x86/nmi/64: Switch stacks on userspace NMI entry
Returning to userspace is tricky: IRET can fail, and ESPFIX can
rearrange the stack prior to IRET.
The NMI nesting fixup relies on a precise stack layout and atomic
IRET. Rather than trying to teach the NMI nesting fixup to handle
ESPFIX and failed IRET, punt: run NMIs that came from user mode on
the normal kernel stack.
This will make some nested NMIs visible to C code, but the C code is
okay with that.
As a side effect, this should speed up perf: it eliminates an RDMSR
when NMIs come from user mode.
Fixes CVE-2015-3290.
Cc: stable@vger.kernel.org
Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
Reviewed-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 062feb4..8668bbd 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1250,19 +1250,73 @@
* a nested NMI that updated the copy interrupt stack frame, a
* jump will be made to the repeat_nmi code that will handle the second
* NMI.
+ *
+ * However, espfix prevents us from directly returning to userspace
+ * with a single IRET instruction. Similarly, IRET to user mode
+ * can fault. We therefore handle NMIs from user space like
+ * other IST entries.
*/
/* Use %rdx as our temp variable throughout */
pushq %rdx
- /*
- * If %cs was not the kernel segment, then the NMI triggered in user
- * space, which means it is definitely not nested.
- */
- cmpl $__KERNEL_CS, 16(%rsp)
- jne first_nmi
+ testb $3, CS-RIP+8(%rsp)
+ jz .Lnmi_from_kernel
/*
+ * NMI from user mode. We need to run on the thread stack, but we
+ * can't go through the normal entry paths: NMIs are masked, and
+ * we don't want to enable interrupts, because then we'll end
+ * up in an awkward situation in which IRQs are on but NMIs
+ * are off.
+ */
+
+ SWAPGS
+ cld
+ movq %rsp, %rdx
+ movq PER_CPU_VAR(cpu_current_top_of_stack), %rsp
+ pushq 5*8(%rdx) /* pt_regs->ss */
+ pushq 4*8(%rdx) /* pt_regs->rsp */
+ pushq 3*8(%rdx) /* pt_regs->flags */
+ pushq 2*8(%rdx) /* pt_regs->cs */
+ pushq 1*8(%rdx) /* pt_regs->rip */
+ pushq $-1 /* pt_regs->orig_ax */
+ pushq %rdi /* pt_regs->di */
+ pushq %rsi /* pt_regs->si */
+ pushq (%rdx) /* pt_regs->dx */
+ pushq %rcx /* pt_regs->cx */
+ pushq %rax /* pt_regs->ax */
+ pushq %r8 /* pt_regs->r8 */
+ pushq %r9 /* pt_regs->r9 */
+ pushq %r10 /* pt_regs->r10 */
+ pushq %r11 /* pt_regs->r11 */
+ pushq %rbx /* pt_regs->rbx */
+ pushq %rbp /* pt_regs->rbp */
+ pushq %r12 /* pt_regs->r12 */
+ pushq %r13 /* pt_regs->r13 */
+ pushq %r14 /* pt_regs->r14 */
+ pushq %r15 /* pt_regs->r15 */
+
+ /*
+ * At this point we no longer need to worry about stack damage
+ * due to nesting -- we're on the normal thread stack and we're
+ * done with the NMI stack.
+ */
+
+ movq %rsp, %rdi
+ movq $-1, %rsi
+ call do_nmi
+
+ /*
+ * Return back to user mode. We must *not* do the normal exit
+ * work, because we don't want to enable interrupts. Fortunately,
+ * do_nmi doesn't modify pt_regs.
+ */
+ SWAPGS
+ jmp restore_c_regs_and_iret
+
+.Lnmi_from_kernel:
+ /*
* Check the special variable on the stack to see if NMIs are
* executing.
*/
