On Thu, May 02, 2019 at 11:43:37AM -0700, Linus Torvalds wrote:
> What would it look like with the "int3-from-kernel is special" modification?
Something like so; it boots; but I could've made some horrible mistake
(again).
---
diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index 7b23431be5cb..4de51cff5b8a 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -67,9 +67,20 @@
# define preempt_stop(clobbers) DISABLE_INTERRUPTS(clobbers);
TRACE_IRQS_OFF
#else
# define preempt_stop(clobbers)
-# define resume_kernel restore_all_kernel
#endif
+.macro RETINT_PREEMPT
+#ifdef CONFIG_PREEMPT
+ DISABLE_INTERRUPTS(CLBR_ANY)
+ cmpl $0, PER_CPU_VAR(__preempt_count)
+ jnz .Lend_\@
+ testl $X86_EFLAGS_IF, PT_EFLAGS(%esp) # interrupts off (exception
path) ?
+ jz .Lend_\@
+ call preempt_schedule_irq
+.Lend_\@:
+#endif
+.endm
+
.macro TRACE_IRQS_IRET
#ifdef CONFIG_TRACE_IRQFLAGS
testl $X86_EFLAGS_IF, PT_EFLAGS(%esp) # interrupts off?
@@ -755,7 +766,7 @@ END(ret_from_fork)
andl $SEGMENT_RPL_MASK, %eax
#endif
cmpl $USER_RPL, %eax
- jb resume_kernel # not returning to v8086 or
userspace
+ jb restore_all_kernel # not returning to v8086 or
userspace
ENTRY(resume_userspace)
DISABLE_INTERRUPTS(CLBR_ANY)
@@ -765,18 +776,6 @@ ENTRY(resume_userspace)
jmp restore_all
END(ret_from_exception)
-#ifdef CONFIG_PREEMPT
-ENTRY(resume_kernel)
- DISABLE_INTERRUPTS(CLBR_ANY)
- cmpl $0, PER_CPU_VAR(__preempt_count)
- jnz restore_all_kernel
- testl $X86_EFLAGS_IF, PT_EFLAGS(%esp) # interrupts off (exception
path) ?
- jz restore_all_kernel
- call preempt_schedule_irq
- jmp restore_all_kernel
-END(resume_kernel)
-#endif
-
GLOBAL(__begin_SYSENTER_singlestep_region)
/*
* All code from here through __end_SYSENTER_singlestep_region is subject
@@ -1027,6 +1026,7 @@ ENTRY(entry_INT80_32)
INTERRUPT_RETURN
restore_all_kernel:
+ RETINT_PREEMPT
TRACE_IRQS_IRET
PARANOID_EXIT_TO_KERNEL_MODE
BUG_IF_WRONG_CR3
@@ -1477,6 +1477,94 @@ END(nmi)
ENTRY(int3)
ASM_CLAC
+
+#ifdef CONFIG_VM86
+ testl $X86_EFLAGS_VM, 8(%esp)
+ jnz .Lfrom_usermode_no_gap
+#endif
+ testl $SEGMENT_RPL_MASK, 4(%esp)
+ jnz .Lfrom_usermode_no_gap
+
+ /*
+ * Here from kernel mode; so the (exception) stack looks like:
+ *
+ * 12(esp) - <previous context>
+ * 8(esp) - flags
+ * 4(esp) - cs
+ * 0(esp) - ip
+ *
+ * Lets build a 5 entry IRET frame after that, such that struct pt_regs
+ * is complete and in particular regs->sp is correct. This gives us
+ * the original 3 enties as gap:
+ *
+ * 32(esp) - <previous context>
+ * 28(esp) - orig_flags / gap
+ * 24(esp) - orig_cs / gap
+ * 20(esp) - orig_ip / gap
+ * 16(esp) - ss
+ * 12(esp) - sp
+ * 8(esp) - flags
+ * 4(esp) - cs
+ * 0(esp) - ip
+ */
+ pushl %ss # ss
+ pushl %esp # sp (points at ss)
+ pushl 4*4(%esp) # flags
+ pushl 4*4(%esp) # cs
+ pushl 4*4(%esp) # ip
+
+ add $16, 12(%esp) # point sp back at the previous context
+
+ pushl $-1 # orig_eax; mark as interrupt
+
+ SAVE_ALL
+ ENCODE_FRAME_POINTER
+ TRACE_IRQS_OFF
+ xorl %edx, %edx # zero error code
+ movl %esp, %eax # pt_regs pointer
+ call do_int3
+
+ RETINT_PREEMPT
+ TRACE_IRQS_IRET
+ /*
+ * If we really never INT3 from entry code, it looks like
+ * we can skip this one.
+ PARANOID_EXIT_TO_KERNEL_MODE
+ */
+ BUG_IF_WRONG_CR3
+ RESTORE_REGS 4 # consume orig_eax
+
+ /*
+ * Reconstruct the 3 entry IRET frame right after the (modified)
+ * regs->sp without lowering %esp in between, such that an NMI in the
+ * middle doesn't scribble our stack.
+ */
+
+ pushl %eax
+ pushl %ecx
+ movl 5*4(%esp), %eax # (modified) regs->sp
+
+ movl 4*4(%esp), %ecx # flags
+ movl %ecx, -4(%eax)
+
+ movl 3*4(%esp), %ecx # cs
+ andl $0x0000ffff, %ecx
+ movl %ecx, -8(%eax)
+
+ movl 2*4(%esp), %ecx # ip
+ movl %ecx, -12(%eax)
+
+ movl 1*4(%esp), %ecx # eax
+ movl %ecx, -16(%eax)
+
+ popl %ecx
+ lea -16(%eax), %esp
+ popl %eax
+
+ jmp .Lirq_return
+
+.Lfrom_usermode_no_gap:
+
pushl $-1 # mark this as an int
SAVE_ALL switch_stacks=1
