fs: move permission hook out of do_iter_write()

In many of the vfs helpers, the rw_verity_area() checks are called before
taking sb_start_write(), making them "start-write-safe".
do_iter_write() is an exception to this rule.

do_iter_write() has two callers - vfs_iter_write() and vfs_writev().
Move rw_verify_area() and other checks from do_iter_write() out to
its callers to make them "start-write-safe".

Move also the fsnotify_modify() hook to align with similar pattern
used in vfs_write() and other vfs helpers.

This is needed for fanotify "pre content" events.
Suggested-by: Jan Kara <jack@suse.cz>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Link: https://lore.kernel.org/r/20231122122715.2561213-12-amir73il@gmail.comReviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/read_write.c

@@ -838,33 +838,6 @@ ssize_t vfs_iter_read(struct file *file, struct iov_iter *iter, loff_t *ppos,
 }
 EXPORT_SYMBOL(vfs_iter_read);
 
-static ssize_t do_iter_write(struct file *file, struct iov_iter *iter,
-			     loff_t *pos, rwf_t flags)
-{
-	size_t tot_len;
-	ssize_t ret = 0;
-
-	if (!(file->f_mode & FMODE_WRITE))
-		return -EBADF;
-	if (!(file->f_mode & FMODE_CAN_WRITE))
-		return -EINVAL;
-
-	tot_len = iov_iter_count(iter);
-	if (!tot_len)
-		return 0;
-	ret = rw_verify_area(WRITE, file, pos, tot_len);
-	if (ret < 0)
-		return ret;
-
-	if (file->f_op->write_iter)
-		ret = do_iter_readv_writev(file, iter, pos, WRITE, flags);
-	else
-		ret = do_loop_readv_writev(file, iter, pos, WRITE, flags);
-	if (ret > 0)
-		fsnotify_modify(file);
-	return ret;
-}
-
 ssize_t vfs_iocb_iter_write(struct file *file, struct kiocb *iocb,
 			    struct iov_iter *iter)
 {
@@ -896,13 +869,28 @@ EXPORT_SYMBOL(vfs_iocb_iter_write);
 ssize_t vfs_iter_write(struct file *file, struct iov_iter *iter, loff_t *ppos,
 		       rwf_t flags)
 {
-	int ret;
+	size_t tot_len;
+	ssize_t ret;
 
+	if (!(file->f_mode & FMODE_WRITE))
+		return -EBADF;
+	if (!(file->f_mode & FMODE_CAN_WRITE))
+		return -EINVAL;
 	if (!file->f_op->write_iter)
 		return -EINVAL;
 
+	tot_len = iov_iter_count(iter);
+	if (!tot_len)
+		return 0;
+
+	ret = rw_verify_area(WRITE, file, ppos, tot_len);
+	if (ret < 0)
+		return ret;
+
 	file_start_write(file);
-	ret = do_iter_write(file, iter, ppos, flags);
+	ret = do_iter_readv_writev(file, iter, ppos, WRITE, flags);
+	if (ret > 0)
+		fsnotify_modify(file);
 	file_end_write(file);
 
 	return ret;
@@ -927,20 +915,42 @@ static ssize_t vfs_readv(struct file *file, const struct iovec __user *vec,
 }
 
 static ssize_t vfs_writev(struct file *file, const struct iovec __user *vec,
-		   unsigned long vlen, loff_t *pos, rwf_t flags)
+			  unsigned long vlen, loff_t *pos, rwf_t flags)
 {
 	struct iovec iovstack[UIO_FASTIOV];
 	struct iovec *iov = iovstack;
 	struct iov_iter iter;
-	ssize_t ret;
+	size_t tot_len;
+	ssize_t ret = 0;
 
-	ret = import_iovec(ITER_SOURCE, vec, vlen, ARRAY_SIZE(iovstack), &iov, &iter);
-	if (ret >= 0) {
-		file_start_write(file);
-		ret = do_iter_write(file, &iter, pos, flags);
-		file_end_write(file);
-		kfree(iov);
-	}
+	if (!(file->f_mode & FMODE_WRITE))
+		return -EBADF;
+	if (!(file->f_mode & FMODE_CAN_WRITE))
+		return -EINVAL;
+
+	ret = import_iovec(ITER_SOURCE, vec, vlen, ARRAY_SIZE(iovstack), &iov,
+			   &iter);
+	if (ret < 0)
+		return ret;
+
+	tot_len = iov_iter_count(&iter);
+	if (!tot_len)
+		goto out;
+
+	ret = rw_verify_area(WRITE, file, pos, tot_len);
+	if (ret < 0)
+		goto out;
+
+	file_start_write(file);
+	if (file->f_op->write_iter)
+		ret = do_iter_readv_writev(file, &iter, pos, WRITE, flags);
+	else
+		ret = do_loop_readv_writev(file, &iter, pos, WRITE, flags);
+	if (ret > 0)
+		fsnotify_modify(file);
+	file_end_write(file);
+out:
+	kfree(iov);
 	return ret;
 }