[PATCH] ovl: fix mmap denywrite

[Miklos Szeredi <miklos@xxxxxxxxxx>]

Overlayfs did not honor positive i_writecount on realfile for VM_DENYWRITE
mappings.  Similarly negative i_mmap_writable counts were ignored for
VM_SHARED mappings.

Fix by making vma_set_file() switch the temporary counts obtained and
released by mmap_region().

Reported-by: Chengguang Xu <cgxu519@xxxxxxxxxxxx>
Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
---
 fs/overlayfs/file.c |    4 +++-
 include/linux/mm.h  |    1 +
 mm/mmap.c           |    2 +-
 mm/util.c           |   38 +++++++++++++++++++++++++++++++++++++-
 4 files changed, 42 insertions(+), 3 deletions(-)

--- a/fs/overlayfs/file.c
+++ b/fs/overlayfs/file.c
@@ -430,7 +430,9 @@ static int ovl_mmap(struct file *file, s
 	if (WARN_ON(file != vma->vm_file))
 		return -EIO;
 
-	vma_set_file(vma, realfile);
+	ret = vma_set_file_checkwrite(vma, realfile);
+	if (ret)
+		return ret;
 
 	old_cred = ovl_override_creds(file_inode(file)->i_sb);
 	ret = call_mmap(vma->vm_file, vma);
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2751,6 +2751,7 @@ static inline void vma_set_page_prot(str
 #endif
 
 void vma_set_file(struct vm_area_struct *vma, struct file *file);
+int vma_set_file_checkwrite(struct vm_area_struct *vma, struct file *file);
 
 #ifdef CONFIG_NUMA_BALANCING
 unsigned long change_prot_numa(struct vm_area_struct *vma,
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1809,6 +1809,7 @@ unsigned long mmap_region(struct file *f
 		 */
 		vma->vm_file = get_file(file);
 		error = call_mmap(file, vma);
+		file = vma->vm_file;
 		if (error)
 			goto unmap_and_free_vma;
 
@@ -1870,7 +1871,6 @@ unsigned long mmap_region(struct file *f
 		if (vm_flags & VM_DENYWRITE)
 			allow_write_access(file);
 	}
-	file = vma->vm_file;
 out:
 	perf_event_mmap(vma);
 
--- a/mm/util.c
+++ b/mm/util.c
@@ -314,12 +314,48 @@ int vma_is_stack_for_current(struct vm_a
 /*
  * Change backing file, only valid to use during initial VMA setup.
  */
-void vma_set_file(struct vm_area_struct *vma, struct file *file)
+int vma_set_file_checkwrite(struct vm_area_struct *vma, struct file *file)
 {
+	vm_flags_t vm_flags = vma->vm_flags;
+	int err = 0;
+
 	/* Changing an anonymous vma with this is illegal */
 	get_file(file);
+
+	/* Get temporary denial counts on replacement */
+	if (vm_flags & VM_DENYWRITE) {
+		err = deny_write_access(file);
+		if (err)
+			goto out_put;
+	}
+	if (vm_flags & VM_SHARED) {
+		err = mapping_map_writable(file->f_mapping);
+		if (err)
+			goto out_allow;
+	}
+
 	swap(vma->vm_file, file);
+
+	/* Undo temporary denial counts on replaced */
+	if (vm_flags & VM_SHARED)
+		mapping_unmap_writable(file->f_mapping);
+out_allow:
+	if (vm_flags & VM_DENYWRITE)
+		allow_write_access(file);
+out_put:
 	fput(file);
+	return err;
+}
+EXPORT_SYMBOL(vma_set_file_checkwrite);
+
+/*
+ * Change backing file, only valid to use during initial VMA setup.
+ */
+void vma_set_file(struct vm_area_struct *vma, struct file *file)
+{
+	int err = vma_set_file_checkwrite(vma, file);
+
+	WARN_ON_ONCE(err);
 }
 EXPORT_SYMBOL(vma_set_file);