fb74a5420f9caa3e001d65004e4b669124283e
Gitweb:     http://git.kernel.org/tip/70fb74a5420f9caa3e001d65004e4b669124283e
Author:     Steven Rostedt <srostedt@xxxxxxxxxx>
AuthorDate: Thu, 7 Jun 2012 11:54:37 -0400
Committer:  Steven Rostedt <rostedt@xxxxxxxxxxx>
CommitDate: Fri, 8 Jun 2012 18:51:12 -0400

x86: Save cr2 in NMI in case NMIs take a page fault (for i386)

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.

This is the i386 version, which has the luxury of doing the work
in C code.

Link: http://lkml.kernel.org/r/4FBB8C40.6080304@xxxxxxxxxx

Reported-by: Avi Kivity <avi@xxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
---
 arch/x86/kernel/nmi.c |   12 ++++++++++++
 1 files changed, 12 insertions(+), 0 deletions(-)

diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c
index a15a888..f84f5c5 100644
--- a/arch/x86/kernel/nmi.c
+++ b/arch/x86/kernel/nmi.c
@@ -395,6 +395,14 @@ static __kprobes void default_do_nmi(struct pt_regs *regs)
  * thus there is no race between the first check of state for NOT_RUNNING
  * and setting it to NMI_EXECUTING. The HW will prevent nested NMIs
  * at this point.
+ *
+ * In case the NMI takes a page fault, we need to save off the CR2
+ * because the NMI could have preempted another page fault and corrupt
+ * the CR2 that is about to be read. As nested NMIs must be restarted
+ * and they can not take breakpoints or page faults, the update of the
+ * CR2 must be done before converting the nmi state back to NOT_RUNNING.
+ * Otherwise, there would be a race of another nested NMI coming in
+ * after setting state to NOT_RUNNING but before updating the nmi_cr2.
  */
 enum nmi_states {
 	NMI_NOT_RUNNING = 0,
@@ -402,6 +410,7 @@ enum nmi_states {
 	NMI_LATCHED,
 };
 static DEFINE_PER_CPU(enum nmi_states, nmi_state);
+static DEFINE_PER_CPU(unsigned long, nmi_cr2);
 
 #define nmi_nesting_preprocess(regs)					\
 	do {								\
@@ -410,11 +419,14 @@ static DEFINE_PER_CPU(enum nmi_states, nmi_state);
 			return;						\
 		}							\
 		this_cpu_write(nmi_state, NMI_EXECUTING);		\
+		this_cpu_write(nmi_cr2, read_cr2());			\
 	} while (0);							\
 	nmi_restart:
 
 #define nmi_nesting_postprocess()					\
 	do {								\
+		if (unlikely(this_cpu_read(nmi_cr2) != read_cr2()))	\
+			write_cr2(this_cpu_read(nmi_cr2));		\
 		if (this_cpu_dec_return(nmi_state))			\
 			goto nmi_restart;				\
 	} while (0)
--
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