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关于截获execve等系统调用，很久以来存在一个问题：新函数不能直接调旧函数，
否则导致stack不平衡，出错。
曾经有高人用一串的汇编代码去平衡堆栈， 但对于偶们这些汇编菜鸟来说， 连阅读都很困难。
而且， 好像gcc4.x下不支持 它使用的一种寻址方式了。
这次使用kprobes来直接找do_execve，直接照搬sys_execve的实现代码。
至于LICENSE的问题，谁用谁负责，我只是从技术角度上说可以这样做。(我的代码是GPL的)
只截获了open和execve作为示例，其它的syscall都是一样的道理。
README:
Can work at i386, linux 2.6.22.
We only intercepted 1) sys_open, and 2) sys_execve, for an illustrating purpose.
New Feautres:
-> Check and verify the WP bit of CR0.
-> Intercept execve with the help of kprobes. Likewise, fork/vfork/clone
can be intercepted in the same way.
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源代码：
#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 (alert7@xfocus.org) \n\t\talbcamus <albcamus@gmail.com>");
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);
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发表于 2007-09-20 18:27
