x86/mm: Teach pte_mkwrite() about stack memory

If a VMA has the VM_SHADOW_STACK flag, it is shadow stack memory. So
when it is made writable with pte_mkwrite(), it should create shadow
stack memory, not conventionally writable memory. Now that all the places
where shadow stack memory might be created pass a VMA into pte_mkwrite(),
it can know when it should do this.

So make pte_mkwrite() create shadow stack memory when the VMA has the
VM_SHADOW_STACK flag. Do the same thing for pmd_mkwrite().

This requires referencing VM_SHADOW_STACK in these functions, which are
currently defined in pgtable.h, however mm.h (where VM_SHADOW_STACK is
located) can't be pulled in without causing problems for files that
reference pgtable.h. So also move pte/pmd_mkwrite() into pgtable.c, where
they can safely reference VM_SHADOW_STACK.

Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
Acked-by: Deepak Gupta <debug@rivosinc.com>
Tested-by: Pengfei Xu <pengfei.xu@intel.com>
Tested-by: John Allen <john.allen@amd.com>
Tested-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/all/20230613001108.3040476-22-rick.p.edgecombe%40intel.com

Author: rpedgeco

arch/x86/mm/pgtable.c

@@ -875,13 +875,19 @@ int pmd_free_pte_page(pmd_t *pmd, unsigned long addr)
 
 pte_t pte_mkwrite(pte_t pte, struct vm_area_struct *vma)
 {
+	if (vma->vm_flags & VM_SHADOW_STACK)
+		return pte_mkwrite_shstk(pte);
+
 	pte = pte_mkwrite_novma(pte);
 
 	return pte_clear_saveddirty(pte);
 }
 
 pmd_t pmd_mkwrite(pmd_t pmd, struct vm_area_struct *vma)
 {
+	if (vma->vm_flags & VM_SHADOW_STACK)
+		return pmd_mkwrite_shstk(pmd);
+
 	pmd = pmd_mkwrite_novma(pmd);
 
 	return pmd_clear_saveddirty(pmd);