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.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Reviewed-by: Steven Rostedt <rostedt@goodmis.org>
Reviewed-by: Borislav Petkov <bp@suse.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 062feb4..8668bbd 100644 (file)
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -1250,18 +1250,72 @@ ENTRY(nmi)
         * 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
 
+       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 */
+
        /*
-        * If %cs was not the kernel segment, then the NMI triggered in user
-        * space, which means it is definitely not nested.
+        * 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.
         */
-       cmpl    $__KERNEL_CS, 16(%rsp)
-       jne     first_nmi
 
+       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.