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Cc:     rick.p.edgecombe@intel.com, Yu-cheng Yu <yu-cheng.yu@intel.com>
Subject: [PATCH 20/35] mm: Update can_follow_write_pte() for shadow stack
Date:   Sun, 30 Jan 2022 13:18:23 -0800
Message-Id: <20220130211838.8382-21-rick.p.edgecombe@intel.com>
In-Reply-To: <20220130211838.8382-1-rick.p.edgecombe@intel.com>
References: <20220130211838.8382-1-rick.p.edgecombe@intel.com>
Precedence: bulk

From: Yu-cheng Yu <yu-cheng.yu@intel.com>

Can_follow_write_pte() ensures a read-only page is COWed by checking the
FOLL_COW flag, and uses pte_dirty() to validate the flag is still valid.

Like a writable data page, a shadow stack page is writable, and becomes
read-only during copy-on-write, but it is always dirty.  Thus, in the
can_follow_write_pte() check, it belongs to the writable page case and
should be excluded from the read-only page pte_dirty() check.  Apply
the same changes to can_follow_write_pmd().

While at it, also split the long line into smaller ones.

Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc: Kees Cook <keescook@chromium.org>
---

Yu-cheng v26:
 - Instead of passing vm_flags, pass down vma pointer to can_follow_write_*().

Yu-cheng v25:
 - Split long line into smaller ones.

Yu-cheng v24:
 - Change arch_shadow_stack_mapping() to is_shadow_stack_mapping().

 mm/gup.c         | 16 ++++++++++++----
 mm/huge_memory.c | 16 ++++++++++++----
 2 files changed, 24 insertions(+), 8 deletions(-)

diff --git a/mm/gup.c b/mm/gup.c
index f0af462ac1e2..95b7d1084c44 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -464,10 +464,18 @@ static int follow_pfn_pte(struct vm_area_struct *vma, unsigned long address,
  * FOLL_FORCE can write to even unwritable pte's, but only
  * after we've gone through a COW cycle and they are dirty.
  */
-static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
+static inline bool can_follow_write_pte(pte_t pte, unsigned int flags,
+					struct vm_area_struct *vma)
 {
-	return pte_write(pte) ||
-		((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
+	if (pte_write(pte))
+		return true;
+	if ((flags & (FOLL_FORCE | FOLL_COW)) != (FOLL_FORCE | FOLL_COW))
+		return false;
+	if (!pte_dirty(pte))
+		return false;
+	if (is_shadow_stack_mapping(vma->vm_flags))
+		return false;
+	return true;
 }
 
 static struct page *follow_page_pte(struct vm_area_struct *vma,
@@ -510,7 +518,7 @@ static struct page *follow_page_pte(struct vm_area_struct *vma,
 	}
 	if ((flags & FOLL_NUMA) && pte_protnone(pte))
 		goto no_page;
-	if ((flags & FOLL_WRITE) && !can_follow_write_pte(pte, flags)) {
+	if ((flags & FOLL_WRITE) && !can_follow_write_pte(pte, flags, vma)) {
 		pte_unmap_unlock(ptep, ptl);
 		return NULL;
 	}
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 3588e9fefbe0..1c7167e6f223 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -1346,10 +1346,18 @@ vm_fault_t do_huge_pmd_wp_page(struct vm_fault *vmf)
  * FOLL_FORCE can write to even unwritable pmd's, but only
  * after we've gone through a COW cycle and they are dirty.
  */
-static inline bool can_follow_write_pmd(pmd_t pmd, unsigned int flags)
+static inline bool can_follow_write_pmd(pmd_t pmd, unsigned int flags,
+					struct vm_area_struct *vma)
 {
-	return pmd_write(pmd) ||
-	       ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pmd_dirty(pmd));
+	if (pmd_write(pmd))
+		return true;
+	if ((flags & (FOLL_FORCE | FOLL_COW)) != (FOLL_FORCE | FOLL_COW))
+		return false;
+	if (!pmd_dirty(pmd))
+		return false;
+	if (is_shadow_stack_mapping(vma->vm_flags))
+		return false;
+	return true;
 }
 
 struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
@@ -1362,7 +1370,7 @@ struct page *follow_trans_huge_pmd(struct vm_area_struct *vma,
 
 	assert_spin_locked(pmd_lockptr(mm, pmd));
 
-	if (flags & FOLL_WRITE && !can_follow_write_pmd(*pmd, flags))
+	if (flags & FOLL_WRITE && !can_follow_write_pmd(*pmd, flags, vma))
 		goto out;
 
 	/* Avoid dumping huge zero page */
-- 
2.17.1