[RFC PATCH 1/3] mm: drop the assumption that VM_SHARED always implies writable

[Lorenzo Stoakes <lstoakes@gmail.com>]

There are places in the kernel where there is an implicit assumption that
VM_SHARED VMAs must either be writable or might become writable via
e.g. mprotect().

We can explicitly check for the writable, shared case while remaining
conservative - If VM_MAYWRITE is not set then, by definition, the memory
can never be written to.

Update these checks to also check for VM_MAYWRITE.

Suggested-by: Andy Lutomirski <luto@amacapital.net>
Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
---
 include/linux/fs.h |  4 ++--
 include/linux/mm.h | 11 +++++++++++
 kernel/fork.c      |  2 +-
 mm/filemap.c       |  2 +-
 mm/madvise.c       |  2 +-
 mm/mmap.c          | 12 ++++++------
 6 files changed, 22 insertions(+), 11 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index c85916e9f7db..373e1edd719c 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -410,7 +410,7 @@ extern const struct address_space_operations empty_aops;
  *   It is also used to block modification of page cache contents through
  *   memory mappings.
  * @gfp_mask: Memory allocation flags to use for allocating pages.
- * @i_mmap_writable: Number of VM_SHARED mappings.
+ * @i_mmap_writable: Number of VM_SHARED, VM_MAYWRITE mappings.
  * @nr_thps: Number of THPs in the pagecache (non-shmem only).
  * @i_mmap: Tree of private and shared mappings.
  * @i_mmap_rwsem: Protects @i_mmap and @i_mmap_writable.
@@ -513,7 +513,7 @@ static inline int mapping_mapped(struct address_space *mapping)
 
 /*
  * Might pages of this file have been modified in userspace?
- * Note that i_mmap_writable counts all VM_SHARED vmas: do_mmap
+ * Note that i_mmap_writable counts all VM_SHARED, VM_MAYWRITE vmas: do_mmap
  * marks vma as VM_SHARED if it is shared, and the file was opened for
  * writing i.e. vma may be mprotected writable even if now readonly.
  *
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 898ece0a3802..8e64041b1703 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -862,6 +862,17 @@ static inline bool vma_is_accessible(struct vm_area_struct *vma)
 	return vma->vm_flags & VM_ACCESS_FLAGS;
 }
 
+static inline bool is_shared_maywrite(vm_flags_t vm_flags)
+{
+	return (vm_flags & (VM_SHARED | VM_MAYWRITE)) ==
+		(VM_SHARED | VM_MAYWRITE);
+}
+
+static inline bool vma_is_shared_maywrite(struct vm_area_struct *vma)
+{
+	return is_shared_maywrite(vma->vm_flags);
+}
+
 static inline
 struct vm_area_struct *vma_find(struct vma_iterator *vmi, unsigned long max)
 {
diff --git a/kernel/fork.c b/kernel/fork.c
index 2066a57786a8..58f257d60fee 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -733,7 +733,7 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
 
 			get_file(file);
 			i_mmap_lock_write(mapping);
-			if (tmp->vm_flags & VM_SHARED)
+			if (vma_is_shared_maywrite(tmp))
 				mapping_allow_writable(mapping);
 			flush_dcache_mmap_lock(mapping);
 			/* insert tmp into the share list, just after mpnt */
diff --git a/mm/filemap.c b/mm/filemap.c
index a34abfe8c654..4d896515032c 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3607,7 +3607,7 @@ int generic_file_mmap(struct file *file, struct vm_area_struct *vma)
  */
 int generic_file_readonly_mmap(struct file *file, struct vm_area_struct *vma)
 {
-	if ((vma->vm_flags & VM_SHARED) && (vma->vm_flags & VM_MAYWRITE))
+	if (vma_is_shared_maywrite(vma))
 		return -EINVAL;
 	return generic_file_mmap(file, vma);
 }
diff --git a/mm/madvise.c b/mm/madvise.c
index 340125d08c03..606c395c4ddd 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -981,7 +981,7 @@ static long madvise_remove(struct vm_area_struct *vma,
 			return -EINVAL;
 	}
 
-	if ((vma->vm_flags & (VM_SHARED|VM_WRITE)) != (VM_SHARED|VM_WRITE))
+	if (!vma_is_shared_maywrite(vma))
 		return -EACCES;
 
 	offset = (loff_t)(start - vma->vm_start)
diff --git a/mm/mmap.c b/mm/mmap.c
index 51cd747884e3..c96dcce90772 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -106,7 +106,7 @@ void vma_set_page_prot(struct vm_area_struct *vma)
 static void __remove_shared_vm_struct(struct vm_area_struct *vma,
 		struct file *file, struct address_space *mapping)
 {
-	if (vma->vm_flags & VM_SHARED)
+	if (vma_is_shared_maywrite(vma))
 		mapping_unmap_writable(mapping);
 
 	flush_dcache_mmap_lock(mapping);
@@ -427,7 +427,7 @@ static unsigned long count_vma_pages_range(struct mm_struct *mm,
 static void __vma_link_file(struct vm_area_struct *vma,
 			    struct address_space *mapping)
 {
-	if (vma->vm_flags & VM_SHARED)
+	if (vma_is_shared_maywrite(vma))
 		mapping_allow_writable(mapping);
 
 	flush_dcache_mmap_lock(mapping);
@@ -2596,7 +2596,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 	vma->vm_pgoff = pgoff;
 
 	if (file) {
-		if (vm_flags & VM_SHARED) {
+		if (is_shared_maywrite(vm_flags)) {
 			error = mapping_map_writable(file->f_mapping);
 			if (error)
 				goto free_vma;
@@ -2671,7 +2671,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 	vma_iter_store(&vmi, vma);
 	mm->map_count++;
 	if (vma->vm_file) {
-		if (vma->vm_flags & VM_SHARED)
+		if (vma_is_shared_maywrite(vma))
 			mapping_allow_writable(vma->vm_file->f_mapping);
 
 		flush_dcache_mmap_lock(vma->vm_file->f_mapping);
@@ -2688,7 +2688,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 
 	/* Once vma denies write, undo our temporary denial count */
 unmap_writable:
-	if (file && vm_flags & VM_SHARED)
+	if (file && is_shared_maywrite(vm_flags))
 		mapping_unmap_writable(file->f_mapping);
 	file = vma->vm_file;
 expanded:
@@ -2734,7 +2734,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 		unmap_region(mm, &mm->mm_mt, vma, prev, next, vma->vm_start,
 			     vma->vm_end, true);
 	}
-	if (file && (vm_flags & VM_SHARED))
+	if (file && is_shared_maywrite(vm_flags))
 		mapping_unmap_writable(file->f_mapping);
 free_vma:
 	vm_area_free(vma);
-- 
2.40.0