9.1. Advisory TFV-1 (CVE-2016-10319)
Title |
Malformed Firmware Update SMC can result in copy of unexpectedly large data into secure memory |
---|---|
CVE ID |
|
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 |
|
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:
Platform code uses TF BL1 with the
TRUSTED_BOARD_BOOT
build option enabled.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.
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 1stif
statement, which would bypass the check, allowing an unclippedblock_size
to be passed intobl1_plat_mem_check()
. Ifbl1_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
andimage_size
are SMC arguments. A very large value ofimage_size
may result in an integer overflow in the 2ndif
statement, which would bypass the check, allowing execution to proceed. Ifbl1_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 ofbl1_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 theblock_size
passed frombl1_fwu_image_copy()
. A very large value ofmem_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.