How to turn off gcc compiler optimization to enable buffer overflow

Question

I m working on a homework problem that requires disabling compiler optimization protection for it to work  I m using gcc 4 4 1 on ubuntu linux  but can t figure out which flags are are the right ones  I realize it s architecture dependant - my machine runs w  32-bit Intel processor   Thanks

User · Answer

Urm  all of the answers so far have been wrong with Rook s answer being correct   Entering   echo 0   sudo tee  proc sys kernel randomize va space   Followed by   gcc -fno-stack-protector -z execstack -o bug bug c   Disables ASLR  SSP Propolice and Ubuntu s NoneXec  which was placed in 9 10  and fairly simple to work around see the mprotect 2  technique to map pages as executable and jmp  should help a little  however these  security features  are by no means infallible   Without the  -z execstack  flag  pages have non-executable stack markings

User · Answer

Try the -fno-stack-protector flag

User · Answer

That s a good problem   In order to solve that problem you will also have to disable ASLR otherwise the address of g   will be unpredictable   Disable ASLR   sudo bash -c  echo 0  gt   proc sys kernel randomize va space    Disable canaries   gcc overflow c -o overflow -fno-stack-protector   After canaries and ASLR are disabled it should be a straight forward attack like the ones described in Smashing the Stack for Fun and Profit   Here is a list of security features used in ubuntu  https   wiki ubuntu com Security Features   You don t have to worry about NX bits  the address of g   will always be in a  executable region of memory because it is within the TEXT memory segment   NX bits only come into play if you are trying to execute shellcode on the stack or heap  which is not required for this assignment    Now go and clobber that EIP

User · Answer

I won t quote the entire page but the whole manual on optimisation is available here  http   gcc gnu org onlinedocs gcc-4 4 3 gcc Optimize-Options html Optimize-Options  From the sounds of it you want at least -O0  the default  and      -fmudflap -fmudflapth -fmudflapir      For front-ends that support it  C and C     instrument all risky   pointer array dereferencing   operations  some standard library   string heap functions  and some other   associated constructs with   range validity tests  Modules so   instrumented should be immune to   buffer overflows  invalid heap use    and some other classes of C C     programming errors  The   instrumentation relies on a separate   runtime library  libmudflap   which   will be linked into a program if   -fmudflap is given at link time  Run-time behavior of the instrumented   program is controlled by the   MUDFLAP OPTIONS environment variable    See env MUDFLAP OPTIONS -help a out   for its options

User · Answer

On newer distros  as of 2016   it seems that PIE is enabled by default so you will need to disable it explicitly when compiling    Here s a little summary of commands which can be helpful when playing locally with buffer overflow exercises in general   Disable canary   gcc vuln c -o vuln disable canary -fno-stack-protector   Disable DEP   gcc vuln c -o vuln disable dep -z execstack   Disable PIE   gcc vuln c -o vuln disable pie -no-pie   Disable all of protection mechanisms listed above  warning  for local testing only    gcc vuln c -o vuln disable all -fno-stack-protector -z execstack -no-pie   For 32-bit machines  you ll need to add the -m32 parameter as well

User · Answer

You don t need to disable ASLR in order to do a buffer overflow  Although ASLR is enabled  kernel randomize va space   2   it will not take effect unless the compiled executable is PIE  So unless you compiled your file with -fPIC -pie flag  ASLR will not take effect   I think only disabling the canaries with -fno-stack-protector is enough  If you want to check if ASLR is working or not  Position independent code must be set   use   hardening-check executable name

[c] How to turn off gcc compiler optimization to enable buffer overflow

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