splice: move permission hook out of splice_direct_to_actor()

vfs_splice_read() has a permission hook inside rw_verify_area() and
it is called from do_splice_direct() -> splice_direct_to_actor().

The callers of do_splice_direct() (e.g. vfs_copy_file_range()) already
call rw_verify_area() for the entire range, but the other caller of
splice_direct_to_actor() (nfsd) does not.

Add the rw_verify_area() checks in nfsd_splice_read() and use a
variant of vfs_splice_read() without rw_verify_area() check in
splice_direct_to_actor() to avoid the redundant rw_verify_area() checks.

This is needed for fanotify "pre content" events.
Acked-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Link: https://lore.kernel.org/r/20231122122715.2561213-4-amir73il@gmail.comReviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/nfsd/vfs.c

@@ -1046,7 +1046,10 @@ __be32 nfsd_splice_read(struct svc_rqst *rqstp, struct svc_fh *fhp,
 	ssize_t host_err;
 
 	trace_nfsd_read_splice(rqstp, fhp, offset, *count);
-	host_err = splice_direct_to_actor(file, &sd, nfsd_direct_splice_actor);
+	host_err = rw_verify_area(READ, file, &offset, *count);
+	if (!host_err)
+		host_err = splice_direct_to_actor(file, &sd,
+						  nfsd_direct_splice_actor);
 	return nfsd_finish_read(rqstp, fhp, file, offset, count, eof, host_err);
 }
 

fs/splice.c

@@ -944,27 +944,15 @@ static void do_splice_eof(struct splice_desc *sd)
 		sd->splice_eof(sd);
 }
 
-/**
- * vfs_splice_read - Read data from a file and splice it into a pipe
- * @in:		File to splice from
- * @ppos:	Input file offset
- * @pipe:	Pipe to splice to
- * @len:	Number of bytes to splice
- * @flags:	Splice modifier flags (SPLICE_F_*)
- *
- * Splice the requested amount of data from the input file to the pipe.  This
- * is synchronous as the caller must hold the pipe lock across the entire
- * operation.
- *
- * If successful, it returns the amount of data spliced, 0 if it hit the EOF or
- * a hole and a negative error code otherwise.
+/*
+ * Callers already called rw_verify_area() on the entire range.
+ * No need to call it for sub ranges.
  */
-long vfs_splice_read(struct file *in, loff_t *ppos,
+static long do_splice_read(struct file *in, loff_t *ppos,
 			   struct pipe_inode_info *pipe, size_t len,
 			   unsigned int flags)
 {
 	unsigned int p_space;
-	int ret;
 
 	if (unlikely(!(in->f_mode & FMODE_READ)))
 		return -EBADF;
@@ -975,10 +963,6 @@ long vfs_splice_read(struct file *in, loff_t *ppos,
 	p_space = pipe->max_usage - pipe_occupancy(pipe->head, pipe->tail);
 	len = min_t(size_t, len, p_space << PAGE_SHIFT);
 
-	ret = rw_verify_area(READ, in, ppos, len);
-	if (unlikely(ret < 0))
-		return ret;
-
 	if (unlikely(len > MAX_RW_COUNT))
 		len = MAX_RW_COUNT;
 
@@ -992,6 +976,34 @@ long vfs_splice_read(struct file *in, loff_t *ppos,
 		return copy_splice_read(in, ppos, pipe, len, flags);
 	return in->f_op->splice_read(in, ppos, pipe, len, flags);
 }
+
+/**
+ * vfs_splice_read - Read data from a file and splice it into a pipe
+ * @in:		File to splice from
+ * @ppos:	Input file offset
+ * @pipe:	Pipe to splice to
+ * @len:	Number of bytes to splice
+ * @flags:	Splice modifier flags (SPLICE_F_*)
+ *
+ * Splice the requested amount of data from the input file to the pipe.  This
+ * is synchronous as the caller must hold the pipe lock across the entire
+ * operation.
+ *
+ * If successful, it returns the amount of data spliced, 0 if it hit the EOF or
+ * a hole and a negative error code otherwise.
+ */
+long vfs_splice_read(struct file *in, loff_t *ppos,
+		     struct pipe_inode_info *pipe, size_t len,
+		     unsigned int flags)
+{
+	int ret;
+
+	ret = rw_verify_area(READ, in, ppos, len);
+	if (unlikely(ret < 0))
+		return ret;
+
+	return do_splice_read(in, ppos, pipe, len, flags);
+}
 EXPORT_SYMBOL_GPL(vfs_splice_read);
 
 /**
@@ -1066,7 +1078,7 @@ ssize_t splice_direct_to_actor(struct file *in, struct splice_desc *sd,
 		size_t read_len;
 		loff_t pos = sd->pos, prev_pos = pos;
 
-		ret = vfs_splice_read(in, &pos, pipe, len, flags);
+		ret = do_splice_read(in, &pos, pipe, len, flags);
 		if (unlikely(ret <= 0))
 			goto read_failure;