security-advisory-tfv-1.rst

Title	Malformed Firmware Update SMC can result in copy of unexpectedly large data into secure memory
CVE ID	CVE-2016-10319
Date	18 Oct 2016
Versions Affected	v1.2 and v1.3 (since commit 48bfb88)
Configurations Affected	Platforms that use AArch64 BL1 plus untrusted normal world firmware update code executing before BL31
Impact	Copy of unexpectedly large data into the free secure memory reported by BL1 platform code
Fix Version	Pull Request #783
Credit	IOActive

Generic Trusted Firmware (TF) BL1 code contains an SMC interface that is briefly available after cold reset to support the Firmware Update (FWU) feature (also known as recovery mode). This allows most FWU functionality to be implemented in the normal world, while retaining the essential image authentication functionality in BL1. When cold boot reaches the EL3 Runtime Software (for example, BL31 on AArch64 systems), the FWU SMC interface is replaced by the EL3 Runtime SMC interface. Platforms may choose how much of this FWU functionality to use, if any.

The BL1 FWU SMC handling code, currently only supported on AArch64, contains several vulnerabilities that may be exploited when all the following conditions apply:

1. Platform code uses TF BL1 with the TRUSTED_BOARD_BOOT build option enabled.
2. Platform code arranges for untrusted normal world FWU code to be executed in the cold boot path, before BL31 starts. Untrusted in this sense means code that is not in ROM or has not been authenticated or has otherwise been executed by an attacker.
3. Platform code copies the insecure pattern described below from the ARM platform version of bl1_plat_mem_check().

The vulnerabilities consist of potential integer overflows in the input validation checks while handling the FWU_SMC_IMAGE_COPY SMC. The SMC implementation is designed to copy an image into secure memory for subsequent authentication, but the vulnerabilities may allow an attacker to copy unexpectedly large data into secure memory. Note that a separate vulnerability is required to leverage these vulnerabilities; for example a way to get the system to change its behaviour based on the unexpected secure memory contents.

Two of the vulnerabilities are in the function bl1_fwu_image_copy() in bl1/bl1_fwu.c. These are listed below, referring to the v1.3 tagged version of the code:

• Line 155:

  /*
   * If last block is more than expected then
   * clip the block to the required image size.
   */
  if (image_desc->copied_size + block_size >
       image_desc->image_info.image_size) {
      block_size = image_desc->image_info.image_size -
          image_desc->copied_size;
      WARN("BL1-FWU: Copy argument block_size > remaining image size."
          " Clipping block_size\n");
  }
  
  /* Make sure the image src/size is mapped. */
  if (bl1_plat_mem_check(image_src, block_size, flags)) {
      WARN("BL1-FWU: Copy arguments source/size not mapped\n");
      return -ENOMEM;
  }
  
  INFO("BL1-FWU: Continuing image copy in blocks\n");
  
  /* Copy image for given block size. */
  base_addr += image_desc->copied_size;
  image_desc->copied_size += block_size;
  memcpy((void *)base_addr, (const void *)image_src, block_size);
  ...

  This code fragment is executed when the image copy operation is performed in blocks over multiple SMCs. block_size is an SMC argument and therefore potentially controllable by an attacker. A very large value may result in an integer overflow in the 1st if statement, which would bypass the check, allowing an unclipped block_size to be passed into bl1_plat_mem_check(). If bl1_plat_mem_check() also passes, this may result in an unexpectedly large copy of data into secure memory.

• Line 206:

  /* Make sure the image src/size is mapped. */
  if (bl1_plat_mem_check(image_src, block_size, flags)) {
      WARN("BL1-FWU: Copy arguments source/size not mapped\n");
      return -ENOMEM;
  }
  
  /* Find out how much free trusted ram remains after BL1 load */
  mem_layout = bl1_plat_sec_mem_layout();
  if ((image_desc->image_info.image_base < mem_layout->free_base) ||
       (image_desc->image_info.image_base + image_size >
        mem_layout->free_base + mem_layout->free_size)) {
      WARN("BL1-FWU: Memory not available to copy\n");
      return -ENOMEM;
  }
  
  /* Update the image size. */
  image_desc->image_info.image_size = image_size;
  
  /* Copy image for given size. */
  memcpy((void *)base_addr, (const void *)image_src, block_size);
  ...

  This code fragment is executed during the 1st invocation of the image copy operation. Both block_size and image_size are SMC arguments. A very large value of image_size may result in an integer overflow in the 2nd if statement, which would bypass the check, allowing execution to proceed. If bl1_plat_mem_check() also passes, this may result in an unexpectedly large copy of data into secure memory.

If the platform's implementation of bl1_plat_mem_check() is correct then it may help prevent the above 2 vulnerabilities from being exploited. However, the ARM platform version of this function contains a similar vulnerability:

• Line 88 of plat/arm/common/arm_bl1_fwu.c in function of bl1_plat_mem_check():

  while (mmap[index].mem_size) {
      if ((mem_base >= mmap[index].mem_base) &&
          ((mem_base + mem_size)
          <= (mmap[index].mem_base +
          mmap[index].mem_size)))
          return 0;
  
      index++;
  }
  ...

  This function checks that the passed memory region is within one of the regions mapped in by ARM platforms. Here, mem_size may be the block_size passed from bl1_fwu_image_copy(). A very large value of mem_size may result in an integer overflow and the function to incorrectly return success. Platforms that copy this insecure pattern will have the same vulnerability.