Buffer overflow protection refers to various techniques used during software development to enhance the security of executable programs by detecting buffer overflows on stack-allocated variables as they occur and preventing them from becoming serious security vulnerabilities. There have been several implementations of buffer overflow protection.
This article deals with stack-based overflow; similar protections also exist against heap-based overflows, but they are implementation-specific.
How it works
Main article: Stack buffer overflow
Typically, buffer overflow protection modifies the organization of data in the stack frame of a function call to include a "canary" value which, when destroyed, shows that a buffer preceding it in memory has been overflowed. This gives the benefit of preventing an entire class of attacks. According to some software vendors[who?], the performance impact of these techniques is negligible.
Canaries
Canaries or canary words are known values that are placed between a buffer and control data on the stack to monitor buffer overflows. When the buffer overflows, the first data to be corrupted will be the canary, and a failed verification of the canary data is therefore an alert of an overflow, which can then be handled, for example, by invalidating the corrupted data.
The terminology is a reference to the historic practice of using canaries in coal mines, since they would be affected by toxic gases earlier than the miners, thus providing a biological warning system.
There are three types of canaries in use: Terminator, Random, and Random XOR. Current versions of StackGuard support all three, while ProPolice supports Terminator and Random canaries.
Terminator canaries
Terminator Canaries use the observation that most buffer overflow attacks are based on certain string operations which end at terminators. The reaction to this observation is that the canaries are built of NULL terminators, CR, LF, and -1. The undesirable result is that the canary is known. Even with the protection, an attacker could potentially overwrite the canary with its known value, and control information with mismatched values, thus passing the canary check code, this latter being executed soon before the specific processor return-from-call instruction.
Random canaries
Random canaries are randomly generated, usually from an entropy-gathering daemon, in order to prevent an attacker from knowing their value. Usually, it is not logically possible or plausible to read the canary for exploiting; the canary is a secure value known only by those who need to know it—the buffer overflow protection code in this case.
Normally, a random canary is generated at program initialization, and stored in a global variable. This variable is usually padded by unmapped pages, so that attempting to read it using any kinds of tricks that exploit bugs to read off RAM cause a segmentation fault, terminating the program. It may still be possible to read the canary, if the attacker knows where it is, or can get the program to read from the stack.
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