This is a note to let you know that I've just added the patch titled
x86/nmi: Enable nested do_nmi() handling for 64-bit kernels
to the 4.1-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
x86-nmi-enable-nested-do_nmi-handling-for-64-bit-kernels.patch
and it can be found in the queue-4.1 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 9d05041679904b12c12421cbcf9cb5f4860a8d7b Mon Sep 17 00:00:00 2001
From: Andy Lutomirski <luto@xxxxxxxxxx>
Date: Wed, 15 Jul 2015 10:29:33 -0700
Subject: x86/nmi: Enable nested do_nmi() handling for 64-bit kernels
From: Andy Lutomirski <luto@xxxxxxxxxx>
commit 9d05041679904b12c12421cbcf9cb5f4860a8d7b upstream.
32-bit kernels handle nested NMIs in C. Enable the exact same
handling on 64-bit kernels as well. This isn't currently
necessary, but it will become necessary once the asm code starts
allowing limited nesting.
Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
Reviewed-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
arch/x86/kernel/nmi.c | 125 +++++++++++++++++++++-----------------------------
1 file changed, 53 insertions(+), 72 deletions(-)
--- a/arch/x86/kernel/nmi.c
+++ b/arch/x86/kernel/nmi.c
@@ -408,15 +408,15 @@ static void default_do_nmi(struct pt_reg
NOKPROBE_SYMBOL(default_do_nmi);
/*
- * NMIs can hit breakpoints which will cause it to lose its
- * NMI context with the CPU when the breakpoint does an iret.
- */
-#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 (preventing nested
- * NMIs if an NMI takes a trap). Simply have 3 states the NMI
- * can be in:
+ * NMIs can hit breakpoints which will cause it to lose its NMI context
+ * with the CPU when the breakpoint or page fault does an IRET.
+ *
+ * As a result, NMIs can nest if NMIs get unmasked due an IRET during
+ * NMI processing. On x86_64, the asm glue protects us from nested NMIs
+ * if the outer NMI came from kernel mode, but we can still nest if the
+ * outer NMI came from user mode.
+ *
+ * To handle these nested NMIs, we have three states:
*
* 1) not running
* 2) executing
@@ -430,15 +430,14 @@ NOKPROBE_SYMBOL(default_do_nmi);
* (Note, the latch is binary, thus multiple NMIs triggering,
* when one is running, are ignored. Only one NMI is restarted.)
*
- * 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 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.
+ * If an NMI 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 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
@@ -461,49 +460,36 @@ enum nmi_states {
static DEFINE_PER_CPU(enum nmi_states, nmi_state);
static DEFINE_PER_CPU(unsigned long, nmi_cr2);
-#define nmi_nesting_preprocess(regs) \
- do { \
- if (this_cpu_read(nmi_state) != NMI_NOT_RUNNING) { \
- this_cpu_write(nmi_state, NMI_LATCHED); \
- 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)
-#else /* x86_64 */
+#ifdef CONFIG_X86_64
/*
- * In x86_64 things are a bit more difficult. This has the same problem
- * where an NMI hitting a breakpoint that calls iret will remove the
- * NMI context, allowing a nested NMI to enter. What makes this more
- * difficult is that both NMIs and breakpoints have their own stack.
- * When a new NMI or breakpoint is executed, the stack is set to a fixed
- * point. If an NMI is nested, it will have its stack set at that same
- * fixed address that the first NMI had, and will start corrupting the
- * stack. This is handled in entry_64.S, but the same problem exists with
- * the breakpoint stack.
- *
- * If a breakpoint is being processed, and the debug stack is being used,
- * if an NMI comes in and also hits a breakpoint, the stack pointer
- * will be set to the same fixed address as the breakpoint that was
- * interrupted, causing that stack to be corrupted. To handle this case,
- * check if the stack that was interrupted is the debug stack, and if
- * so, change the IDT so that new breakpoints will use the current stack
- * and not switch to the fixed address. On return of the NMI, switch back
- * to the original IDT.
+ * In x86_64, we need to handle breakpoint -> NMI -> breakpoint. Without
+ * some care, the inner breakpoint will clobber the outer breakpoint's
+ * stack.
+ *
+ * If a breakpoint is being processed, and the debug stack is being
+ * used, if an NMI comes in and also hits a breakpoint, the stack
+ * pointer will be set to the same fixed address as the breakpoint that
+ * was interrupted, causing that stack to be corrupted. To handle this
+ * case, check if the stack that was interrupted is the debug stack, and
+ * if so, change the IDT so that new breakpoints will use the current
+ * stack and not switch to the fixed address. On return of the NMI,
+ * switch back to the original IDT.
*/
static DEFINE_PER_CPU(int, update_debug_stack);
+#endif
-static inline void nmi_nesting_preprocess(struct pt_regs *regs)
+dotraplinkage notrace void
+do_nmi(struct pt_regs *regs, long error_code)
{
+ if (this_cpu_read(nmi_state) != NMI_NOT_RUNNING) {
+ this_cpu_write(nmi_state, NMI_LATCHED);
+ return;
+ }
+ this_cpu_write(nmi_state, NMI_EXECUTING);
+ this_cpu_write(nmi_cr2, read_cr2());
+nmi_restart:
+
+#ifdef CONFIG_X86_64
/*
* If we interrupted a breakpoint, it is possible that
* the nmi handler will have breakpoints too. We need to
@@ -514,22 +500,8 @@ static inline void nmi_nesting_preproces
debug_stack_set_zero();
this_cpu_write(update_debug_stack, 1);
}
-}
-
-static inline void nmi_nesting_postprocess(void)
-{
- if (unlikely(this_cpu_read(update_debug_stack))) {
- debug_stack_reset();
- this_cpu_write(update_debug_stack, 0);
- }
-}
#endif
-dotraplinkage notrace void
-do_nmi(struct pt_regs *regs, long error_code)
-{
- nmi_nesting_preprocess(regs);
-
nmi_enter();
inc_irq_stat(__nmi_count);
@@ -539,8 +511,17 @@ do_nmi(struct pt_regs *regs, long error_
nmi_exit();
- /* On i386, may loop back to preprocess */
- nmi_nesting_postprocess();
+#ifdef CONFIG_X86_64
+ if (unlikely(this_cpu_read(update_debug_stack))) {
+ debug_stack_reset();
+ this_cpu_write(update_debug_stack, 0);
+ }
+#endif
+
+ 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;
}
NOKPROBE_SYMBOL(do_nmi);
Patches currently in stable-queue which might be from luto@xxxxxxxxxx are
queue-4.1/x86-nmi-enable-nested-do_nmi-handling-for-64-bit-kernels.patch
queue-4.1/x86-nmi-64-switch-stacks-on-userspace-nmi-entry.patch
queue-4.1/x86-nmi-64-remove-asm-code-that-saves-cr2.patch
queue-4.1/x86-nmi-64-use-df-to-avoid-userspace-rsp-confusing-nested-nmi-detection.patch
queue-4.1/x86-asm-entry-64-remove-pointless-jump-to-irq_return.patch
queue-4.1/x86-nmi-64-reorder-nested-nmi-checks.patch
queue-4.1/x86-nmi-64-improve-nested-nmi-comments.patch
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