286 lines
7.5 KiB
HTML
286 lines
7.5 KiB
HTML
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>Dynamically Linked Libraries</TITLE
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>Secure Programming for Linux and Unix HOWTO</TH
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>Chapter 3. Summary of Linux and Unix Security Features</TD
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><H1
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><A
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NAME="DLLS"
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></A
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>3.7. Dynamically Linked Libraries</H1
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><P
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>Practically all programs depend on libraries to execute.
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In most modern Unix-like systems, including Linux,
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programs are by default compiled to use <EM
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>dynamically linked libraries</EM
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>
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(DLLs).
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That way, you can update a library and all the programs using that library
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will use the new (hopefully improved) version if they can.</P
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><P
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>Dynamically linked libraries are typically placed in one a few special
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directories. The usual directories include
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<TT
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CLASS="FILENAME"
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>/lib</TT
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>, <TT
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CLASS="FILENAME"
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>/usr/lib</TT
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>, <TT
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CLASS="FILENAME"
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>/lib/security</TT
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>
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for PAM modules,
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<TT
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CLASS="FILENAME"
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>/usr/X11R6/lib</TT
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> for X-windows, and <TT
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CLASS="FILENAME"
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>/usr/local/lib</TT
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>.
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You should use these standard conventions in your programs, in particular,
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except during debugging you shouldn't use value computed from the
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current directory as a source for dynamically linked libraries (an
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attacker may be able to add their own choice ``library'' values).</P
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><P
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>There are special conventions for naming libraries and having symbolic
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links for them, with the result that you can update libraries and still
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support programs that want to use old, non-backward-compatible versions
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of those libraries.
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There are also ways to override specific libraries or even just
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specific functions in a library when executing a particular program.
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This is a real advantage of Unix-like systems over
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Windows-like systems; I believe Unix-like systems have a much better system
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for handling library updates, one reason that Unix and Linux systems are reputed
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to be more stable than Windows-based systems.</P
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><P
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>On GNU glibc-based systems, including all Linux systems,
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the list of directories automatically searched during program start-up is
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stored in the file /etc/ld.so.conf.
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Many Red Hat-derived distributions don't normally
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include <TT
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CLASS="FILENAME"
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>/usr/local/lib</TT
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>
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in the file <TT
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CLASS="FILENAME"
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>/etc/ld.so.conf</TT
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>.
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I consider this a bug, and adding <TT
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CLASS="FILENAME"
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>/usr/local/lib</TT
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> to
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<TT
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CLASS="FILENAME"
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>/etc/ld.so.conf</TT
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>
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is a common ``fix'' required to run many programs on Red Hat-derived systems.
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If you want to just override a few functions in a library, but keep the
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rest of the library, you can enter the names of overriding libraries
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(.o files) in <TT
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CLASS="FILENAME"
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>/etc/ld.so.preload</TT
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>;
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these ``preloading'' libraries will take precedence over the standard set.
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This preloading file is typically used for emergency patches;
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a distribution usually won't include such a file when delivered.
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Searching all of these directories at program start-up would be too
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time-consuming, so a caching arrangement is actually used.
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The program ldconfig(8) by default reads in the file /etc/ld.so.conf,
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sets up the appropriate symbolic links in the dynamic link directories
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(so they'll follow the standard conventions),
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and then writes a cache to /etc/ld.so.cache that's then used by other
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programs.
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So, ldconfig has to be run whenever a DLL is added, when a DLL is removed,
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or when the set of DLL directories changes; running ldconfig is often
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one of the steps performed by package managers
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when installing a library.
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On start-up, then, a program uses the dynamic loader to
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read the file /etc/ld.so.cache and then load the libraries it needs.</P
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><P
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>Various environment variables can control this process, and in fact
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there are environment variables that permit you to
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override this process (so, for example, you can temporarily
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substitute a different library for this particular execution).
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In Linux,
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the environment variable
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LD_LIBRARY_PATH is a colon-separated set of directories where libraries
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are searched for first, before the standard set of directories;
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this is useful when debugging a new library or using a nonstandard
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library for special purposes, but be sure you trust those who can
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control those directories.
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The variable LD_PRELOAD lists object files with functions that override
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the standard set, just as /etc/ld.so.preload does.
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The variable LD_DEBUG, displays debugging information; if set
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to ``all'', voluminous information about the dynamic linking process
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is displayed while it's occurring.</P
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><P
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>Permitting user control over dynamically linked libraries
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would be disastrous for setuid/setgid programs if special measures
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weren't taken.
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Therefore, in the GNU glibc implementation, if the program is setuid or setgid
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these variables (and other similar variables) are ignored or greatly
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limited in what they can do.
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The GNU glibc library determines if a program is setuid or setgid
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by checking the program's credentials;
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if the UID and EUID differ, or the GID and the EGID differ, the
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library presumes the program is setuid/setgid (or descended from one)
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and therefore greatly limits its abilities to control linking.
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If you load the GNU glibc libraries, you can see this; see especially
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the files elf/rtld.c and sysdeps/generic/dl-sysdep.c.
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This means that if you cause the UID and GID to equal the EUID and EGID,
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and then call a program, these variables will have full effect.
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Other Unix-like systems handle the situation differently but for the
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same reason: a setuid/setgid program should not be unduly affected
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by the environment variables set.
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Note that graphical user interface toolkits generally do permit
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user control over dynamically linked libraries, because
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executables that directly invoke graphical user inteface toolkits
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should never, ever, be setuid (or have other special privileges) at all.
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For more about how to develop secure GUI applications, see
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<A
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HREF="minimize-privileges.html#MINIMIZE-PRIVILEGED-MODULES"
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>Section 7.4.4</A
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>.</P
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><P
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>For Linux systems, you can get more information from my document, the
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<A
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><EM
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