sdoc -- guide to adding security considerations sections to manual pages
This document presents guidelines for adding security considerations sections
to manual pages. It provides two typical examples.
The guidelines for writing FreeBSD manual pages in groff_mdoc(7) mandate
that each manual page describing a feature of the FreeBSD system should
contain a security considerations section describing what security
requirements can be broken through the misuse of that feature. When
writing these sections, authors should attempt to achieve a happy medium
between two conflicting goals: brevity and completeness. On one hand,
security consideration sections must not be too verbose, or busy readers
might be dissuaded from reading them. On the other hand, security consideration
sections must not be incomplete, or they will fail in their
purpose of instructing the reader on how to avoid all insecure uses.
This document provides guidelines for balancing brevity and completeness
in the security consideration section for a given feature of the FreeBSD
system.
Where to Start [Toc] [Back]
Begin by listing those general security requirements that can be violated
through the misuse of the feature. As described in the FreeBSD Security
Architecture (FSA), there are four classes of security requirements:
integrity (example: non-administrators should not modify system
binaries),
confidentiality (example: non-administrators should not view the
shadow password file),
availability (example: the web server should respond to client
requests in a timely fashion), and
correctness (example: the ps program should provide exactly the
process table information listing functionality described
in its documentation - no more, no less.)
The FSA contains a list of integrity, confidentiality, availability, and
correctness requirements for the base FreeBSD system. Many commands,
tools, and utilities documented in sections 1, 6, and 8 of the manual are
partly responsible for meeting these base system requirements. Consequently,
borrowing entries from the list in the FSA is a good way to
begin the list of requirements for these commands, tools, and utilities.
Complex servers and subsystems may have their own integrity, confidentiality,
availability and correctness requirements in addition to the
system-wide ones listed in the FSA. Listing these additional requirements
will require some thought and analysis. Correctness requirements
will most often deal with configuration issues, especially in cases of
programs that can load modules containing arbitrary functionality during
run-time.
For low-level features, such as the individual functions documented in
sections 2, 3, and 9 of the manual, it is generally sufficient to proceed
with only a single correctness requirement: simply that the function
behaves as advertised.
A good security considerations section should explain how the feature can
be misused to violate each general security requirement in the list.
Each explanation should be accompanied by instructions the reader should
follow in order to avoid a violation. For the sake of brevity, assume
the reader is familiar with all of the concepts in the FSA. When referencing
potential vulnerabilities described in the Secure Programming
Practices man page, sprog(7), likewise cross-reference that document
rather than replicating information. Whenever possible, refer to this
document rather than reproducing the material it contains.
Where to Stop [Toc] [Back]
Security problems are often interrelated; individual problems often have
far-reaching implications. For example, the correctness of virtually any
dynamically-linked program is dependent on the correct implementation and
configuration of the run-time linker. The correctness of this program,
in turn, depends on the correctness of its libraries, the compiler used
to build it, the correctness of the preceding compiler that was used to
build that compiler, and so on, as described by Thompson (see SEE ALSO,
below).
Due to the need for brevity, security consideration sections should
describe only those issues directly related to the feature that is the
subject of the manual page. Refer to other manual pages rather than
duplicating the material found there. Refer to generalized descriptions
of problems in the FSA rather than referring to specific instances of
those problems in other manual pages. Ideally, each specific securityrelevant
issue should be described in exactly one manual page, preferably
as a specific instance of a general problem described in the FSA.
Security considerations sections for most individual functions can follow
this simple formula:
1. Provide one or two sentences describing each potential security
problem, referencing the FSA to provide details whenever
possible.
2. Provide one or two sentences describing how to avoid each
potential security problem.
3. Provide a short example in code.
This is an example security considerations section for the strcpy(3) manual
page:
The strcpy() function is easily misused in a manner which enables malicious
users to arbitrarily change a running program's functionality
through a buffer overflow attack. (See the FSA.)
Avoid using strcpy(). Instead, use strncpy() and ensure that no more
characters are copied to the destination buffer than it can hold. Do not
forget to NUL-terminate the destination buffer, as strncpy() will not
terminate the destination string if it is truncated.
Note that strncpy() can also be problematic. It may be a security concern
for a string to be truncated at all. Since the truncated string
will not be as long as the original, it may refer to a completely different
resource and usage of the truncated resource could result in very
incorrect behavior. Example:
void
foo(const char *arbitrary_string)
{
char onstack[8];
#if defined(BAD)
/*
* This first strcpy is bad behavior. Do not use strcpy()!
*/
(void)strcpy(onstack, arbitrary_string); /* BAD! */
#elif defined(BETTER)
/*
* The following two lines demonstrate better use of
* strncpy().
*/
(void)strncpy(onstack, arbitrary_string, sizeof(onstack) - 1);
onstack[sizeof(onstack - 1)] = '\0';
#elif defined(BEST)
/*
* These lines are even more robust due to testing for
* truncation.
*/
if (strlen(arbitrary_string) + 1 > sizeof(onstack))
err(1, "onstack would be truncated");
(void)strncpy(onstack, arbitrary_string, sizeof(onstack));
#endif
}
Security considerations sections for tools and commands are apt to be
less formulaic. Let your list of potentially-violated security requirements
be your guide; explain each one and list a solution in as concise a
manner as possible.
This is an example security considerations section for the rtld(1) manual
page:
Using the LD_LIBRARY_PATH and LD_PRELOAD environment variables, malicious
users can cause the dynamic linker to link shared libraries of their own
devising into the address space of processes running non-set-userID/group-ID
programs. These shared libraries can arbitrarily change the
functionality of the program by replacing calls to standard library functions
with calls to their own. Although this feature is disabled for
set-user-ID and set-group-ID programs, it can still be used to create
Trojan horses in other programs. (See the FSA.)
All users should be aware that the correct operation of non set-userID/group-ID
dynamically-linked programs depends on the proper configuration
of these environment variables, and take care to avoid actions that
might set them to values which would cause the run-time linker to link in
shared libraries of unknown pedigree.
groff_mdoc(7), security(7), sprog(7)
"The FreeBSD Security Architecture", file:///usr/share/doc/{to be
determined}.
Edward Amoroso, AT&T Bell Laboratories, Fundamentals of Computer Security
Technology, P T R Prentice Hall, 1994.
Ken Thompson, "Reflections on Trusting Trust", Association for Computing
Machinery, Inc., Communications of the ACM, Vol. 27, No. 8, 761-763,
August, 1984.
The sdoc manual page first appeared in FreeBSD 5.0.
Tim Fraser, NAI Labs CBOSS project. <[email protected]>
Brian Feldman, NAI Labs CBOSS project. <[email protected]>
FreeBSD 5.2.1 October 12, 2001 FreeBSD 5.2.1 [ Back ] |
