Re: [PATCH] autofs - fix autofs_sbi() does not check super block type

Ian Kent
2018-08-29T06:35:23Z

On Mon, 2018-08-27 at 02:03 +0100, Al Viro wrote:
> On Mon, Aug 20, 2018 at 04:37:09PM +0800, Ian Kent wrote:
> > The autofs_sbi() inline function does not check the super block
> > magic number to verify it has been given an autofs super block.
>
> IMO it's the wrong way to fix it. The one and only caller where that
> check might trigger is
>
> if (!fp) {
> if (cmd == AUTOFS_DEV_IOCTL_ISMOUNTPOINT_CMD)
> goto cont;
> err = -EBADF;
> goto out;
> }
>
> sbi = autofs_dev_ioctl_sbi(fp);
> if (!sbi || sbi->magic != AUTOFS_SBI_MAGIC) {
> err = -EINVAL;
> fput(fp);
> goto out;
> }
> with
> static struct autofs_sb_info *autofs_dev_ioctl_sbi(struct file *f)
> {
> struct autofs_sb_info *sbi = NULL;
> struct inode *inode;
>
> if (f) {
> inode = file_inode(f);
> sbi = autofs_sbi(inode->i_sb);
> }
> return sbi;
> }
>
> First of all, what is that `if (f)' doing in there? We have just checked
> that in the only caller.
>
> Next, dereferencing the result of autofs_sbi() does need to be preceded
> by making sure that superblock is autofs one, all right... and what are
> we doing in that first dereferencing, again?
>
> IOW, turn that into
>
> if (!fp) {
> ....
> goto out;
> }
> sb = file_inode(fp)->i_sb;
> if (sb->s_type != &autofs_fs_type)
> bugger off
> sbi = autofs_sbi(sb);
> ....
>
> and be done with that. Other callers of autofs_sbi() really shouldn't
> happen to other filesystem's superblocks...

Yes, adding it to the inline does add a little extra for other
callers that won't get a non-autofs super block.

I was tempted to just change autofs_dev_ioctl_sbi() in case other
callers were added but your suggestion is somewhat simpler and
really only requires due attention if changes are made.

I'll send a patch to Andrew based on what you recommend.

Thanks
Ian