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>
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 @@
* 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 @@
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 @@
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)
