btrfs: dev-replace: properly validate device names

commit 9845664b9ee47ce7ee7ea93caf47d39a9d4552c4 upstream.

There's a syzbot report that device name buffers passed to device
replace are not properly checked for string termination which could lead
to a read out of bounds in getname_kernel().

Add a helper that validates both source and target device name buffers.
For devid as the source initialize the buffer to empty string in case
something tries to read it later.

This was originally analyzed and fixed in a different way by Edward Adam
Davis (see links).

Link: https://lore.kernel.org/linux-btrfs/[email protected]/
Link: https://lore.kernel.org/linux-btrfs/[email protected]/
CC: [email protected] # 4.19+
CC: Edward Adam Davis <[email protected]>
Reported-and-tested-by: [email protected]
Reviewed-by: Boris Burkov <[email protected]>
Signed-off-by: David Sterba <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

fs/btrfs/dev-replace.c

@@ -535,6 +535,23 @@ int btrfs_dev_replace_start(struct btrfs_fs_info *fs_info,
 	return ret;
 }
 
+static int btrfs_check_replace_dev_names(struct btrfs_ioctl_dev_replace_args *args)
+{
+	if (args->start.srcdevid == 0) {
+		if (memchr(args->start.srcdev_name, 0,
+			   sizeof(args->start.srcdev_name)) == NULL)
+			return -ENAMETOOLONG;
+	} else {
+		args->start.srcdev_name[0] = 0;
+	}
+
+	if (memchr(args->start.tgtdev_name, 0,
+		   sizeof(args->start.tgtdev_name)) == NULL)
+	    return -ENAMETOOLONG;
+
+	return 0;
+}
+
 int btrfs_dev_replace_by_ioctl(struct btrfs_fs_info *fs_info,
 			    struct btrfs_ioctl_dev_replace_args *args)
 {
@@ -547,10 +564,9 @@ int btrfs_dev_replace_by_ioctl(struct btrfs_fs_info *fs_info,
 	default:
 		return -EINVAL;
 	}
-
-	if ((args->start.srcdevid == 0 && args->start.srcdev_name[0] == '\0') ||
-	    args->start.tgtdev_name[0] == '\0')
-		return -EINVAL;
+	ret = btrfs_check_replace_dev_names(args);
+	if (ret < 0)
+		return ret;
 
 	ret = btrfs_dev_replace_start(fs_info, args->start.tgtdev_name,
 					args->start.srcdevid,