CVE-2017-17806

The HMAC implementation (crypto/hmac.c) in the Linux kernel before 4.14.8 does not validate that the underlying cryptographic hash algorithm is unkeyed, allowing a local attacker able to use the AF_ALG-based hash interface (CONFIG_CRYPTO_USER_API_HASH) and the SHA-3 hash algorithm (CONFIG_CRYPTO_SHA3) to cause a kernel stack buffer overflow by executing a crafted sequence of system calls that encounter a missing SHA-3 initialization.

Weakness

The product writes data past the end, or before the beginning, of the intended buffer.

Affected Software

Potential Mitigations

• Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.

• For example, many languages that perform their own memory management, such as Java and Perl, are not subject to buffer overflows. Other languages, such as Ada and C#, typically provide overflow protection, but the protection can be disabled by the programmer.

• Be wary that a language’s interface to native code may still be subject to overflows, even if the language itself is theoretically safe.

• Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.

• Examples include the Safe C String Library (SafeStr) by Messier and Viega [REF-57], and the Strsafe.h library from Microsoft [REF-56]. These libraries provide safer versions of overflow-prone string-handling functions.

• Use automatic buffer overflow detection mechanisms that are offered by certain compilers or compiler extensions. Examples include: the Microsoft Visual Studio /GS flag, Fedora/Red Hat FORTIFY_SOURCE GCC flag, StackGuard, and ProPolice, which provide various mechanisms including canary-based detection and range/index checking.

• D3-SFCV (Stack Frame Canary Validation) from D3FEND [REF-1334] discusses canary-based detection in detail.

• Consider adhering to the following rules when allocating and managing an application’s memory:

• Run or compile the software using features or extensions that randomly arrange the positions of a program’s executable and libraries in memory. Because this makes the addresses unpredictable, it can prevent an attacker from reliably jumping to exploitable code.

• Examples include Address Space Layout Randomization (ASLR) [REF-58] [REF-60] and Position-Independent Executables (PIE) [REF-64]. Imported modules may be similarly realigned if their default memory addresses conflict with other modules, in a process known as “rebasing” (for Windows) and “prelinking” (for Linux) [REF-1332] using randomly generated addresses. ASLR for libraries cannot be used in conjunction with prelink since it would require relocating the libraries at run-time, defeating the whole purpose of prelinking.

• For more information on these techniques see D3-SAOR (Segment Address Offset Randomization) from D3FEND [REF-1335].

• Use a CPU and operating system that offers Data Execution Protection (using hardware NX or XD bits) or the equivalent techniques that simulate this feature in software, such as PaX [REF-60] [REF-61]. These techniques ensure that any instruction executed is exclusively at a memory address that is part of the code segment.

• For more information on these techniques see D3-PSEP (Process Segment Execution Prevention) from D3FEND [REF-1336].

References

• http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=af3ff8045bbf3e32f1a448542e73abb4c8ceb6f1
• http://lists.opensuse.org/opensuse-security-announce/2018-01/msg00006.html
• http://lists.opensuse.org/opensuse-security-announce/2018-01/msg00007.html
• http://lists.opensuse.org/opensuse-security-announce/2018-01/msg00008.html
• http://lists.opensuse.org/opensuse-security-announce/2018-01/msg00014.html
• http://lists.opensuse.org/opensuse-security-announce/2018-01/msg00016.html
• http://www.securityfocus.com/bid/102293
• https://access.redhat.com/errata/RHSA-2018:2948
• https://github.com/torvalds/linux/commit/af3ff8045bbf3e32f1a448542e73abb4c8ceb6f1
• https://lists.debian.org/debian-lts-announce/2018/01/msg00004.html
• https://usn.ubuntu.com/3583-1/
• https://usn.ubuntu.com/3583-2/
• https://usn.ubuntu.com/3617-1/
• https://usn.ubuntu.com/3617-2/
• https://usn.ubuntu.com/3617-3/
• https://usn.ubuntu.com/3619-1/
• https://usn.ubuntu.com/3619-2/
• https://usn.ubuntu.com/3632-1/
• https://www.debian.org/security/2017/dsa-4073
• https://www.debian.org/security/2018/dsa-4082
• https://www.kernel.org/pub/linux/kernel/v4.x/ChangeLog-4.14.8