Re: [PATCH] ntfs: Ensure $Extend is a directory

[Greg KH <gregkh@linuxfoundation.org>]

On Sun, Jul 24, 2022 at 06:21:07AM -0700, Soumya Negi wrote:
> Fixes Syzbot bug: kernel BUG in ntfs_lookup_inode_by_name
> https://syzkaller.appspot.com/bug?id=32cf53b48c1846ffc25a185a2e92e170d1a95d71
> 
> Check whether $Extend is a directory or not( for NTFS3.0+) while loading
> system files. If it isn't(as in the case of this bug where the mft record for
> $Extend contains a regular file), load_system_files() returns false.

Please wrap your changelog text at 72 columns like your editor asked you
to when writing this :)

> 
> Reported-by: syzbot+30b7f850c6d98ea461d2@syzkaller.appspotmail.com
> Signed-off-by: Soumya Negi <soumya.negi97@gmail.com>

What commit caused this problem?  What Fixes: tag should go here?
Should it go to stable kernels?  If so, how far back?

> ---
>  fs/ntfs/super.c | 9 +++++++--
>  1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ntfs/super.c b/fs/ntfs/super.c
> index 5ae8de09b271..18e2902531f9 100644
> --- a/fs/ntfs/super.c
> +++ b/fs/ntfs/super.c
> @@ -2092,10 +2092,15 @@ static bool load_system_files(ntfs_volume *vol)
>  	// TODO: Initialize security.
>  	/* Get the extended system files' directory inode. */
>  	vol->extend_ino = ntfs_iget(sb, FILE_Extend);
> -	if (IS_ERR(vol->extend_ino) || is_bad_inode(vol->extend_ino)) {
> +	if (IS_ERR(vol->extend_ino) || is_bad_inode(vol->extend_ino) ||
> +	    !S_ISDIR(vol->extend_ino->i_mode)) {
> +		static const char *es1 = "$Extend is not a directory";
> +		static const char *es2 = "Failed to load $Extend";
> +		const char *es = !S_ISDIR(vol->extend_ino->i_mode) ? es1 : es2;
> +
>  		if (!IS_ERR(vol->extend_ino))
>  			iput(vol->extend_ino);
> -		ntfs_error(sb, "Failed to load $Extend.");
> +		ntfs_error(sb, "%s.", es);

Are you allowing the system log to be spammed by an untrusted user with
this change?

thanks,

greg k-h