restore_c_regs_and_iret:
RESTORE_C_REGS
REMOVE_PT_GPREGS_FROM_STACK 8
-
-irq_return:
INTERRUPT_RETURN
ENTRY(native_iret)
* If the variable is not set and the stack is not the NMI
* stack then:
* o Set the special variable on the stack
- * o Copy the interrupt frame into a "saved" location on the stack
- * o Copy the interrupt frame into a "copy" location on the stack
+ * o Copy the interrupt frame into an "outermost" location on the
+ * stack
+ * o Copy the interrupt frame into an "iret" location on the stack
* o Continue processing the NMI
* If the variable is set or the previous stack is the NMI stack:
- * o Modify the "copy" location to jump to the repeate_nmi
+ * o Modify the "iret" location to jump to the repeat_nmi
* o return back to the first NMI
*
* Now on exit of the first NMI, we first clear the stack variable
* 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_cfi %rdx
CFI_REL_OFFSET rdx, 0
+ testb $3, CS-RIP+8(%rsp)
+ jz .Lnmi_from_kernel
+
/*
- * If %cs was not the kernel segment, then the NMI triggered in user
- * space, which means it is definitely not nested.
+ * 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.
*/
- cmpl $__KERNEL_CS, 16(%rsp)
- jne first_nmi
+
+ SWAPGS
+ cld
+ movq %rsp, %rdx
+ movq PER_CPU_VAR(kernel_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 */
/*
- * Check the special variable on the stack to see if NMIs are
- * executing.
+ * 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:
+ /*
+ * Here's what our stack frame will look like:
+ * +---------------------------------------------------------+
+ * | original SS |
+ * | original Return RSP |
+ * | original RFLAGS |
+ * | original CS |
+ * | original RIP |
+ * +---------------------------------------------------------+
+ * | temp storage for rdx |
+ * +---------------------------------------------------------+
+ * | "NMI executing" variable |
+ * +---------------------------------------------------------+
+ * | iret SS } Copied from "outermost" frame |
+ * | iret Return RSP } on each loop iteration; overwritten |
+ * | iret RFLAGS } by a nested NMI to force another |
+ * | iret CS } iteration if needed. |
+ * | iret RIP } |
+ * +---------------------------------------------------------+
+ * | outermost SS } initialized in first_nmi; |
+ * | outermost Return RSP } will not be changed before |
+ * | outermost RFLAGS } NMI processing is done. |
+ * | outermost CS } Copied to "iret" frame on each |
+ * | outermost RIP } iteration. |
+ * +---------------------------------------------------------+
+ * | pt_regs |
+ * +---------------------------------------------------------+
+ *
+ * The "original" frame is used by hardware. Before re-enabling
+ * NMIs, we need to be done with it, and we need to leave enough
+ * space for the asm code here.
+ *
+ * We return by executing IRET while RSP points to the "iret" frame.
+ * That will either return for real or it will loop back into NMI
+ * processing.
+ *
+ * The "outermost" frame is copied to the "iret" frame on each
+ * iteration of the loop, so each iteration starts with the "iret"
+ * frame pointing to the final return target.
+ */
+
+ /*
+ * Determine whether we're a nested NMI.
+ *
+ * If we interrupted kernel code between repeat_nmi and
+ * end_repeat_nmi, then we are a nested NMI. We must not
+ * modify the "iret" frame because it's being written by
+ * the outer NMI. That's okay; the outer NMI handler is
+ * about to about to call do_nmi anyway, so we can just
+ * resume the outer NMI.
+ */
+
+ movq $repeat_nmi, %rdx
+ cmpq 8(%rsp), %rdx
+ ja 1f
+ movq $end_repeat_nmi, %rdx
+ cmpq 8(%rsp), %rdx
+ ja nested_nmi_out
+1:
+
+ /*
+ * Now check "NMI executing". If it's set, then we're nested.
+ * This will not detect if we interrupted an outer NMI just
+ * before IRET.
*/
cmpl $1, -8(%rsp)
je nested_nmi
/*
- * Now test if the previous stack was an NMI stack.
