Re: [PATCH v2] usb: gadget: configfs: Fix KASAN use-after-free

[Kees Cook]

On Tue, Jan 17, 2017 at 1:29 AM, Jim Lin <jilin@xxxxxxxxxx> wrote:
> When gadget is disconnected, running sequence is like this.
> . composite_disconnect
> . Call trace:
>   usb_string_copy+0xd0/0x128
>   gadget_config_name_configuration_store+0x4
>   gadget_config_name_attr_store+0x40/0x50
>   configfs_write_file+0x198/0x1f4
>   vfs_write+0x100/0x220
>   SyS_write+0x58/0xa8
> . configfs_composite_unbind
> . configfs_composite_bind
>
> In configfs_composite_bind, it has
> "cn->strings.s = cn->configuration;"
>
> When usb_string_copy is invoked. it would
> allocate memory, copy input string, release previous pointed memory space,
> and use new allocated memory.
>
> When gadget is connected, host sends down request to get information.
> Call trace:
>   usb_gadget_get_string+0xec/0x168
>   lookup_string+0x64/0x98
>   composite_setup+0xa34/0x1ee8
>   android_setup+0xb4/0x140
>
> If gadget is disconnected and connected quickly, in the failed case,
> cn->configuration memory has been released by usb_string_copy kfree but
> configfs_composite_bind hasn't been run in time to assign new allocated
> "cn->configuration" pointer to "cn->strings.s".
>
> When "strlen(s->s) of usb_gadget_get_string is being executed, the dangling
> memory is accessed, "BUG: KASAN: use-after-free" error occurs.
>
> Signed-off-by: Jim Lin <jilin@xxxxxxxxxx>

Hi! What's the current state of this patch?

> ---
> Changes in v2:
>  Rephrase commit description
>
>  drivers/usb/gadget/configfs.c | 15 +++++++++++----
>  1 file changed, 11 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/usb/gadget/configfs.c b/drivers/usb/gadget/configfs.c
> index 78c4497..39fea62 100644
> --- a/drivers/usb/gadget/configfs.c
> +++ b/drivers/usb/gadget/configfs.c
> @@ -106,6 +106,9 @@ struct gadget_config_name {
>         struct list_head list;
>  };
>
> +#define MAX_USB_STRING_LEN     126
> +#define MAX_USB_STRING_WITH_NULL_LEN   (MAX_USB_STRING_LEN+1)
> +
>  static int usb_string_copy(const char *s, char **s_copy)
>  {
>         int ret;
> @@ -115,12 +118,16 @@ static int usb_string_copy(const char *s, char **s_copy)
>         if (ret > 126)

This should be MAX_USB_STRING_LEN, yes?

>                 return -EOVERFLOW;
>
> -       str = kstrdup(s, GFP_KERNEL);
> -       if (!str)
> -               return -ENOMEM;
> +       if (copy) {
> +               str = copy;
> +       } else {
> +               str = kmalloc(MAX_USB_STRING_WITH_NULL_LEN, GFP_KERNEL);
> +               if (!str)
> +                       return -ENOMEM;
> +       }
> +       strcpy(str, s);
>         if (str[ret - 1] == '\n')
>                 str[ret - 1] = '\0';
> -       kfree(copy);
>         *s_copy = str;
>         return 0;
>  }
> --
> 2.7.4
>

-Kees


-- 
Kees Cook
Pixel Security