On Tue, 2018-11-06 at 10:43 -0800, Dave Hansen wrote:

On 10/11/18 8:15 AM, Yu-cheng Yu wrote:

quoted

--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c

@@ -1305,6 +1305,15 @@ __do_page_fault(struct pt_regs *regs, unsigned long

error_code,
 		error_code |= X86_PF_USER;
 		flags |= FAULT_FLAG_USER;
 	} else {
+		/*
+		 * WRUSS is a kernel instruction and but writes
+		 * to user shadow stack.  When a fault occurs,
+		 * both X86_PF_USER and X86_PF_SHSTK are set.
+		 * Clear X86_PF_USER here.
+		 */
+		if ((error_code & (X86_PF_USER | X86_PF_SHSTK)) ==
+		    (X86_PF_USER | X86_PF_SHSTK))
+			error_code &= ~X86_PF_USER;

This hunk of code basically points out that the architecture of WRUSS is
broken for Linux.  The setting of X86_PF_USER for a ring-0 instruction
really is a mis-feature of the architecture for us and we *undo* it in
software which is unfortunate.  Wish I would have caught this earlier.

Andy, note that this is another case where hw_error_code and
sw_error_code will diverge, unfortunately.

Anyway, this is going to necessitate some comment updates in the page
fault code.  Yu-cheng, you are going to collide with some recent changes
I made to the page fault code.  Please be careful with the context when
you do the merge and make sure that all the new comments stay correct.

Ok.  Thanks!

Yu-cheng