fuse: prevent overflow in copy_file_range return value

commit 1e08938 upstream.

The FUSE protocol uses struct fuse_write_out to convey the return value of
copy_file_range, which is restricted to uint32_t.  But the COPY_FILE_RANGE
interface supports a 64-bit size copies.

Currently the number of bytes copied is silently truncated to 32-bit, which
may result in poor performance or even failure to copy in case of
truncation to zero.

Reported-by: Florian Weimer <[email protected]>
Closes: https://lore.kernel.org/all/[email protected]/
Fixes: 88bc7d5 ("fuse: add support for copy_file_range()")
Cc: <[email protected]> # v4.20
Signed-off-by: Miklos Szeredi <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

Author: Miklos Szeredi

fs/fuse/file.c

@@ -3106,7 +3106,7 @@ static ssize_t __fuse_copy_file_range(struct file *file_in, loff_t pos_in,
 		.nodeid_out = ff_out->nodeid,
 		.fh_out = ff_out->fh,
 		.off_out = pos_out,
-		.len = len,
+		.len = min_t(size_t, len, UINT_MAX & PAGE_MASK),
 		.flags = flags
 	};
 	struct fuse_write_out outarg;