Fissure Blue Fissure Blue - 3 months ago 6
C Question

How do I prevent an include of files out of system directories?

I have to prevent the

#include
of any files out of system directories because of security reasons. Is there any restrictions that can prevent
#include<...>
and
#include"..."
from including unsafe files like
#include </dev/tty>
or
#include "/dev/random"
?

I have read the header files chapter of Docs of C Preprocessor as well as a similar question How do I prevent a quoted include from searching the directory of the current source file? but can't find a proper way to do so.

Since
-I-
command is obsolete and there is no other way that can accomplish the same function, can I use jailkit to compile the code as a low-privilege user? Or is there other way to ensure the security of compile process?

Answer

godbolt.org solves a similar problem by forbidding absolute and relative paths in #includes:

Input:

#include "/etc/passwd"

Output:

<stdin>:1:1: no absolute or relative includes please
Compilation failed

This can be achieved by running a simple checker on the sources, before invoking the compiler. The checker only has to grep for #include directives and check the paths against violations of such restrictions. A draft version of such a script (usable with a single source file) follows:

check_includes:

#!/bin/bash

if (( $# != 1 ))
then
    echo "Usage: $(basename "$0") source_file"
    exit 1
fi

join_lines()
{
    sed '/\\$/ {N;s/\\\n//;s/^/\n/;D}' "$1"
}

absolute_includes()
{
    join_lines "$1"|grep '^\s*#\s*include\s*["<]\s*/'
}

relative_includes()
{
    join_lines "$1"|grep '^\s*#\s*include'|fgrep '../'
}

includes_via_defines()
{
    join_lines "$1"|grep '^\s*#\s*include\s*[^"< \t]'
}

show_and_count()
{
    tee /dev/stderr|wc -l
}

exit_status=0
if (( 0 != $(absolute_includes "$1"|show_and_count) ))
then
    echo 1>&2 "ERROR: $1 contains absolute includes"
    exit_status=1
fi
if (( 0 != $(relative_includes "$1"|show_and_count) ))
then
    echo 1>&2 "ERROR: $1 contains relative includes"
    exit_status=1
fi
if (( 0 != $(includes_via_defines "$1"|show_and_count) ))
then
    echo 1>&2 "ERROR: $1 contains includes via defines"
    exit_status=1
fi
exit $exit_status

Assuming that the input file can be compiled without errors, this script identifies #includes that contain absolute or relative paths. It also detects includes done via a macro like below (this can be abused to work around the path checking):

#define HEADER "/etc/password"
#include HEADER
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