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Advisory TFV-4 (CVE-2017-9607)

Title Malformed Firmware Update SMC can result in copy or authentication of unexpected data in secure memory in AArch32 state
CVE ID CVE-2017-9607
Date 20 Jun 2017
Versions Affected None (only between 22 May 2017 and 14 June 2017)
Configurations Affected Platforms that use AArch32 BL1 plus untrusted normal world firmware update code executing before BL31
Impact Copy or authentication of unexpected data in the secure memory
Fix Version Pull Request #979 (merged on 14 June 2017)
Credit ARM

The include/lib/utils_def.h header file provides the check_uptr_overflow() macro, which aims at detecting arithmetic overflows that may occur when computing the sum of a base pointer and an offset. This macro evaluates to 1 if the sum of the given base pointer and offset would result in a value large enough to wrap around, which may lead to unpredictable behaviour.

The macro code is at line 52, referring to the version of the code as of commit c396b73:

/*
 * Evaluates to 1 if (ptr + inc) overflows, 0 otherwise.
 * Both arguments must be unsigned pointer values (i.e. uintptr_t).
 */
#define check_uptr_overflow(ptr, inc)       \
    (((ptr) > UINTPTR_MAX - (inc)) ? 1 : 0)

This macro does not work correctly for AArch32 images. It fails to detect overflows when the sum of its two parameters fall into the [2^32, 2^64 - 1] range. Therefore, any AArch32 code relying on this macro to detect such integer overflows is actually not protected.

The buggy code has been present in ARM Trusted Firmware (TF) since Pull Request #678 was merged (on 18 August 2016). However, the upstream code was not vulnerable until Pull Request #939 was merged (on 22 May 2017), which introduced AArch32 support for the Trusted Board Boot (TBB) feature. Before then, the check_uptr_overflow() macro was not used in AArch32 code.

The vulnerability resides in the BL1 FWU SMC handling code and it may be exploited when all the following conditions apply:

  • Platform code uses TF BL1 with the TRUSTED_BOARD_BOOT build option.
  • Platform code uses the Firmware Update (FWU) code provided in bl1/bl1_fwu.c, which is part of the TBB support.
  • TF BL1 is compiled with the ARCH=aarch32 build option.

In this context, the AArch32 BL1 image might fail to detect potential integer overflows in the input validation checks while handling the FWU_SMC_IMAGE_COPY and FWU_SMC_IMAGE_AUTH SMCs.

The FWU_SMC_IMAGE_COPY SMC handler is designed to copy an image into secure memory for subsequent authentication. This is implemented by the bl1_fwu_image_copy() function, which has the following function prototype:

static int bl1_fwu_image_copy(unsigned int image_id,
                   uintptr_t image_src,
                   unsigned int block_size,
                   unsigned int image_size,
                   unsigned int flags)

image_src is an SMC argument and therefore potentially controllable by an attacker. A very large 32-bit value, for example 2^32 -1, may result in the sum of image_src and block_size overflowing a 32-bit type, which check_uptr_overflow() will fail to detect. Depending on its implementation, the platform-specific function bl1_plat_mem_check() might get defeated by these unsanitized values and allow the following memory copy operation, that would wrap around. This may allow an attacker to copy unexpected data into secure memory if the memory is mapped in BL1's address space, or cause a fatal exception if it's not.

The FWU_SMC_IMAGE_AUTH SMC handler is designed to authenticate an image resident in secure memory. This is implemented by the bl1_fwu_image_auth() function, which has the following function prototype:

static int bl1_fwu_image_auth(unsigned int image_id,
                    uintptr_t image_src,
                    unsigned int image_size,
                    unsigned int flags)

Similarly, if an attacker has control over the image_src or image_size arguments through the SMC interface and injects high values whose sum overflows, they might defeat the bl1_plat_mem_check() function and make the authentication module read data outside of what's normally allowed by the platform code or crash the platform.

Note that in both cases, a separate vulnerability is required to leverage this vulnerability; for example a way to get the system to change its behaviour based on the unexpected secure memory accesses. Moreover, the normal world FWU code would need to be compromised in order to send a malformed FWU SMC that triggers an integer overflow.

The vulnerability is known to affect all ARM standard platforms when enabling the TRUSTED_BOARD_BOOT and ARCH=aarch32 build options. Other platforms may also be affected if they fulfil the above conditions.