On Mon, 2022-10-03 at 15:51 -0700, Andy Lutomirski wrote:

On 9/29/22 15:29, Rick Edgecombe wrote:

quoted

From: Yu-cheng Yu <redacted>

+static void do_user_control_protection_fault(struct pt_regs *regs,
+                                          unsigned long
error_code)
   {
-     if (!cpu_feature_enabled(X86_FEATURE_IBT)) {
-             pr_err("Unexpected #CP\n");
-             BUG();
+     struct task_struct *tsk;
+     unsigned long ssp;
+
+     /* Read SSP before enabling interrupts. */
+     rdmsrl(MSR_IA32_PL3_SSP, ssp); > +
+     cond_local_irq_enable(regs);

I feel like I'm missing something.  Either PL3_SSL is context
switched 
correctly and reading it with IRQs off is useless, or it's not
context 
switched, and I'm very confused.

Please either improve the comment or move it after the 
cond_local_irq_enable().

The thinking was, we were just in userspace and we took a #CP. Since we
were in userspace, we had a live SSP. After we re-enable interrupts we
could get scheduled and it would be in the xsave buffer. So we can grab
it for free now, otherwise we would have to force restore it and read
it after we re-enable interrupts.

I can clarify the comments, unless there is something wrong with that
reasoning.