Re: [PATCH] btrfs: fix oob Read in getname_kernel

[David Sterba]

On Tue, Dec 19, 2023 at 06:19:10PM +0800, Edward Adam Davis wrote:
> If ioctl does not pass in the correct tgtdev_name string, oob will occur because
> "\0" cannot be found.
> 
> Reported-and-tested-by: syzbot+33f23b49ac24f986c9e8@xxxxxxxxxxxxxxxxxxxxxxxxx
> Signed-off-by: Edward Adam Davis <eadavis@xxxxxx>
> ---
>  fs/btrfs/dev-replace.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/btrfs/dev-replace.c b/fs/btrfs/dev-replace.c
> index f9544fda38e9..e7e96e57f682 100644
> --- a/fs/btrfs/dev-replace.c
> +++ b/fs/btrfs/dev-replace.c
> @@ -730,7 +730,7 @@ static int btrfs_dev_replace_start(struct btrfs_fs_info *fs_info,
>  int btrfs_dev_replace_by_ioctl(struct btrfs_fs_info *fs_info,
>  			    struct btrfs_ioctl_dev_replace_args *args)
>  {
> -	int ret;
> +	int ret, len;
>  
>  	switch (args->start.cont_reading_from_srcdev_mode) {
>  	case BTRFS_IOCTL_DEV_REPLACE_CONT_READING_FROM_SRCDEV_MODE_ALWAYS:
> @@ -740,8 +740,10 @@ int btrfs_dev_replace_by_ioctl(struct btrfs_fs_info *fs_info,
>  		return -EINVAL;
>  	}
>  
> +	len = strnlen(args->start.tgtdev_name, BTRFS_DEVICE_PATH_NAME_MAX + 1);
>  	if ((args->start.srcdevid == 0 && args->start.srcdev_name[0] == '\0') ||
> -	    args->start.tgtdev_name[0] == '\0')
> +	    args->start.tgtdev_name[0] == '\0' ||
> +	    len == BTRFS_DEVICE_PATH_NAME_MAX + 1)

I think srcdev_name would have to be checked the same way, but instead
of strnlen I'd do memchr(name, 0, BTRFS_DEVICE_PATH_NAME_MAX). The check
for 0 in [0] is probably pointless, it's just a shortcut for an empty
buffer. We expect a valid 0-terminated string, which could be an invalid
path but that will be found out later when opening the block device.