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
