1Advisory TFV-4 (CVE-2017-9607) 2============================== 3 4+----------------+-------------------------------------------------------------+ 5| Title | Malformed Firmware Update SMC can result in copy or | 6| | authentication of unexpected data in secure memory in | 7| | AArch32 state | 8+================+=============================================================+ 9| CVE ID | `CVE-2017-9607`_ | 10+----------------+-------------------------------------------------------------+ 11| Date | 20 Jun 2017 | 12+----------------+-------------------------------------------------------------+ 13| Versions | None (only between 22 May 2017 and 14 June 2017) | 14| Affected | | 15+----------------+-------------------------------------------------------------+ 16| Configurations | Platforms that use AArch32 BL1 plus untrusted normal world | 17| Affected | firmware update code executing before BL31 | 18+----------------+-------------------------------------------------------------+ 19| Impact | Copy or authentication of unexpected data in the secure | 20| | memory | 21+----------------+-------------------------------------------------------------+ 22| Fix Version | `Pull Request #979`_ (merged on 14 June 2017) | 23+----------------+-------------------------------------------------------------+ 24| Credit | ARM | 25+----------------+-------------------------------------------------------------+ 26 27The ``include/lib/utils_def.h`` header file provides the 28``check_uptr_overflow()`` macro, which aims at detecting arithmetic overflows 29that may occur when computing the sum of a base pointer and an offset. This 30macro evaluates to 1 if the sum of the given base pointer and offset would 31result in a value large enough to wrap around, which may lead to unpredictable 32behaviour. 33 34The macro code is at line 52, referring to the version of the code as of `commit 35c396b73`_: 36 37.. code:: c 38 39 /* 40 * Evaluates to 1 if (ptr + inc) overflows, 0 otherwise. 41 * Both arguments must be unsigned pointer values (i.e. uintptr_t). 42 */ 43 #define check_uptr_overflow(ptr, inc) \ 44 (((ptr) > UINTPTR_MAX - (inc)) ? 1 : 0) 45 46This macro does not work correctly for AArch32 images. It fails to detect 47overflows when the sum of its two parameters fall into the ``[2^32, 2^64 - 1]`` 48range. Therefore, any AArch32 code relying on this macro to detect such integer 49overflows is actually not protected. 50 51The buggy code has been present in ARM Trusted Firmware (TF) since `Pull Request 52#678`_ was merged (on 18 August 2016). However, the upstream code was not 53vulnerable until `Pull Request #939`_ was merged (on 22 May 2017), which 54introduced AArch32 support for the Trusted Board Boot (TBB) feature. Before 55then, the ``check_uptr_overflow()`` macro was not used in AArch32 code. 56 57The vulnerability resides in the BL1 FWU SMC handling code and it may be 58exploited when *all* the following conditions apply: 59 60- Platform code uses TF BL1 with the ``TRUSTED_BOARD_BOOT`` build option. 61 62- Platform code uses the Firmware Update (FWU) code provided in 63 ``bl1/bl1_fwu.c``, which is part of the TBB support. 64 65- TF BL1 is compiled with the ``ARCH=aarch32`` build option. 66 67In this context, the AArch32 BL1 image might fail to detect potential integer 68overflows in the input validation checks while handling the 69``FWU_SMC_IMAGE_COPY`` and ``FWU_SMC_IMAGE_AUTH`` SMCs. 70 71The ``FWU_SMC_IMAGE_COPY`` SMC handler is designed to copy an image into secure 72memory for subsequent authentication. This is implemented by the 73``bl1_fwu_image_copy()`` function, which has the following function prototype: 74 75.. code:: c 76 77 static int bl1_fwu_image_copy(unsigned int image_id, 78 uintptr_t image_src, 79 unsigned int block_size, 80 unsigned int image_size, 81 unsigned int flags) 82 83``image_src`` is an SMC argument and therefore potentially controllable by an 84attacker. A very large 32-bit value, for example ``2^32 -1``, may result in the 85sum of ``image_src`` and ``block_size`` overflowing a 32-bit type, which 86``check_uptr_overflow()`` will fail to detect. Depending on its implementation, 87the platform-specific function ``bl1_plat_mem_check()`` might get defeated by 88these unsanitized values and allow the following memory copy operation, that 89would wrap around. This may allow an attacker to copy unexpected data into 90secure memory if the memory is mapped in BL1's address space, or cause a fatal 91exception if it's not. 92 93The ``FWU_SMC_IMAGE_AUTH`` SMC handler is designed to authenticate an image 94resident in secure memory. This is implemented by the ``bl1_fwu_image_auth()`` 95function, which has the following function prototype: 96 97.. code:: c 98 99 static int bl1_fwu_image_auth(unsigned int image_id, 100 uintptr_t image_src, 101 unsigned int image_size, 102 unsigned int flags) 103 104Similarly, if an attacker has control over the ``image_src`` or ``image_size`` 105arguments through the SMC interface and injects high values whose sum overflows, 106they might defeat the ``bl1_plat_mem_check()`` function and make the 107authentication module read data outside of what's normally allowed by the 108platform code or crash the platform. 109 110Note that in both cases, a separate vulnerability is required to leverage this 111vulnerability; for example a way to get the system to change its behaviour based 112on the unexpected secure memory accesses. Moreover, the normal world FWU code 113would need to be compromised in order to send a malformed FWU SMC that triggers 114an integer overflow. 115 116The vulnerability is known to affect all ARM standard platforms when enabling 117the ``TRUSTED_BOARD_BOOT`` and ``ARCH=aarch32`` build options. Other platforms 118may also be affected if they fulfil the above conditions. 119 120.. _CVE-2017-9607: http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2017-9607 121.. _commit c396b73: https://github.com/ARM-software/arm-trusted-firmware/commit/c396b73 122.. _Pull Request #678: https://github.com/ARM-software/arm-trusted-firmware/pull/678 123.. _Pull Request #939: https://github.com/ARM-software/arm-trusted-firmware/pull/939 124.. _Pull Request #979: https://github.com/ARM-software/arm-trusted-firmware/pull/979 125