我正在尝试为 Kali Linux 2021-W1（Linux 内核版本 5） 上的 bind() 系统调用设置一个挂钩，但由于某种原因，调用了原始系统调用失败并发生错误。
这是我的代码：
/* includes, license, author... */
void **sys_call_table_addr = (void **) 0xffffffff9e0002c0;
int enable_page_rw(void *ptr){
unsigned int level;
pte_t *pte = lookup_address((unsigned long) ptr, &level);
if(pte->pte &~_PAGE_RW){
pte->pte |=_PAGE_RW;
}
return 0;
}
int disable_page_rw(void *ptr){
unsigned int level;
pte_t *pte = lookup_address((unsigned long) ptr, &level);
pte->pte = pte->pte &~_PAGE_RW;
return 0;
}
asmlinkage int (*original_bind) (int, const struct sockaddr *, int);
asmlinkage int log_bind(int sockfd, const struct sockaddr *addr, int addrlen) {
int ret;
printk(KERN_INFO SOCKETLOG "bind was called");
return (*original_bind)(sockfd, addr, addrlen);
}
static int __init socketlog_init(void) {
printk(KERN_INFO SOCKETLOG "socketlog module has been loaded\n");
enable_page_rw(sys_call_table_addr);
original_bind = sys_call_table_addr[__NR_bind];
if (!original_bind) return -1;
sys_call_table_addr[__NR_bind] = log_bind;
disable_page_rw(sys_call_table_addr);
printk(KERN_INFO SOCKETLOG "original_bind = %p", original_bind);
return 0;
}
static void __exit socketlog_exit(void) {
printk(KERN_INFO SOCKETLOG "socketlog module has been unloaded\n");
enable_page_rw(sys_call_table_addr);
sys_call_table_addr[__NR_bind] = original_bind;
disable_page_rw(sys_call_table_addr);
}
module_init(socketlog_init);
module_exit(socketlog_exit);
执行sudo insmod socketlog.ko后，可以看到预期的输出：
[ +0.000488] [SOCKETLOG] socketlog module has been loaded
[ +0.000002] [SOCKETLOG] original_bind = 00000000bbf288f1
但是每次调用 bind() 时，我都会得到奇怪的行为：
[ +0.000488] [SOCKETLOG] bind was called
[ +0.000005] BUG: unable to handle page fault for address: 0000000040697fb8
[ +0.000002] #PF: supervisor read access in kernel mode
[ +0.000001] #PF: error_code(0x0000) - not-present page
正如预期的那样，0x0000000040697fb8 是 0x00000000bbf288f1 指向的地址：原始系统调用的内容。我错过了什么？
答案 0 :(得分：0)
也许您包装系统调用的方式不起作用。例如，在 Linux 5.4.0-59-generic x86_64 架构上，内核中的系统调用是通过一个名为 do_syscall_64() 的通用包装器调用的。它通过pt_regs结构将参数传递给sys_call_table[]中的条目：
__visible void do_syscall_64(unsigned long nr, struct pt_regs *regs)
{
struct thread_info *ti;
enter_from_user_mode();
local_irq_enable();
ti = current_thread_info();
if (READ_ONCE(ti->flags) & _TIF_WORK_SYSCALL_ENTRY)
nr = syscall_trace_enter(regs);
if (likely(nr < NR_syscalls)) {
nr = array_index_nospec(nr, NR_syscalls);
regs->ax = sys_call_table[nr](regs); <-------- Call to the entry with pt_regs structure
#ifdef CONFIG_X86_X32_ABI
} else if (likely((nr & __X32_SYSCALL_BIT) &&
(nr & ~__X32_SYSCALL_BIT) < X32_NR_syscalls)) {
nr = array_index_nospec(nr & ~__X32_SYSCALL_BIT,
X32_NR_syscalls);
regs->ax = x32_sys_call_table[nr](regs);
#endif
}
syscall_return_slowpath(regs);
}
pt_regs 结构嵌入了用户传递给系统调用的参数。所以这可以解释你崩溃的原因：printk(..."bind was called") 工作，因为它不访问参数但在调用原始系统调用条目后不符合预期参数。
如果查看net/socket.c中bind()系统调用的源码，它定义为：
SYSCALL_DEFINE3(bind, int, fd, struct sockaddr __user *, umyaddr, int, addrlen)
{
return __sys_bind(fd, umyaddr, addrlen);
}
上述宏 SYSCALL_DEFINE3() 扩展为一些包装器，这些包装器从 pt_regs 结构中提取参数。
所以，这里是一个例子，你的模块中的一些修复适用于我的 5.4.0-60-generic Ubuntu x86_64：
#include <linux/version.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <asm/ptrace.h>
#include <linux/socket.h>
#include <linux/kallsyms.h>
MODULE_LICENSE("Dual BSD/GPL");
typedef int (* syscall_wrapper)(struct pt_regs *);
unsigned long sys_call_table_addr;
#define SOCKETLOG "[SOCKETLOG]"
int enable_page_rw(void *ptr){
unsigned int level;
pte_t *pte = lookup_address((unsigned long) ptr, &level);
if(pte->pte &~_PAGE_RW){
pte->pte |=_PAGE_RW;
}
return 0;
}
int disable_page_rw(void *ptr){
unsigned int level;
pte_t *pte = lookup_address((unsigned long) ptr, &level);
pte->pte = pte->pte &~_PAGE_RW;
return 0;
}
syscall_wrapper original_bind;
//asmlinkage int log_bind(int sockfd, const struct sockaddr *addr, int addrlen) {
