Let’s check where the GOT entry for printf is: $ objdump -R test | grep -i printf 0000000000600fe0 R_X86_64_GLOB_DAT how the GOT entry changed from R_X86_64_JUMP_SLOT to R_X86_64_GLOB_DAT when full RELRO was used.
Now let’s compile the same program using full RELRO as follows: $ gcc -g -Wl,-z,relro,-z,now -o test testcase.cĬheck if full RELRO was enabled: ~]$ checksec test This demonstrates how partial RELRO is not really effective in preventing GOT overwrite attacks.
When printf is called, arbitrary code is executed. We can see that the GOT is overwritten such that printf now points to attacker-controlled address. (gdb) run 0000000000601018 Starting program: /home/huzaifas/test 0000000000601018 Missing separate debuginfos, use: dnf debuginfo-install 86_64 Program received signal SIGSEGV, Segmentation fault. Let’s check where the GOT entry for printf is located using objdump: $ objdump -R test | grep -i printf 0000000000601018 R_X86_64_JUMP_SLOT let’s run the code via gdb: $ gdb -q. This is an oversimplified form of a buffer overflow, where an attacker has control over the overwrite address.Ĭompile this code with partial RELRO: gcc -g -Wl,-z,relro -o test testcase.cĬheck the binary using checksec (this package is available in Fedora and EPEL): $ checksec -f test The above code attempts to write the hex constant, 0xdeadbeef, at an address provided by the user on the command line. Let us consider the following example code: #include #include int main(int argc, int *argv) Example of exploit mitigation when using RELRO Using Partial RELRO: got.plt both) is marked as read-only.īoth partial and full RELRO reorder the ELF internal data sections to protect them from being overwritten in the event of a buffer-overflow, but only full RELRO mitigates the above mentioned popular technique of overwriting the GOT entry to get control of program execution. Whereas in complete RELRO, the entire GOT (.got and. In partial RELRO, the non-PLT part of the GOT section (.got from readelf output) is read only but. It’s also possible to compile with partial RELRO, which can be achieved by using the "- z,relro " option and not the " -z,now " on the gcc command line. RELRO can be turned on when compiling a program by using the following options: gcc -g -O0 -Wl,-z,relro,-z,now -o This technique is called RELRO and ensures that the GOT cannot be overwritten in vulnerable ELF binaries. To prevent the above mentioned security weakness, we need to ensure that the linker resolves all dynamically linked functions at the beginning of the execution, and then makes the GOT read-only. Since GOT exists at a predefined place in memory, a program that contains a vulnerability allowing an attacker to write 4 bytes at a controlled place in memory (such as some integer overflows leading to out-of-bounds write), may be exploited to allow arbitrary code execution. Lastly, and more importantly, because the GOT is lazily bound it needs to be writable. Secondly, since GOT contains data used by different parts of the program directly, it needs to be allocated at a known static address in memory. Firstly, PLT needs to be located at a fixed offset from the. There are a few implications of the above. This is called "lazy binding." This is because it is unlikely that the location of the shared function has changed and it saves some CPU cycles as well. The second time a function is called, the GOT contains the known location of the function. The location found is then written to the GOT. The first time a shared function is called, the GOT contains a pointer back to the PLT, where the dynamic linker is called to find the actual location of the function in question. The GOT is populated dynamically as the program is running. GOT normally contains pointers that point to the actual location of these functions in the shared libraries in memory.
plt section contains x86 instructions that point directly to the GOT, which lives in the. Such calls point to the Procedure Linkage Table (PLT), which is present in the.
Visit our Red Hat Enterprise Linux (RHEL) Performance Series pageĪ dynamically linked ELF binary uses a look-up table called the Global Offset Table (GOT) to dynamically resolve functions that are located in shared libraries.