- * We need the double check. We check the NMI stack to satisfy the
- * race when the first NMI clears the variable before returning.
- * We check the variable because the first NMI could be in a
- * breakpoint routine using a breakpoint stack.
+ * Now test if the previous stack was an NMI stack. This covers
+ * the case where we interrupt an outer NMI after it clears
+ * "NMI executing" but before IRET. We need to be careful, though:
+ * there is one case in which RSP could point to the NMI stack
+ * despite there being no NMI active: naughty userspace controls
+ * RSP at the very beginning of the SYSCALL targets. We can
+ * pull a fast one on naughty userspace, though: we program
+ * SYSCALL to mask DF, so userspace cannot cause DF to be set
+ * if it controls the kernel's RSP. We set DF before we clear
+ * "NMI executing".
*/
lea 6*8(%rsp), %rdx
/* Compare the NMI stack (rdx) with the stack we came from (4*8(%rsp)) */
cmpq %rdx, 4*8(%rsp)
/* If it is below the NMI stack, it is a normal NMI */
jb first_nmi
- /* Ah, it is within the NMI stack, treat it as nested */
+
+ /* Ah, it is within the NMI stack. */
+
+ testb $(X86_EFLAGS_DF >> 8), (3*8 + 1)(%rsp)
+ jz first_nmi /* RSP was user controlled. */
+
+ /* This is a nested NMI. */
CFI_REMEMBER_STATE
nested_nmi:
/*
- * Do nothing if we interrupted the fixup in repeat_nmi.
- * It's about to repeat the NMI handler, so we are fine
- * with ignoring this one.
+ * Modify the "iret" frame to point to repeat_nmi, forcing another
+ * iteration of NMI handling.
*/
- movq $repeat_nmi, %rdx
- cmpq 8(%rsp), %rdx
- ja 1f
- movq $end_repeat_nmi, %rdx
- cmpq 8(%rsp), %rdx
- ja nested_nmi_out
-
-1:
- /* Set up the interrupted NMIs stack to jump to repeat_nmi */
leaq -1*8(%rsp), %rdx
movq %rdx, %rsp
CFI_ADJUST_CFA_OFFSET 1*8
popq_cfi %rdx
CFI_RESTORE rdx
- /* No need to check faults here */
+ /* We are returning to kernel mode, so this cannot result in a fault. */
INTERRUPT_RETURN
CFI_RESTORE_STATE
first_nmi:
- /*
- * Because nested NMIs will use the pushed location that we
- * stored in rdx, we must keep that space available.
- * Here's what our stack frame will look like:
- * +-------------------------+
- * | original SS |
- * | original Return RSP |
- * | original RFLAGS |
- * | original CS |
- * | original RIP |
- * +-------------------------+
- * | temp storage for rdx |
- * +-------------------------+
- * | NMI executing variable |
- * +-------------------------+
- * | copied SS |
- * | copied Return RSP |
- * | copied RFLAGS |
- * | copied CS |
- * | copied RIP |
- * +-------------------------+
- * | Saved SS |
- * | Saved Return RSP |
- * | Saved RFLAGS |
- * | Saved CS |
- * | Saved RIP |
- * +-------------------------+
- * | pt_regs |
- * +-------------------------+
- *
- * The saved stack frame is used to fix up the copied stack frame
- * that a nested NMI may change to make the interrupted NMI iret jump
- * to the repeat_nmi. The original stack frame and the temp storage
- * is also used by nested NMIs and can not be trusted on exit.
- */
- /* Do not pop rdx, nested NMIs will corrupt that part of the stack */
+ /* Restore rdx. */
movq (%rsp), %rdx
CFI_RESTORE rdx
- /* Set the NMI executing variable on the stack. */
+ /* Set "NMI executing" on the stack. */
pushq_cfi $1
- /*
- * Leave room for the "copied" frame
- */
+ /* Leave room for the "iret" frame */
subq $(5*8), %rsp
CFI_ADJUST_CFA_OFFSET 5*8
- /* Copy the stack frame to the Saved frame */
+ /* Copy the "original" frame to the "outermost" frame */
.rept 5
pushq_cfi 11*8(%rsp)
.endr
/* Everything up to here is safe from nested NMIs */
+repeat_nmi:
/*
* If there was a nested NMI, the first NMI's iret will return
* here. But NMIs are still enabled and we can take another
* it will just return, as we are about to repeat an NMI anyway.