int log_bind(struct pt_regs *regs) {
printk(KERN_INFO SOCKETLOG "bind was called");
return (*original_bind)(regs);
}
static int __init socketlog_init(void) {
printk(KERN_INFO SOCKETLOG "socketlog module has been loaded\n");
sys_call_table_addr = kallsyms_lookup_name("sys_call_table");
printk(KERN_INFO SOCKETLOG "sys_call_table@%lx\n", sys_call_table_addr);
enable_page_rw((void *)sys_call_table_addr);
original_bind = ((syscall_wrapper *)sys_call_table_addr)[__NR_bind];
if (!original_bind) return -1;
((syscall_wrapper *)sys_call_table_addr)[__NR_bind] = log_bind;
disable_page_rw((void *)sys_call_table_addr);
printk(KERN_INFO SOCKETLOG "original_bind = %p", original_bind);
return 0;
}
static void __exit socketlog_exit(void) {
printk(KERN_INFO SOCKETLOG "socketlog module has been unloaded\n");
enable_page_rw((void *)sys_call_table_addr);
((syscall_wrapper *)sys_call_table_addr)[__NR_bind] = original_bind;
disable_page_rw((void *)sys_call_table_addr);
}
module_init(socketlog_init);
module_exit(socketlog_exit);
通过测试：
$ sudo insmod ./bind_ovl.ko
$ dmesg
[ 2253.201888] [SOCKETLOG]socketlog module has been loaded
[ 2253.209486] [SOCKETLOG]sys_call_table@ffffffff88c013a0
[ 2253.209489] [SOCKETLOG]original_bind = 00000000f54304a9
例如，在重新加载 WEB 页面后，我得到：
$ dmesg
[ 2136.946042] [SOCKETLOG]socketlog module has been unloaded
[ 2253.201888] [SOCKETLOG]socketlog module has been loaded
[ 2253.209486] [SOCKETLOG]sys_call_table@ffffffff88c013a0
[ 2253.209489] [SOCKETLOG]original_bind = 00000000f54304a9
[ 2281.716581] [SOCKETLOG]bind was called
[ 2295.607476] [SOCKETLOG]bind was called
[ 2301.947866] [SOCKETLOG]bind was called
[ 2304.088116] [SOCKETLOG]bind was called
[ 2309.599634] [SOCKETLOG]bind was called
[ 2310.946833] [SOCKETLOG]bind was called
卸载模块后：
$ sudo rmmod bind_ovl
$ dmesg
[...]
[ 2390.908456] [SOCKETLOG]bind was called
[ 2398.921475] [SOCKETLOG]bind was called
[ 2398.928855] [SOCKETLOG]socketlog module has been unloaded
您当然可以通过显示传递给系统调用的参数来增强过载。在 x86_64 上，系统调用最多通过处理器寄存器传递 6 个参数。我们可以在 pt_regs 结构中检索它们。后者在 arch/x86/include/asm/ptrace.h 中定义为：
struct pt_regs {
/*
* C ABI says these regs are callee-preserved. They aren't saved on kernel entry
* unless syscall needs a complete, fully filled "struct pt_regs".
*/
unsigned long r15;
unsigned long r14;
unsigned long r13;
unsigned long r12;
unsigned long bp;
unsigned long bx;
/* These regs are callee-clobbered. Always saved on kernel entry. */
unsigned long r11;
unsigned long r10;
unsigned long r9;
unsigned long r8;
unsigned long ax;
unsigned long cx;
unsigned long dx;
unsigned long si;
unsigned long di;
/*
* On syscall entry, this is syscall#. On CPU exception, this is error code.
* On hw interrupt, it's IRQ number:
*/
unsigned long orig_ax;
/* Return frame for iretq */
unsigned long ip;
unsigned long cs;
unsigned long flags;
unsigned long sp;
unsigned long ss;
/* top of stack page */
};
系统调用的参数传递约定是：param#0 到 param#5 分别传入RDI、RSI、RDX、R10、R8 和 R9 寄存器。
根据这个规则，对于bind()系统调用，参数在以下寄存器中：
然后您可以使用以下内容增强日志功能：
int log_bind(struct pt_regs *regs) {
printk(KERN_INFO SOCKETLOG "bind was called(%d, %p, %u)", (int)(regs->di), (void *)(regs->si), (unsigned int)(regs->dx));
return (*original_bind)(regs);
}
来自模块的痕迹变得更加详细：
[ 3259.589915] [SOCKETLOG]socketlog module has been loaded
[ 3259.594631] [SOCKETLOG]sys_call_table@ffffffff88c013a0
[ 3259.594634] [SOCKETLOG]original_bind = 00000000f54304a9
[ 3274.368906] [SOCKETLOG]bind was called(149, 0000000091c163d5, 12)
[ 3276.040330] [SOCKETLOG]bind was called(149, 0000000075b17cb4, 12)
[ 3278.203942] [SOCKETLOG]bind was called(188, 0000000091c163d5, 12)
[ 3287.014980] [SOCKETLOG]bind was called(214, 0000000075b17cb4, 12)
[ 3287.021167] [SOCKETLOG]bind was called(214, 0000000091c163d5, 12)
[ 3298.395713] [SOCKETLOG]bind was called(3, 000000008c2a9103, 12)
[ 3298.403249] [SOCKETLOG]socketlog module has been unloaded