CVE-2022-23222：Linux 内核 eBPF 本地权限提升

近期在对Linux eBPF进行代码审计的过程中，发现了一枚权限提升漏洞CVE-2022-23222。    此漏洞影响Linux Kernel 5.8 - 5.16，并在5.10.92 / 5.15.15 / 5.16.1中修复。完整利用代码详见：https://github.com/tr3ee/CVE-2022-23222----[ 1.1 - eBPF verifer

我们在写eBPF程序时，会发现调用bpf_map_lookup_elem()返回的结果，一定要判断是否为NULL，否则就会被拒

绝加载。因为，bpf_map_lookup_elem()运行结果的结果，可能是有效的指针，但也可能返回NULL来表示没有查找到与key相关的值。在eBPF中有很多类似的用法，那么eBPF verifi er是如何跟踪这些值的类型呢？* bpf.h *

/* types of values stored in eBPF registers */
/* Pointer types represent:
 * pointer
 * pointer + imm
 * pointer + (u16) var
 * pointer + (u16) var + imm
 * if (range > 0) then [ptr, ptr + range - off) is safe to access
 * if (id > 0) means that some 'var' was added
 * if (off > 0) means that 'imm' was added
 */
enum bpf_reg_type {
    NOT_INIT = 0,         /* nothing was written into register */
    SCALAR_VALUE,         /* reg doesn't contain a valid pointer */
    PTR_TO_CTX,           /* reg points to bpf_context */
    CONST_PTR_TO_MAP,     /* reg points to struct bpf_map */
    PTR_TO_MAP_VALUE,     /* reg points to map element value */
    PTR_TO_MAP_VALUE_OR_NULL,  /* points to map elem value or NULL */
    PTR_TO_STACK,         /* reg == frame_pointer + offset */
    PTR_TO_PACKET_META,   /* skb->data - meta_len */
    PTR_TO_PACKET,        /* reg points to skb->data */
    PTR_TO_PACKET_END,    /* skb->data + headlen */
    PTR_TO_FLOW_KEYS,     /* reg points to bpf_flow_keys */
    PTR_TO_SOCKET,        /* reg points to struct bpf_sock */
    PTR_TO_SOCKET_OR_NULL,      /* reg points to struct bpf_sock or NULL */
    PTR_TO_SOCK_COMMON,   /* reg points to sock_common */
    PTR_TO_SOCK_COMMON_OR_NULL, /* reg points to sock_common or NULL */
    PTR_TO_TCP_SOCK,      /* reg points to struct tcp_sock */
    PTR_TO_TCP_SOCK_OR_NULL,    /* reg points to struct tcp_sock or NULL */
    PTR_TO_TP_BUFFER,     /* reg points to a writable raw tp's buffer */
    PTR_TO_XDP_SOCK,      /* reg points to struct xdp_sock */
    /* PTR_TO_BTF_ID points to a kernel struct that does not need
     * to be null checked by the BPF program. This does not imply the
     * pointer is _not_ null and in practice this can easily be a null
     * pointer when reading pointer chains. The assumption is program
     * context will handle null pointer dereference typically via fault
     * handling. The verifier must keep this in mind and can make no
     * assumptions about null or non-null when doing branch analysis.
     * Further, when passed into helpers the helpers can not, without
     * additional context, assume the value is non-null.
     */
    PTR_TO_BTF_ID,
    /* PTR_TO_BTF_ID_OR_NULL points to a kernel struct that has not
     * been checked for null. Used primarily to inform the verifier
     * an explicit null check is required for this struct.
     */
    PTR_TO_BTF_ID_OR_NULL,
    PTR_TO_MEM,           /* reg points to valid memory region */
    PTR_TO_MEM_OR_NULL,   /* reg points to valid memory region or NULL */
    PTR_TO_RDONLY_BUF,    /* reg points to a readonly buffer */
    PTR_TO_RDONLY_BUF_OR_NULL,  /* reg points to a readonly buffer or NULL */
    PTR_TO_RDWR_BUF,      /* reg points to a read/write buffer */
    PTR_TO_RDWR_BUF_OR_NULL,    /* reg points to a read/write buffer or NULL */
    PTR_TO_PERCPU_BTF_ID,       /* reg points to a percpu kernel variable */
};

    上面是eBPF中寄存器的完整类型列表，可以知道它通过`*_OR_NULL`类型来表示一个未知的指针类型。当寄存器的类型是`*_OR_NULL`时，它能做的操作是很有限的，一般只能进行NULL比较或者作为参数调用辅助函数。    一个类型为`*_OR_NULL`的寄存器做完!=NULL的比较后，才可能变为`PTR_TO_*`类型，或者`SCALAR_VALUE`也就是数值0。----[ 1.2 - 漏洞分析* C *

/* Handles arithmetic on a pointer and a scalar: computes new min/max and var_off.
 * Caller should also handle BPF_MOV case separately.
 * If we return -EACCES, caller may want to try again treating pointer as a
 * scalar.  So we only emit a diagnostic if !env->allow_ptr_leaks.
 */
static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,
                   struct bpf_insn *insn,
                   const struct bpf_reg_state *ptr_reg,
                   const struct bpf_reg_state *off_reg)
{
...
    switch (ptr_reg->type) {
    case PTR_TO_MAP_VALUE_OR_NULL:
        verbose(env, "R%d pointer arithmetic on %s prohibited, null-check it first\n",
            dst, reg_type_str[ptr_reg->type]);
        return -EACCES;
    case CONST_PTR_TO_MAP:
        /* smin_val represents the known value */
        if (known && smin_val == 0 && opcode == BPF_ADD)
            break;
        fallthrough;
    case PTR_TO_PACKET_END:
    case PTR_TO_SOCKET:
    case PTR_TO_SOCKET_OR_NULL:
    case PTR_TO_SOCK_COMMON:
    case PTR_TO_SOCK_COMMON_OR_NULL:
    case PTR_TO_TCP_SOCK:
    case PTR_TO_TCP_SOCK_OR_NULL:
    case PTR_TO_XDP_SOCK:
        verbose(env, "R%d pointer arithmetic on %s prohibited\n",
            dst, reg_type_str[ptr_reg->type]);
        return -EACCES;
    default:
        break;
    }
...
    return 0;
}

    上面的代码中adjust_ptr_min_max_vals()是eBPF verifier用于检验指针加减运算的函数。其中的switch分支用于过滤不支持加减运算的指针类型，比如各种OR_NULL类型。但是这个switch分支却少了很多类型的判断，比如`PTR_TO_MEM_OR_NULL`, `PTR_TO_RDONLY_BUF_OR_NULL`,`PTR_TO_RDWR_BUF_OR_NULL`。这意味着，我们可以对一些OR_NULL类型做加减运算！

参考资料

[1] Advisory: https://www.openwall.com/lists/oss-security/2022/01/13/1[2] Exploit Overview: https://www.openwall.com/lists/oss-security/2022/01/18/2[3] Patch: []()[4] Source: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/tree/kernel/bpf/verifier.c?h=v5.10.83