f2fs: enforce the immutable flag on open files

This patch ports commit 02b016ca ("ext4: enforce the immutable
flag on open files") to f2fs.

According to the chattr man page, "a file with the 'i' attribute
cannot be modified..."  Historically, this was only enforced when the
file was opened, per the rest of the description, "... and the file
can not be opened in write mode".

There is general agreement that we should standardize all file systems
to prevent modifications even for files that were opened at the time
the immutable flag is set.  Eventually, a change to enforce this at
the VFS layer should be landing in mainline.

Cc: stable@kernel.org
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/file.c

@@ -60,6 +60,9 @@ static vm_fault_t f2fs_vm_page_mkwrite(struct vm_fault *vmf)
 	bool need_alloc = true;
 	int err = 0;
 
+	if (unlikely(IS_IMMUTABLE(inode)))
+		return VM_FAULT_SIGBUS;
+
 	if (unlikely(f2fs_cp_error(sbi))) {
 		err = -EIO;
 		goto err;
@@ -865,6 +868,14 @@ int f2fs_setattr(struct dentry *dentry, struct iattr *attr)
 	if (unlikely(f2fs_cp_error(F2FS_I_SB(inode))))
 		return -EIO;
 
+	if (unlikely(IS_IMMUTABLE(inode)))
+		return -EPERM;
+
+	if (unlikely(IS_APPEND(inode) &&
+			(attr->ia_valid & (ATTR_MODE | ATTR_UID |
+				  ATTR_GID | ATTR_TIMES_SET))))
+		return -EPERM;
+
 	if ((attr->ia_valid & ATTR_SIZE) &&
 		!f2fs_is_compress_backend_ready(inode))
 		return -EOPNOTSUPP;
@@ -4351,6 +4362,11 @@ static ssize_t f2fs_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
 		inode_lock(inode);
 	}
 
+	if (unlikely(IS_IMMUTABLE(inode))) {
+		ret = -EPERM;
+		goto unlock;
+	}
+
 	ret = generic_write_checks(iocb, from);
 	if (ret > 0) {
 		bool preallocated = false;
@@ -4415,6 +4431,7 @@ static ssize_t f2fs_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
 		if (ret > 0)
 			f2fs_update_iostat(F2FS_I_SB(inode), APP_WRITE_IO, ret);
 	}
+unlock:
 	inode_unlock(inode);
 out:
 	trace_f2fs_file_write_iter(inode, iocb->ki_pos,