Part 6 Buffer Overflow

Understanding the Crash and the Structure of the Function Frame on the Stack

In this part, we are going to see what exactly happens behind the scenes when a buffer overflow causes a crash. After this part, we should
understand why overwriting data on the stack changes the return address.
What are the components of a function frame?
And how can these components be viewed and analyzed with gdb.

When a function is called in C, the system creates a space on the stack for that function. We call this space the function frame. Each frame contains these parts.

Local variables of the function

Parameter values

Saved RBP or base pointer
for return

Saved return address
which the function returns to after the function completes

If more data is written to the local buffer than the limit, these important values ​​are overwritten in the frame
And as a result, the program crashes on return or jumps to the wrong address

This code helps us see the function frame and crash and analyze it in gdb

#include <stdio.h>
#include <string.h>

void crash(char *input) {
char buffer[16];
printf("address of buffer: %p\n", buffer);
strcpy(buffer, input);
printf("done copying\n");
}

int main(int argc, char **argv) {
if (argc < 2) {
printf("usage: %s input\n", argv[0]);
return 1;
}
crash(argv[1]);
printf("returned safely\n");
return 0;
}

Execution and analysis commands

gcc -g file4.c -o file4

gdb --args ./file4 $(python3 -c "print('A'*40)")

After running the program in gdb, perform these steps

break crash

run

info frame

x/32x $rbp

x/32x $rsp

Here you can see that the buffer is below the saved RBP

Every byte you write out of the buffer will eventually reach the saved RBP and then the return address

To view the crash

continue

The program crashes with a segmentation fault error

See the return path with the following command

backtrace

And check the last return address with this command

info registers rip

Full explanation

When executing the crash function, the system first saves the current RBP

Then it moves the RSP down to provide the necessary space for local variables

In this new buffer space, Yes
When we write data larger than the buffer size, first the data is written to local variables, then to RBP, and then to the return address
At the moment the function wants to return, the wrong value is read from the stack and RIP jumps to the wrong address, which causes a segmentation fault

Interesting part

You can see the difference before and after the overflow in gdb with this command

Before strcpy
x/32x $rbp-32

After strcpy
x/32x $rbp-32

You can see that the A bytes have filled all the space between the buffer and the return address

This is the reason for the program crash

@reverseengine

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