* This makes it safe to copy to the stack frame that a nested
* NMI will update.
- */
-repeat_nmi:
- /*
- * Update the stack variable to say we are still in NMI (the update
- * is benign for the non-repeat case, where 1 was pushed just above
- * to this very stack slot).
+ *
+ * RSP is pointing to "outermost RIP". gsbase is unknown, but, if
+ * we're repeating an NMI, gsbase has the same value that it had on
+ * the first iteration. paranoid_entry will load the kernel
+ * gsbase if needed before we call do_nmi.
+ *
+ * Set "NMI executing" in case we came back here via IRET.
*/
movq $1, 10*8(%rsp)
- /* Make another copy, this one may be modified by nested NMIs */
+ /*
+ * Copy the "outermost" frame to the "iret" frame. NMIs that nest
+ * here must not modify the "iret" frame while we're writing to
+ * it or it will end up containing garbage.
+ */
addq $(10*8), %rsp
CFI_ADJUST_CFA_OFFSET -10*8
.rept 5
end_repeat_nmi:
/*
- * Everything below this point can be preempted by a nested
- * NMI if the first NMI took an exception and reset our iret stack
- * so that we repeat another NMI.
+ * Everything below this point can be preempted by a nested NMI.
+ * If this happens, then the inner NMI will change the "iret"
+ * frame to point back to repeat_nmi.
*/
pushq_cfi $-1 /* ORIG_RAX: no syscall to restart */
ALLOC_PT_GPREGS_ON_STACK
call paranoid_entry
DEFAULT_FRAME 0
- /*
- * Save off the CR2 register. If we take a page fault in the NMI then
- * it could corrupt the CR2 value. If the NMI preempts a page fault
- * handler before it was able to read the CR2 register, and then the
- * NMI itself takes a page fault, the page fault that was preempted
- * will read the information from the NMI page fault and not the
- * origin fault. Save it off and restore it if it changes.
- * Use the r12 callee-saved register.
- */
- movq %cr2, %r12
-
/* paranoidentry do_nmi, 0; without TRACE_IRQS_OFF */
movq %rsp,%rdi
movq $-1,%rsi
call do_nmi
- /* Did the NMI take a page fault? Restore cr2 if it did */
- movq %cr2, %rcx
- cmpq %rcx, %r12
- je 1f
- movq %r12, %cr2
-1:
-
testl %ebx,%ebx /* swapgs needed? */
jnz nmi_restore
nmi_swapgs:
nmi_restore:
RESTORE_EXTRA_REGS
RESTORE_C_REGS
- /* Pop the extra iret frame at once */
+
+ /* Point RSP at the "iret" frame. */
REMOVE_PT_GPREGS_FROM_STACK 6*8
- /* Clear the NMI executing stack variable */
- movq $0, 5*8(%rsp)
- jmp irq_return
+ /*
+ * Clear "NMI executing". Set DF first so that we can easily
+ * distinguish the remaining code between here and IRET from
+ * the SYSCALL entry and exit paths. On a native kernel, we
+ * could just inspect RIP, but, on paravirt kernels,
+ * INTERRUPT_RETURN can translate into a jump into a
+ * hypercall page.
+ */
+ std
+ movq $0, 5*8(%rsp) /* clear "NMI executing" */
+
+ /*
+ * INTERRUPT_RETURN reads the "iret" frame and exits the NMI
+ * stack in a single instruction. We are returning to kernel
+ * mode, so this cannot result in a fault.
+ */
+ INTERRUPT_RETURN
CFI_ENDPROC
END(nmi)
Code Review / kvmfornfv.git / blobdiff