Re: usb: A use-after-free Write in put_dev

[Gerald Lee <sundaywind2004@gmail.com>]

Hi Alan,

I've tested this patch. It resolves the problem.

Thanks.


On Tue, Dec 13, 2022 at 12:24 AM Alan Stern <stern@rowland.harvard.edu> wrote:
>
> On Mon, Dec 12, 2022 at 04:07:43PM +0800, Gerald Lee wrote:
> > Hi, all
> >
> > I found a vulnerability when fuzzing linux kernel by syzkaller. The
> > KASAN reports that use-after-free Write in put_dev. Then I tried to
> > reproduce and got the C source file. I compiled it and executed the
> > binary program, but the kernel crashed as expected. This vulnerability
> > could be used to LPE as UAF, I thought.
>
> I think this use-after-free violation is caused by a race among the
> superblock operations in the gadgetfs driver.  Please try running your
> test after applying the patch below, to see if it fixes the problem.
>
> Alan Stern
>
>
> --- usb-devel.orig/drivers/usb/gadget/legacy/inode.c
> +++ usb-devel/drivers/usb/gadget/legacy/inode.c
> @@ -229,6 +229,7 @@ static void put_ep (struct ep_data *data
>   */
>
>  static const char *CHIP;
> +static DEFINE_MUTEX(sb_mutex);         /* Serialize superblock maintenance */
>
>  /*----------------------------------------------------------------------*/
>
> @@ -2010,13 +2011,20 @@ gadgetfs_fill_super (struct super_block
>  {
>         struct inode    *inode;
>         struct dev_data *dev;
> +       int             rc;
>
> -       if (the_device)
> -               return -ESRCH;
> +       mutex_lock(&sb_mutex);
> +
> +       if (the_device) {
> +               rc = -ESRCH;
> +               goto Done;
> +       }
>
>         CHIP = usb_get_gadget_udc_name();
> -       if (!CHIP)
> -               return -ENODEV;
> +       if (!CHIP) {
> +               rc = -ENODEV;
> +               goto Done;
> +       }
>
>         /* superblock */
>         sb->s_blocksize = PAGE_SIZE;
> @@ -2053,13 +2061,17 @@ gadgetfs_fill_super (struct super_block
>          * from binding to a controller.
>          */
>         the_device = dev;
> -       return 0;
> +       rc = 0;
> +       goto Done;
>
> -Enomem:
> + Enomem:
>         kfree(CHIP);
>         CHIP = NULL;
> +       rc = -ENOMEM;
>
> -       return -ENOMEM;
> + Done:
> +       mutex_unlock(&sb_mutex);
> +       return rc;
>  }
>
>  /* "mount -t gadgetfs path /dev/gadget" ends up here */
> @@ -2081,6 +2093,7 @@ static int gadgetfs_init_fs_context(stru
>  static void
>  gadgetfs_kill_sb (struct super_block *sb)
>  {
> +       mutex_lock(&sb_mutex);
>         kill_litter_super (sb);
>         if (the_device) {
>                 put_dev (the_device);
> @@ -2088,6 +2101,7 @@ gadgetfs_kill_sb (struct super_block *sb
>         }
>         kfree(CHIP);
>         CHIP = NULL;
> +       mutex_unlock(&sb_mutex);
>  }
>
>  /*----------------------------------------------------------------------*/