Re: [PATCH v3 15/37] x86/mm: Check Shadow Stack page fault errors

On Tue, 2022-11-15 at 12:47 +0100, Peter Zijlstra wrote:

On Fri, Nov 04, 2022 at 03:35:42PM -0700, Rick Edgecombe wrote:

quoted

@@ -1331,6 +1345,18 @@ void do_user_addr_fault(struct pt_regs

*regs,
  
       perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
  
+     /*
+      * To service shadow stack read faults, unlike normal read
faults, the
+      * fault handler needs to create a type of memory that will
also be
+      * writable (with instructions that generate shadow stack
writes).
+      * In the case of COW memory, the COW needs to take place
even with
+      * a shadow stack read. Otherwise the shared page will be
left (shadow
+      * stack) writable in userspace. So to trigger the
appropriate behavior
+      * by setting FAULT_FLAG_WRITE for shadow stack accesses,
even if the
+      * access was a shadow stack read.
+      */

Clear as mud... So SS pages are 'Write=0,Dirty=1', which, per
construction, lack a RW bit. And these pages are writable (WRUSS).

pte_wrprotect() seems to do: _PAGE_DIRTY->_PAGE_COW (which is really
weird in this situation), resulting in: 'Write=0,Dirty=0,Cow=1'.

That's regular RO memory and won't raise read-faults.

But I'm thinking RET will trip #PF here when it tries to read the SS
because the SSP is not a proper shadow stack page?

And in that case you want to tickle pte_mkwrite() to undo the
pte_wrprotect() above?

So while the #PF is a 'read' fault due to RET not actually writing to
the shadow stack, you want to force a write fault so it will re-
instate
the SS page.

Did I get that right?

That's right. I think the assumption that needs to be broken in the
readers head is that you can satisfy a read fault with read-only PTE.
This is kind of baked in all over the place with the zero-pfn, COW,
etc. Maybe I should try to start with that.