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Date: Mon, 28 Mar 2022 20:28:21 +0200
From: David Bouman <davidbouman35@...il.com>
To: oss-security@...ts.openwall.com
Subject: Linux kernel: CVE-2022-1015,CVE-2022-1016 in nf_tables cause
 privilege escalation, information leak

Hello list,

I'm reporting two linux kernel vulnerabilities in the nf_tables 
component of the netfilter subsystem that I found.

CVE-2022-1015 pertains to an out of bounds access in nf_tables 
expression evaluation due to validation of user register indices. It 
leads to local privilege escalation, for example by overwriting a stack 
return address OOB with a crafted nft_expr_payload.

CVE-2022-1015 is exploitable starting from commit 345023b0db3 
("netfilter: nftables: add nft_parse_register_store() and use it"), 
v5.12 and has been fixed in commit 6e1acfa387b9 ("netfilter: nf_tables: 
validate registers coming from userspace.").

The bug has been present since commit 49499c3e6e18 ("netfilter: 
nf_tables: switch registers to 32 bit addressing"), but to my knowledge 
has not been exploitable until v5.12.

CVE-2022-1016 pertains to uninitialized stack data in the nft_do_chain 
routine. CVE-2022-1016 is exploitable starting from commit 96518518cc41 
(original merge of nf_tables), v3.13-rc1, and has been fixed in commit 
4c905f6740a3 ("netfilter: nf_tables: initialize registers in 
nft_do_chain()").

I will be releasing a detailed blog post and exploit code for both 
vulnerabilities in a few days.

Root cause CVE-2022-1016: (it is the shortest, so I will begin with it)

The nft_do_chain routine in net/netfilter/nf_tables_core.c does not 
initialize the register data that nf_tables expressions can read from- 
and write to. These expressions inherently exhibit side effects that can 
be used to determine the register data, which can contain kernel image 
pointers, module pointers, and allocation pointers depending on the code 
path taken to end up at nft_do_chain.

```
unsigned int
nft_do_chain(struct nft_pktinfo *pkt, void *priv)
{
	const struct nft_chain *chain = priv, *basechain = chain;
	const struct net *net = nft_net(pkt);
	struct nft_rule *const *rules;
	const struct nft_rule *rule;
	const struct nft_expr *expr, *last;
	struct nft_regs regs; // <-------- VULNERABLE! NOT INITIALIZED.
	unsigned int stackptr = 0;
	struct nft_jumpstack jumpstack[NFT_JUMP_STACK_SIZE];
	bool genbit = READ_ONCE(net->nft.gencursor);
	struct nft_traceinfo info;

	info.trace = false;
	if (static_branch_unlikely(&nft_trace_enabled))
		nft_trace_init(&info, pkt, &regs.verdict, basechain);
do_chain:
	if (genbit)
		rules = rcu_dereference(chain->rules_gen_1);
	else
		rules = rcu_dereference(chain->rules_gen_0);

next_rule:
	rule = *rules;
	regs.verdict.code = NFT_CONTINUE;
	for (; *rules ; rules++) {
		rule = *rules;
		nft_rule_for_each_expr(expr, last, rule) {
			if (expr->ops == &nft_cmp_fast_ops)
				nft_cmp_fast_eval(expr, &regs);
			else if (expr->ops == &nft_bitwise_fast_ops)
				nft_bitwise_fast_eval(expr, &regs);
			else if (expr->ops != &nft_payload_fast_ops ||
				 !nft_payload_fast_eval(expr, &regs, pkt))
				expr_call_ops_eval(expr, &regs, pkt);
				...
```

Root cause CVE-2022-1015:

(below is pasted from my original security@...nel.org report)

Hello, I'm mailing to report a vulnerability I found in nf_tables 
component of the netfilter subsystem. The vulnerability gives an 
attacker a powerful primitive that can be used to both read from and 
write to relative stack data. This can lead to arbitrary code execution 
by an attacker.

In order for an unprivileged attacker to exploit this issue, 
unprivileged user- and network namespaces access is required 
(CLONE_NEWUSER | CLONE_NEWNET). The bug relies on a compiler 
optimization that introduces behavior that the maintainer did not 
account for, and most likely only occurs on kernels with 
`CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE=y`. I successfully exploited the bug 
on x86_64 kernel version 5.16-rc3, but I believe this vulnerability 
exists across different kernel versions and architectures (more on this 
later).

