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SubjectRe: [PATCH 12/31] x86/entry/32: Add PTI cr3 switch to non-NMI entry/exit points
On Tue, Feb 27, 2018 at 02:18:36PM -0500, Waiman Long wrote:
> > + /* Make sure we are running on kernel cr3 */
> > + SWITCH_TO_KERNEL_CR3 scratch_reg=%eax
> > +
> > xorl %edx, %edx # error code 0
> > movl %esp, %eax # pt_regs pointer
> >
>
> The debug exception calls ret_from_exception on exit. If coming from
> userspace, the C function prepare_exit_to_usermode() will be called.
> With the PTI-32 code, it means that function will be called with the
> entry stack instead of the task stack. This can be problematic as macro
> like current won't work anymore.

Okay, I had another look at the debug handler. As I said before, it
already handles the from-entry-stack case, but with these patches it
gets more likely that we actually hit that path.

Also, with the special handling for from-kernel-with-entry-stack
situations we can simplify the debug handler and make it more robust
with the diff below. Thoughts?

diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index 8c149f5..844aff1 100644
--- a/arch/x86/entry/entry_32.S
+++ b/arch/x86/entry/entry_32.S
@@ -1318,33 +1318,14 @@ ENTRY(debug)
ASM_CLAC
pushl $-1 # mark this as an int

- SAVE_ALL
+ SAVE_ALL switch_stacks=1
ENCODE_FRAME_POINTER

- /* Make sure we are running on kernel cr3 */
- SWITCH_TO_KERNEL_CR3 scratch_reg=%eax
-
xorl %edx, %edx # error code 0
movl %esp, %eax # pt_regs pointer

- /* Are we currently on the SYSENTER stack? */
- movl PER_CPU_VAR(cpu_entry_area), %ecx
- addl $CPU_ENTRY_AREA_entry_stack + SIZEOF_entry_stack, %ecx
- subl %eax, %ecx /* ecx = (end of entry_stack) - esp */
- cmpl $SIZEOF_entry_stack, %ecx
- jb .Ldebug_from_sysenter_stack
-
- TRACE_IRQS_OFF
- call do_debug
- jmp ret_from_exception
-
-.Ldebug_from_sysenter_stack:
- /* We're on the SYSENTER stack. Switch off. */
- movl %esp, %ebx
- movl PER_CPU_VAR(cpu_current_top_of_stack), %esp
TRACE_IRQS_OFF
call do_debug
- movl %ebx, %esp
jmp ret_from_exception
END(debug)


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