1+----------------+-------------------------------------------------------------+ 2| Title | Malformed Firmware Update SMC can result in copy of | 3| | unexpectedly large data into secure memory | 4+================+=============================================================+ 5| CVE ID | `CVE-2016-10319`_ | 6+----------------+-------------------------------------------------------------+ 7| Date | 18 Oct 2016 | 8+----------------+-------------------------------------------------------------+ 9| Versions | v1.2 and v1.3 (since commit `48bfb88`_) | 10| Affected | | 11+----------------+-------------------------------------------------------------+ 12| Configurations | Platforms that use AArch64 BL1 plus untrusted normal world | 13| Affected | firmware update code executing before BL31 | 14+----------------+-------------------------------------------------------------+ 15| Impact | Copy of unexpectedly large data into the free secure memory | 16| | reported by BL1 platform code | 17+----------------+-------------------------------------------------------------+ 18| Fix Version | `Pull Request #783`_ | 19+----------------+-------------------------------------------------------------+ 20| Credit | IOActive | 21+----------------+-------------------------------------------------------------+ 22 23Generic Trusted Firmware (TF) BL1 code contains an SMC interface that is briefly 24available after cold reset to support the Firmware Update (FWU) feature (also 25known as recovery mode). This allows most FWU functionality to be implemented in 26the normal world, while retaining the essential image authentication 27functionality in BL1. When cold boot reaches the EL3 Runtime Software (for 28example, BL31 on AArch64 systems), the FWU SMC interface is replaced by the EL3 29Runtime SMC interface. Platforms may choose how much of this FWU functionality 30to use, if any. 31 32The BL1 FWU SMC handling code, currently only supported on AArch64, contains 33several vulnerabilities that may be exploited when *all* the following 34conditions apply: 35 361. Platform code uses TF BL1 with the ``TRUSTED_BOARD_BOOT`` build option 37 enabled. 38 392. Platform code arranges for untrusted normal world FWU code to be executed in 40 the cold boot path, before BL31 starts. Untrusted in this sense means code 41 that is not in ROM or has not been authenticated or has otherwise been 42 executed by an attacker. 43 443. Platform code copies the insecure pattern described below from the ARM 45 platform version of ``bl1_plat_mem_check()``. 46 47The vulnerabilities consist of potential integer overflows in the input 48validation checks while handling the ``FWU_SMC_IMAGE_COPY`` SMC. The SMC 49implementation is designed to copy an image into secure memory for subsequent 50authentication, but the vulnerabilities may allow an attacker to copy 51unexpectedly large data into secure memory. Note that a separate vulnerability 52is required to leverage these vulnerabilities; for example a way to get the 53system to change its behaviour based on the unexpected secure memory contents. 54 55Two of the vulnerabilities are in the function ``bl1_fwu_image_copy()`` in 56``bl1/bl1_fwu.c``. These are listed below, referring to the v1.3 tagged version 57of the code: 58 59- Line 155: 60 61 .. code:: c 62 63 /* 64 * If last block is more than expected then 65 * clip the block to the required image size. 66 */ 67 if (image_desc->copied_size + block_size > 68 image_desc->image_info.image_size) { 69 block_size = image_desc->image_info.image_size - 70 image_desc->copied_size; 71 WARN("BL1-FWU: Copy argument block_size > remaining image size." 72 " Clipping block_size\n"); 73 } 74 75 /* Make sure the image src/size is mapped. */ 76 if (bl1_plat_mem_check(image_src, block_size, flags)) { 77 WARN("BL1-FWU: Copy arguments source/size not mapped\n"); 78 return -ENOMEM; 79 } 80 81 INFO("BL1-FWU: Continuing image copy in blocks\n"); 82 83 /* Copy image for given block size. */ 84 base_addr += image_desc->copied_size; 85 image_desc->copied_size += block_size; 86 memcpy((void *)base_addr, (const void *)image_src, block_size); 87 ... 88 89 This code fragment is executed when the image copy operation is performed in 90 blocks over multiple SMCs. ``block_size`` is an SMC argument and therefore 91 potentially controllable by an attacker. A very large value may result in an 92 integer overflow in the 1st ``if`` statement, which would bypass the check, 93 allowing an unclipped ``block_size`` to be passed into 94 ``bl1_plat_mem_check()``. If ``bl1_plat_mem_check()`` also passes, this may 95 result in an unexpectedly large copy of data into secure memory. 96 97- Line 206: 98 99 .. code:: c 100 101 /* Make sure the image src/size is mapped. */ 102 if (bl1_plat_mem_check(image_src, block_size, flags)) { 103 WARN("BL1-FWU: Copy arguments source/size not mapped\n"); 104 return -ENOMEM; 105 } 106 107 /* Find out how much free trusted ram remains after BL1 load */ 108 mem_layout = bl1_plat_sec_mem_layout(); 109 if ((image_desc->image_info.image_base < mem_layout->free_base) || 110 (image_desc->image_info.image_base + image_size > 111 mem_layout->free_base + mem_layout->free_size)) { 112 WARN("BL1-FWU: Memory not available to copy\n"); 113 return -ENOMEM; 114 } 115 116 /* Update the image size. */ 117 image_desc->image_info.image_size = image_size; 118 119 /* Copy image for given size. */ 120 memcpy((void *)base_addr, (const void *)image_src, block_size); 121 ... 122 123 This code fragment is executed during the 1st invocation of the image copy 124 operation. Both ``block_size`` and ``image_size`` are SMC arguments. A very 125 large value of ``image_size`` may result in an integer overflow in the 2nd 126 ``if`` statement, which would bypass the check, allowing execution to proceed. 127 If ``bl1_plat_mem_check()`` also passes, this may result in an unexpectedly 128 large copy of data into secure memory. 129 130If the platform's implementation of ``bl1_plat_mem_check()`` is correct then it 131may help prevent the above 2 vulnerabilities from being exploited. However, the 132ARM platform version of this function contains a similar vulnerability: 133 134- Line 88 of ``plat/arm/common/arm_bl1_fwu.c`` in function of 135 ``bl1_plat_mem_check()``: 136 137 .. code:: c 138 139 while (mmap[index].mem_size) { 140 if ((mem_base >= mmap[index].mem_base) && 141 ((mem_base + mem_size) 142 <= (mmap[index].mem_base + 143 mmap[index].mem_size))) 144 return 0; 145 146 index++; 147 } 148 ... 149 150 This function checks that the passed memory region is within one of the 151 regions mapped in by ARM platforms. Here, ``mem_size`` may be the 152 ``block_size`` passed from ``bl1_fwu_image_copy()``. A very large value of 153 ``mem_size`` may result in an integer overflow and the function to incorrectly 154 return success. Platforms that copy this insecure pattern will have the same 155 vulnerability. 156 157.. _CVE-2016-10319: http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2016-10319 158.. _48bfb88: https://github.com/ARM-software/arm-trusted-firmware/commit/48bfb88 159.. _Pull Request #783: https://github.com/ARM-software/arm-trusted-firmware/pull/783 160