Re: [syzbot] [btrfs?] KASAN: slab-out-of-bounds Read in getname_kernel (2)

[Edward Adam Davis]

On Mon, 15 Jan 2024 20:08:25 +0100, David Sterba 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.
> 
> Please let me know if you're going to send an updated fix. I'd like to
> get this fixed to close the syzbot report but also want to give you the
> credit for debugging and fix.
> 
> The preferred fix is something like that:
> 
> --- a/fs/btrfs/dev-replace.c
> +++ b/fs/btrfs/dev-replace.c
> @@ -741,6 +741,8 @@ int btrfs_dev_replace_by_ioctl(struct btrfs_fs_info *fs_info,
>         if ((args->start.srcdevid == 0 && args->start.srcdev_name[0] == '\0') ||
>             args->start.tgtdev_name[0] == '\0')
>                 return -EINVAL;
> +       args->start.srcdev_name[BTRFS_PATH_NAME_MAX] = 0;
> +       args->start.tgtdev_name[BTRFS_PATH_NAME_MAX] = 0;
This is not correct,
1. The maximum length of tgtdev_name is BTRFS_DEVICE_PATH_NAME_MAX + 1
2. strnlen should be used to confirm the presence of \0 in tgtdev_name
3. Input values should not be subjectively updated
4. The current issue only involves tgtdev_name
> 
>         ret = btrfs_dev_replace_start(fs_info, args->start.tgtdev_name,
>                                         args->start.srcdevid,