#include <linux/kernel.h> #include <linux/init.h> #include <linux/module.h> #include <linux/kprobes.h> #include <linux/kallsyms.h> #include <linux/sched.h> #include <linux/ptrace.h> #include <linux/mm.h> #include <linux/smp.h> #include <linux/user.h> #include <linux/errno.h> #include <linux/cpu.h> #include <asm/uaccess.h> #include <asm/fcntl.h> #include <asm/unistd.h> MODULE_DESCRIPTION("Intercept the system call table in Linux"); MODULE_AUTHOR("alert7 ([email protected]) \n\t\talbcamus <[email protected]>"); MODULE_LICENSE("GPL"); /* comment the following line to shut me up */ #define INTERCEPT_DEBUG #ifdef INTERCEPT_DEBUG #define dbgprint(format,args...) \ printk("intercept: function:%s-L%d: "format, __FUNCTION__, __LINE__, ##args); #else #define dbgprint(format,args...) do {} while(0); #endif /** * the system call table */ void **my_table; unsigned int orig_cr0; /** * the original syscall functions */ asmlinkage long (*old_open) (char __user *filename, int flags, int mode); asmlinkage int (*old_execve) (struct pt_regs regs); /** do_execve and do_fork */ unsigned int can_exec_fork = 0; int (*new_do_execve) (char * filename, char __user *__user *argv, char __user *__user *envp, struct pt_regs * regs); struct idtr { unsigned short limit; unsigned int base; } __attribute__ ((packed)); struct idt { unsigned short off1; unsigned short sel; unsigned char none, flags; unsigned short off2; } __attribute__ ((packed)); /** * check if we can intercept fork/vfork/clone/execve or not * * return : 0 for no, 1 for yes */ struct kprobe kp_exec; unsigned int can_intercept_fork_exec(void) { int ret = 0; #ifndef CONFIG_KPROBES return ret; #endif kp_exec.symbol_name = "do_execve"; ret = register_kprobe(&kp_exec); if (ret != 0 ) { dbgprint("cannot find do_execve by kprobe.\n"); return 0; } new_do_execve = ( int (*) (char *, char __user * __user *, char __user * __user *, struct pt_regs * ) ) kp_exec.addr; dbgprint("do_execve at %p\n", (void *)kp_exec.addr); unregister_kprobe(&kp_exec); return 1; } /** * clear WP bit of CR0, and return the original value */ unsigned int clear_and_return_cr0(void) { unsigned int cr0 = 0; unsigned int ret; asm volatile ("movl %%cr0, %%eax" : "=a"(cr0) ); ret = cr0; /* clear the 20 bit of CR0, a.k.a WP bit */ cr0 &= 0xfffeffff; asm volatile ("movl %%eax, %%cr0" : : "a"(cr0) ); return ret; } /** set CR0 with new value * * @val : new value to set in cr0 */ void setback_cr0(unsigned int val) { asm volatile ("movl %%eax, %%cr0" : : "a"(val) ); } /** * Return the first appearence of NEEDLE in HAYSTACK. * */ static void *memmem(const void *haystack, size_t haystack_len, const void *needle, size_t needle_len) {/*{{{*/ const char *begin; const char *const last_possible = (const char *) haystack + haystack_len - needle_len; if (needle_len == 0) /* The first occurrence of the empty string is deemed to occur at the beginning of the string. */ return (void *) haystack; /* Sanity check, otherwise the loop might search through the whole memory. */ if (__builtin_expect(haystack_len < needle_len, 0)) return NULL; for (begin = (const char *) haystack; begin <= last_possible; ++begin) if (begin[0] == ((const char *) needle)[0] && !memcmp((const void *) &begin[1], (const void *) ((const char *) needle + 1), needle_len - 1)) return (void *) begin; return NULL; }/*}}}*/ /** * Find the location of sys_call_table */ static unsigned long get_sys_call_table(void) {/*{{{*/ /* we'll read first 100 bytes of int $0x80 */ #define OFFSET_SYSCALL 100 struct idtr idtr; struct idt idt; unsigned sys_call_off; unsigned retval; char sc_asm[OFFSET_SYSCALL], *p; /* well, let's read IDTR */ asm("sidt %0":"=m"(idtr) : :"memory" ); dbgprint("idtr base at 0x%X\n", (unsigned int)idtr.base); /* Read in IDT for vector 0x80 (syscall) */ memcpy(&idt, (char *) idtr.base + 8 * 0x80, sizeof(idt)); sys_call_off = (idt.off2 << 16) | idt.off1; dbgprint("idt80: flags=%X sel=%X off=%X\n", (unsigned) idt.flags, (unsigned) idt.sel, sys_call_off); /* we have syscall routine address now, look for syscall table dispatch (indirect call) */ memcpy(sc_asm, (void *)sys_call_off, OFFSET_SYSCALL); /** * Search opcode of `call sys_call_table(,eax,4)' */ p = (char *) memmem(sc_asm, OFFSET_SYSCALL, "\xff\x14\x85", 3); if (p == NULL) return 0; retval = *(unsigned *) (p + 3); if (p) { dbgprint("sys_call_table at 0x%x, call dispatch at 0x%x\n", retval, (unsigned int) p); } return retval; #undef OFFSET_SYSCALL }/*}}}*/ /** * new_open - replace the original sys_open when initilazing, * as well as be got rid of when removed */ asmlinkage long new_open(char *filename, int flags, int mode) { dbgprint("hello\n"); return old_open (filename, flags, mode); } /** * new_execve - you should change this function whenever the kernel's sys_execve() * changes */ asmlinkage int new_execve(struct pt_regs regs) { int error; char *filename; dbgprint("Hello\n"); filename = getname( (char __user *) regs.ebx ); error = PTR_ERR(filename); if ( IS_ERR(filename) ) goto out; dbgprint("file to execve: %s\n", filename); error = new_do_execve(filename, (char __user * __user *) regs.ecx, (char __user * __user *) regs.edx, ®s); if (error == 0) { task_lock(current); current->ptrace &= ~PT_DTRACE; task_unlock(current); set_thread_flag(TIF_IRET); } putname (filename); out: return error; } static int intercept_init(void) { my_table = (void **)get_sys_call_table(); if (my_table == NULL) return -1; dbgprint("sys call table address %p\n", (void *) my_table); #define REPLACE(x) old_##x = my_table[__NR_##x];\ my_table[__NR_##x] = new_##x REPLACE(open); can_exec_fork = can_intercept_fork_exec(); if(can_exec_fork == 1) REPLACE(execve); #undef REPLACE return 0; } static int __init this_init(void) { int ret; printk("syscall intercept: Hi, poor linux!\n"); orig_cr0 = clear_and_return_cr0(); ret = intercept_init(); setback_cr0(orig_cr0); return ret; } static void __exit this_fini(void) { printk("syscall intercept: bye, poor linux!\n"); #define RESTORE(x) my_table[__NR_##x] = old_##x orig_cr0 = clear_and_return_cr0(); RESTORE(open); if(can_exec_fork == 1) RESTORE(execve); setback_cr0(orig_cr0); #undef RESTORE } module_init(this_init); module_exit(this_fini); |