Btrfs: fix memory and mount leak in btrfs_ioctl_rm_dev_v2()

If we have invalid flags set, when we error out we must drop our writer
counter and free the buffer we allocated for the arguments. This bug is
trivially reproduced with the following program on 4.7+:

	#include <fcntl.h>
	#include <stdint.h>
	#include <stdio.h>
	#include <stdlib.h>
	#include <unistd.h>
	#include <sys/ioctl.h>
	#include <sys/stat.h>
	#include <sys/types.h>
	#include <linux/btrfs.h>
	#include <linux/btrfs_tree.h>

	int main(int argc, char **argv)
	{
		struct btrfs_ioctl_vol_args_v2 vol_args = {
			.flags = UINT64_MAX,
		};
		int ret;
		int fd;

		if (argc != 2) {
			fprintf(stderr, "usage: %s PATH\n", argv[0]);
			return EXIT_FAILURE;
		}

		fd = open(argv[1], O_WRONLY);
		if (fd == -1) {
			perror("open");
			return EXIT_FAILURE;
		}

		ret = ioctl(fd, BTRFS_IOC_RM_DEV_V2, &vol_args);
		if (ret == -1)
			perror("ioctl");

		close(fd);
		return EXIT_SUCCESS;
	}

When unmounting the filesystem, we'll hit the
WARN_ON(mnt_get_writers(mnt)) in cleanup_mnt() and also may prevent the
filesystem to be remounted read-only as the writer count will stay
lifted.

Fixes: 6b526ed70cf1 ("btrfs: introduce device delete by devid")
CC: stable@vger.kernel.org # 4.9+
Signed-off-by: Omar Sandoval <osandov@fb.com>
Reviewed-by: Su Yue <suy.fnst@cn.fujitsu.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index b9b779a4ab6e..3b49b5eb98f1 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -2613,8 +2613,10 @@ static long btrfs_ioctl_rm_dev_v2(struct file *file, void __user *arg)
 	}
 
 	/* Check for compatibility reject unknown flags */
-	if (vol_args->flags & ~BTRFS_VOL_ARG_V2_FLAGS_SUPPORTED)
-		return -EOPNOTSUPP;
+	if (vol_args->flags & ~BTRFS_VOL_ARG_V2_FLAGS_SUPPORTED) {
+		ret = -EOPNOTSUPP;
+		goto out;
+	}
 
 	if (test_and_set_bit(BTRFS_FS_EXCL_OP, &fs_info->flags)) {
 		ret = BTRFS_ERROR_DEV_EXCL_RUN_IN_PROGRESS;