[PATCH v7 08/18] fsnotify: generate pre-content permission event on truncate

[Josef Bacik <josef@toxicpanda.com>]

From: Amir Goldstein <amir73il@gmail.com>

Generate FS_PRE_ACCESS event before truncate, without sb_writers held.

Move the security hooks also before sb_start_write() to conform with
other security hooks (e.g. in write, fallocate).

The event will have a range info of the page surrounding the new size
to provide an opportunity to fill the conetnt at the end of file before
truncating to non-page aligned size.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---
 fs/open.c                | 31 +++++++++++++++++++++----------
 include/linux/fsnotify.h | 32 ++++++++++++++++++++++----------
 2 files changed, 43 insertions(+), 20 deletions(-)

diff --git a/fs/open.c b/fs/open.c
index e6911101fe71..e75456cda440 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -81,14 +81,18 @@ long vfs_truncate(const struct path *path, loff_t length)
 	if (!S_ISREG(inode->i_mode))
 		return -EINVAL;
 
-	error = mnt_want_write(path->mnt);
-	if (error)
-		goto out;
-
 	idmap = mnt_idmap(path->mnt);
 	error = inode_permission(idmap, inode, MAY_WRITE);
 	if (error)
-		goto mnt_drop_write_and_out;
+		return error;
+
+	error = fsnotify_truncate_perm(path, length);
+	if (error)
+		return error;
+
+	error = mnt_want_write(path->mnt);
+	if (error)
+		return error;
 
 	error = -EPERM;
 	if (IS_APPEND(inode))
@@ -114,7 +118,7 @@ long vfs_truncate(const struct path *path, loff_t length)
 	put_write_access(inode);
 mnt_drop_write_and_out:
 	mnt_drop_write(path->mnt);
-out:
+
 	return error;
 }
 EXPORT_SYMBOL_GPL(vfs_truncate);
@@ -175,11 +179,18 @@ long do_ftruncate(struct file *file, loff_t length, int small)
 	/* Check IS_APPEND on real upper inode */
 	if (IS_APPEND(file_inode(file)))
 		return -EPERM;
-	sb_start_write(inode->i_sb);
+
 	error = security_file_truncate(file);
-	if (!error)
-		error = do_truncate(file_mnt_idmap(file), dentry, length,
-				    ATTR_MTIME | ATTR_CTIME, file);
+	if (error)
+		return error;
+
+	error = fsnotify_truncate_perm(&file->f_path, length);
+	if (error)
+		return error;
+
+	sb_start_write(inode->i_sb);
+	error = do_truncate(file_mnt_idmap(file), dentry, length,
+			    ATTR_MTIME | ATTR_CTIME, file);
 	sb_end_write(inode->i_sb);
 
 	return error;
diff --git a/include/linux/fsnotify.h b/include/linux/fsnotify.h
index 2d1c13df112c..f1ef072a3b2f 100644
--- a/include/linux/fsnotify.h
+++ b/include/linux/fsnotify.h
@@ -154,17 +154,14 @@ static inline int fsnotify_file(struct file *file, __u32 mask)
 }
 
 #ifdef CONFIG_FANOTIFY_ACCESS_PERMISSIONS
-static inline int fsnotify_pre_content(const struct file *file,
+static inline int fsnotify_pre_content(const struct path *path,
 				       const loff_t *ppos, size_t count)
 {
-	struct inode *inode = file_inode(file);
+	struct inode *inode = d_inode(path->dentry);
 	struct file_range range;
 	const void *data;
 	int data_type;
 
-	if (!fsnotify_file_watchable(file, FS_PRE_ACCESS))
-		return 0;
-
 	/*
 	 * Pre-content events are only reported for regular files and dirs
 	 * if there are any pre-content event watchers on this sb.
@@ -177,18 +174,17 @@ static inline int fsnotify_pre_content(const struct file *file,
 
 	/* Report page aligned range only when pos is known */
 	if (ppos) {
-		range.path = &file->f_path;
+		range.path = path;
 		range.pos = PAGE_ALIGN_DOWN(*ppos);
 		range.count = PAGE_ALIGN(*ppos + count) - range.pos;
 		data = &range;
 		data_type = FSNOTIFY_EVENT_FILE_RANGE;
 	} else {
-		data = &file->f_path;
+		data = path;
 		data_type = FSNOTIFY_EVENT_PATH;
 	}
 
-	return fsnotify_parent(file->f_path.dentry, FS_PRE_ACCESS,
-			       data, data_type);
+	return fsnotify_parent(path->dentry, FS_PRE_ACCESS, data, data_type);
 }
 
 /*
@@ -206,11 +202,14 @@ static inline int fsnotify_file_area_perm(struct file *file, int perm_mask,
 	 */
 	lockdep_assert_once(file_write_not_started(file));
 
+	if (!fsnotify_file_watchable(file, FS_PRE_ACCESS | FS_ACCESS_PERM))
+		return 0;
+
 	/*
 	 * read()/write and other types of access generate pre-content events.
 	 */
 	if (perm_mask & (MAY_READ | MAY_WRITE | MAY_ACCESS | MAY_OPEN)) {
-		int ret = fsnotify_pre_content(file, ppos, count);
+		int ret = fsnotify_pre_content(&file->f_path, ppos, count);
 
 		if (ret)
 			return ret;
@@ -226,6 +225,14 @@ static inline int fsnotify_file_area_perm(struct file *file, int perm_mask,
 	return fsnotify_file(file, FS_ACCESS_PERM);
 }
 
+/*
+ * fsnotify_truncate_perm - permission hook before file truncate
+ */
+static inline int fsnotify_truncate_perm(const struct path *path, loff_t length)
+{
+	return fsnotify_pre_content(path, &length, 0);
+}
+
 /*
  * fsnotify_file_perm - permission hook before file access (unknown range)
  */
@@ -254,6 +261,11 @@ static inline int fsnotify_file_area_perm(struct file *file, int perm_mask,
 	return 0;
 }
 
+static inline int fsnotify_truncate_perm(const struct path *path, loff_t length)
+{
+	return 0;
+}
+
 static inline int fsnotify_file_perm(struct file *file, int perm_mask)
 {
 	return 0;
-- 
2.43.0