Shadow stack memory is writable only in very specific, controlled ways.
However, since it is writable, the kernel treats it as such. As a result
there remain many ways for userspace to trigger the kernel to write to
shadow stack's via get_user_pages(, FOLL_WRITE) operations. To make this a
little less exposed, block writable GUPs for shadow stack VMAs.

Still allow FOLL_FORCE to write through shadow stack protections, as it
does for read-only protections.

Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
---

v2:
 - New patch

 arch/x86/include/asm/pgtable.h | 3 +++
 mm/gup.c                       | 2 +-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
index 7a769c4dbc1c..2e6a5ee70034 100644
--- a/arch/x86/include/asm/pgtable.h
+++ b/arch/x86/include/asm/pgtable.h

@@ -1633,6 +1633,9 @@ static inline bool __pte_access_permitted(unsigned long pteval, bool write)
 {
 	unsigned long need_pte_bits = _PAGE_PRESENT|_PAGE_USER;
 
+	if (write && (pteval & (_PAGE_RW | _PAGE_DIRTY)) == _PAGE_DIRTY)
+		return 0;
+
 	if (write)
 		need_pte_bits |= _PAGE_RW;
 

diff --git a/mm/gup.c b/mm/gup.c
index 5abdaf487460..56da98f3335c 100644
--- a/mm/gup.c
+++ b/mm/gup.c

@@ -1043,7 +1043,7 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
 		return -EFAULT;
 
 	if (write) {
-		if (!(vm_flags & VM_WRITE)) {
+		if (!(vm_flags & VM_WRITE) || (vm_flags & VM_SHADOW_STACK)) {
 			if (!(gup_flags & FOLL_FORCE))
 				return -EFAULT;
 			/*

-- 
2.17.1