MSC16-C. Consider encrypting function pointers

The location in memory of a function is computed at compile time and then stored for later use by the program. If an attacker can overwrite certain function pointers, they may be able to execute arbitrary code. To mitigate the effects of such attacks, pointers to functions may be encrypted at run time based on some characteristics of the execution process such that only a running process will be able to decode them.

Noncompliant Code Example

This noncompliant code example assigns the address of the printf() function to the log_fn function pointer, which may be in allocated in the stack or data segment.

int (*log_fn)(const char *, ...) = printf;
/* ... */
log_fn("foo");

If a vulnerability exists in this program that allows an attacker to overwrite the log_fn function pointer, such as a buffer overflow or arbitrary memory write, the attacker may be able to overwrite the value of printf with the location of an arbitrary function.

Compliant Solution (Windows)

Microsoft Windows provides the EncodePointer() and DecodePointer() function that encrypt and decrypt pointers using a secret that is unique to the given process.

int (*log_fn)(const char *, ...) = EncodePointer(printf);
/* ... */
DecodePointer(log_fn)("foo");

Compliant Solution (C1X)

Two similar functions are under consideration for the C1X major revision to the C Standard.

C1X defines encode_pointer() to have the following behavior:

The encode_pointer function shall perform a transformation on the pf argument, such that the decode_pointer function shall reverse that transformation. Thus, for any pointer to function pfun,

decode_pointer(encode_pointer( (void(*)()) pfun )

when converted to the type of pfun, shall equal pfun.

int (*log_fn)(const char *, ...) = encode_pointer(printf);
/* ... */
decode_pointer(log_fn)("foo");

Risk Assessment

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

C++ Secure Coding Standard: MSC16-CPP. Consider encrypting function pointers

Bibliography

[MSDN] EncodePointer(), DecodePointer()
[MITRE 2007] CWE-311, "Missing Encryption of Sensitive Data"

      49. Miscellaneous (MSC)