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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@redhat.com

Reported-by: Avi Kivity <avi@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
This commit is contained in:
Steven Rostedt 2012-06-07 11:54:37 -04:00 committed by Steven Rostedt
parent c7d65a78fc
commit 70fb74a542
1 changed files with 12 additions and 0 deletions
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12
arch/x86/kernel/nmi.c
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@ -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)