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

Peter Zijlstra
2022-11-15T19:34:19Z

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

> + if (error_code & X86_PF_SHSTK)
> + flags |= FAULT_FLAG_WRITE;
> if (error_code & X86_PF_WRITE)
> flags |= FAULT_FLAG_WRITE;
> if (error_code & X86_PF_INSTR)
> --
> 2.17.1
>