Difference between revisions of "Unsafe function call from a signal handler"

Revision as of 16:36, 16 April 2006

Overview

There are several functions which - under certain circumstances, if used in a signal handler - may result in the corruption of memory, allowing for exploitation of the process.

Consequences

• Access control: It may be possible to execute arbitrary code through the use of a write-what-where condition.

• Integrity: Signal race conditions often result in data corruption.

Exposure period

• Requirements specification: A language might be chosen which is not subject to this flaw.

• Design: Signal handlers with complicated functionality may result in this issue.

• Implementation: The use of any number of non-reentrant functions will result in this issue.

Platform

• Languages: C, C++, Assembly

• Platforms: All

Required resources

Any

Severity

High

Likelihood of exploit

Low

Avoidance and mitigation

• Requirements specification: A language might be chosen, which is not subject to this flaw, through a guarantee of reentrant code.

• Design: Design signal handlers to only set flags rather than perform complex functionality.

• Implementation: Ensure that non-reentrant functions are not found in signal handlers. Also, use sanity checks to ensure that state is consistently performing asynchronous actions which effect the state of execution.

Discussion

This flaw is a subset of race conditions occurring in signal handler calls which is concerned primarily with memory corruption caused by calls to non-reentrant functions in signal handlers.

Non-reentrant functions are functions that cannot safely be called, interrupted, and then recalled before the first call has finished without resulting in memory corruption. The function call syslog() is an example of this. In order to perform its functionality, it allocates a small amount of memory as "scratch space." If syslog() is suspended by a signal call and the signal handler calls syslog(), the memory used by both of these functions enters an undefined, and possibly, exploitable state.

Examples

See Race condition in signal handler, for an example usage of free() in a signal handler which is exploitable.

Related problems

• Race condition in signal handler

• Write-what-where

Categories