ASA-2018-00021 – U-Boot: Insufficient boundary checks in filesystem image load


Allele Security Alert

ASA-2018-00021

Identifier(s)

ASA-2018-00021, CVE-2018-18440

Title

Insufficient boundary checks in filesystem image load

Vendor(s)

DENX

Product(s)

U-Boot

Affected version(s)

All released U-Boot versions

Fixed version(s)

Unknown

Proof of concept

Unknown

Description

The U-Boot bootloader supports kernel loading from a variety of filesystem formats, through the `load` command or its filesystem specific equivalents (e.g. `ext2load`, `ext4load`, `fatload`, etc.)

These commands do not protect system memory from being overwritten when loading files of a length that exceeds the boundaries of the relocated U-Boot memory region, filled with the loaded file starting from the passed `addr` variable.

Therefore an excessively large boot image, saved on the filesystem, can be crafted to overwrite all U-Boot static and runtime memory segments, and in general all device addressable memory starting from the `addr` load address argument.

The memory overwrite can directly lead to arbitrary code execution, fully controlled by the contents of the loaded image.

When verified boot is implemented, the issue allows to bypass its intended validation as the memory overwrite happens before any validation can take place.

Reference(s)

Security advisory: U-Boot verified boot bypass
https://github.com/inversepath/usbarmory/blob/master/software/secure_boot/Security_Advisory-Ref_IPVR2018-0001.txt

CVE-2018-18439, CVE-2018-18440 – U-Boot verified boot bypass vulnerabilities
https://seclists.org/oss-sec/2018/q4/125

CVE-2018-18440
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2018-18440

CVE-2018-18440
https://nvd.nist.gov/vuln/detail/CVE-2018-18440

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Last modified: February 1, 2019

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