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-rw-r--r--arch/x86/kernel/nmi.c47
1 files changed, 33 insertions, 14 deletions
diff --git a/arch/x86/kernel/nmi.c b/arch/x86/kernel/nmi.c
index a0b2f84..f84f5c5 100644
--- a/arch/x86/kernel/nmi.c
+++ b/arch/x86/kernel/nmi.c
@@ -365,8 +365,9 @@ static __kprobes void default_do_nmi(struct pt_regs *regs)
#ifdef CONFIG_X86_32
/*
* For i386, NMIs use the same stack as the kernel, and we can
- * add a workaround to the iret problem in C. Simply have 3 states
- * the NMI can be in.
+ * add a workaround to the iret problem in C (preventing nested
+ * NMIs if an NMI takes a trap). Simply have 3 states the NMI
+ * can be in:
*
* 1) not running
* 2) executing
@@ -383,32 +384,50 @@ static __kprobes void default_do_nmi(struct pt_regs *regs)
* If an NMI hits a breakpoint that executes an iret, another
* NMI can preempt it. We do not want to allow this new NMI
* to run, but we want to execute it when the first one finishes.
- * We set the state to "latched", and the first NMI will perform
- * an cmpxchg on the state, and if it doesn't successfully
- * reset the state to "not running" it will restart the next
- * NMI.
+ * We set the state to "latched", and the exit of the first NMI will
+ * perform a dec_return, if the result is zero (NOT_RUNNING), then
+ * it will simply exit the NMI handler. If not, the dec_return
+ * would have set the state to NMI_EXECUTING (what we want it to
+ * be when we are running). In this case, we simply jump back
+ * to rerun the NMI handler again, and restart the 'latched' NMI.
+ *
+ * No trap (breakpoint or page fault) should be hit before nmi_restart,
+ * 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,
+ NMI_NOT_RUNNING = 0,
NMI_EXECUTING,
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 { \
- if (__get_cpu_var(nmi_state) != NMI_NOT_RUNNING) { \
- __get_cpu_var(nmi_state) = NMI_LATCHED; \
+ if (this_cpu_read(nmi_state) != NMI_NOT_RUNNING) { \
+ this_cpu_write(nmi_state, NMI_LATCHED); \
return; \
} \
- nmi_restart: \
- __get_cpu_var(nmi_state) = NMI_EXECUTING; \
- } while (0)
+ this_cpu_write(nmi_state, NMI_EXECUTING); \
+ this_cpu_write(nmi_cr2, read_cr2()); \
+ } while (0); \
+ nmi_restart:
#define nmi_nesting_postprocess() \
do { \
- if (cmpxchg(&__get_cpu_var(nmi_state), \
- NMI_EXECUTING, NMI_NOT_RUNNING) != NMI_EXECUTING) \
+ 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)
#else /* x86_64 */