Re: [PATCH 03/11] MdeModulePkg/UsbBus: Fix out-of-bound read access to descriptors

["Zeng, Star" <star.zeng@intel.com>]

On 2018/10/15 14:38, Ruiyu Ni wrote:
> Today's implementation reads the Type/Length field in the USB
> descriptors data without checking whether the offset to read is
> beyond the data boundary.
> 
> The patch fixes this issue.
> 
> Contributed-under: TianoCore Contribution Agreement 1.1
> Signed-off-by: Ruiyu Ni <ruiyu.ni@intel.com>
> Cc: Star Zeng <star.zeng@intel.com>
> Cc: Jiewen Yao <jiewen.yao@intel.com>

Ray,

Thanks for the patch.
I have two minor comments.
You can take them as you wish as I have no strong opinion for them. :)
Reviewed-by: Star Zeng <star.zeng@intel.com>

> ---
>   MdeModulePkg/Bus/Usb/UsbBusDxe/UsbDesc.c | 50 ++++++++++++++++++++++++--------
>   1 file changed, 38 insertions(+), 12 deletions(-)
> 
> diff --git a/MdeModulePkg/Bus/Usb/UsbBusDxe/UsbDesc.c b/MdeModulePkg/Bus/Usb/UsbBusDxe/UsbDesc.c
> index 061e4622e8..a93060deea 100644
> --- a/MdeModulePkg/Bus/Usb/UsbBusDxe/UsbDesc.c
> +++ b/MdeModulePkg/Bus/Usb/UsbBusDxe/UsbDesc.c
> @@ -177,6 +177,17 @@ UsbCreateDesc (
>       DescLen = sizeof (EFI_USB_ENDPOINT_DESCRIPTOR);
>       CtrlLen = sizeof (USB_ENDPOINT_DESC);
>       break;
> +
> +  default:
> +    ASSERT (FALSE);
> +    return NULL;
> +  }
> +
> +  //
> +  // Total length is too small that cannot hold the single descriptor header plus data.
> +  //
> +  if (Len <= sizeof (USB_DESC_HEAD)) {

Add debug message here like others below for error cases?

> +    return NULL;
>     }
>   
>     //
> @@ -184,24 +195,39 @@ UsbCreateDesc (
>     // format. Skip the descriptor that isn't of this Type
>     //
>     Offset = 0;
> -  Head   = (USB_DESC_HEAD*)DescBuf;
> +  Head   = (USB_DESC_HEAD *)DescBuf;
> +  while (Offset < Len - sizeof (USB_DESC_HEAD)) {
> +    //
> +    // Above condition make sure Head->Len and Head->Type are safe to access
> +    //
> +    Head = (USB_DESC_HEAD *)&DescBuf[Offset];
>   
> -  while ((Offset < Len) && (Head->Type != Type)) {
> -    Offset += Head->Len;
> -    if (Len <= Offset) {
> -      DEBUG (( EFI_D_ERROR, "UsbCreateDesc: met mal-format descriptor, Beyond boundary!\n"));
> +    if (Head->Len == 0) {
> +      DEBUG ((DEBUG_ERROR, "UsbCreateDesc: met mal-format descriptor, Head->Len = 0!\n"));
>         return NULL;
>       }
> -    Head    = (USB_DESC_HEAD*)(DescBuf + Offset);
> -    if (Head->Len == 0) {
> -      DEBUG (( EFI_D_ERROR, "UsbCreateDesc: met mal-format descriptor, Head->Len = 0!\n"));
> +
> +    //
> +    // Make sure no overflow when adding Head->Len to Offset.
> +    //
> +    if (Head->Len > MAX_UINTN - Offset) {
> +      DEBUG ((DEBUG_ERROR, "UsbCreateDesc: met mal-format descriptor, Head->Len = %d!\n", Head->Len));
>         return NULL;
>       }
> +
> +    Offset += Head->Len;
> +
> +    if (Head->Type == Type) {
> +      break;
> +    }
>     }
>   
> -  if ((Len <= Offset)      || (Len < Offset + Head->Len) ||
> -      (Head->Type != Type) || (Head->Len < DescLen)) {
> -    DEBUG (( EFI_D_ERROR, "UsbCreateDesc: met mal-format descriptor\n"));
> +  //
> +  // Head->Len is invalid resulting data beyond boundary, or
> +  // Descriptor cannot be found: No such type.
> +  //
> +  if ((Len < Offset) || (Head->Type != Type) || (Head->Len < DescLen)) {
> +    DEBUG (( EFI_D_ERROR, "UsbCreateDesc: met mal-format descriptor. Offset/Len = %d/%d, Header(T/L) = %d/%d\n", Offset, Len, Head->Type, Head->Len));

How about splitting the condition check and debug message to two? :)

if (Len < Offset) { // It is boundary check.

if ((Head->Type != Type) || (Head->Len < DescLen)) { // It is content check.


Thanks,
Star

>       return NULL;
>     }
>   
> @@ -212,7 +238,7 @@ UsbCreateDesc (
>   
>     CopyMem (Desc, Head, (UINTN) DescLen);
>   
> -  *Consumed = Offset + Head->Len;
> +  *Consumed = Offset;
>   
>     return Desc;
>   }
>