Without further ado:

The bug resides in `linux/net/netfilter/nf_tables_api.c`, in the 
`nft_validate_register_store` and `nft_validate_register_load` routines. 
These routines are used to check if nft expression parameters supplied 
by the user are sound and won't cause OOB stack accesses when evaluating 
the expression.

 From my 5.16-rc3 kernel source 
(d58071a8a76d779eedab38033ae4c821c30295a5: Linux 5.16-rc3):

nft_validate_register_store:

```
static int nft_validate_register_store(const struct nft_ctx *ctx,
       enum nft_registers reg,
       const struct nft_data *data,
       enum nft_data_types type,
       unsigned int len)
{
int err;

switch (reg) {
         ...
default:
if (reg < NFT_REG_1 * NFT_REG_SIZE / NFT_REG32_SIZE)
return -EINVAL;
if (len == 0)
return -EINVAL;
if (reg * NFT_REG32_SIZE + len >
    sizeof_field(struct nft_regs, data))
return -ERANGE;

if (data != NULL && type != NFT_DATA_VALUE)
return -EINVAL;
return 0;
}
}
```

nft_validate_register_load:

```
static int nft_validate_register_load(enum nft_registers reg, unsigned 
int len)
{
if (reg < NFT_REG_1 * NFT_REG_SIZE / NFT_REG32_SIZE)
return -EINVAL;
if (len == 0)
return -EINVAL;
if (reg * NFT_REG32_SIZE + len > sizeof_field(struct nft_regs, data))
return -ERANGE;

return 0;
}
```

The problem lies in the fact that `enum nft_registers reg` is not 
guaranteed only be a single byte. As per the C89 specification, 3.1.3.3 
Enumeration constants: `An identifier declared as an enumeration 
constant has type int.`.

Effectively this implies that the compiler is free to emit code that 
operates on `reg` as if it were a 32-bit value. If this is the case (and 
it is on the kernel I tested), a user can forge an expression register 
value that will overflow upon multiplication with `NFT_REG32_SIZE` (4) 
and upon addition with `len`, will be a value smaller than 
`sizeof_field(struct nft_regs, data)` (0x50). Once this check passes, 
the least significant byte of `reg` can still contain a value that will 
index outside of the bounds of the `struct nft_regs regs` that it will 
later be used with.

Take for example a `reg` value of `0xfffffff8` and a `len` value of 
`0x40`. The expression `reg * 4 + len` will then result in `0xffffffe0 + 
0x40 = 0x20`, which is lower than `0x50`. This makes that a value of 
`0xf8` is recognized as a valid index, and is subsequently assigned to a 
register value in the expression info structs.


Here is a snippet of the x86_64 assembly code that these functions might 
generate:

```
Disassembly of section .text:

0000000000002ed0 <nft_validate_register_store>:
     2ed0: e8 00 00 00 00       callq  2ed5 
<nft_validate_register_store+0x5>
     2ed5: 55                   push   %rbp
     2ed6: 48 89 e5             mov    %rsp,%rbp
     2ed9: 41 54                 push   %r12
     2edb: 85 f6                 test   %esi,%esi
     2edd: 75 2b                 jne    2f0a 
<nft_validate_register_store+0x3a>
     2edf: 81 f9 00 ff ff ff     cmp    $0xffffff00,%ecx
     2ee5: 75 49                 jne    2f30 
<nft_validate_register_store+0x60>
     2ee7: 45 31 e4             xor    %r12d,%r12d
     2eea: 48 85 d2             test   %rdx,%rdx
     2eed: 74 3a                 je     2f29 
<nft_validate_register_store+0x59>
     2eef: 8b 02                 mov    (%rdx),%eax
     2ef1: 83 c0 04             add    $0x4,%eax
     2ef4: 83 f8 01             cmp    $0x1,%eax
     2ef7: 77 30                 ja     2f29 
<nft_validate_register_store+0x59>
     2ef9: 48 8b 72 08           mov    0x8(%rdx),%rsi
     2efd: e8 7e da ff ff       callq  980 <nf_tables_check_loops>
     2f02: 85 c0                 test   %eax,%eax
     2f04: 44 0f 4e e0           cmovle %eax,%r12d
     2f08: eb 1f                 jmp    2f29 
<nft_validate_register_store+0x59>
     2f0a: 83 fe 03             cmp    $0x3,%esi
     2f0d: 76 21                 jbe    2f30 
<nft_validate_register_store+0x60>
     2f0f: 45 85 c0             test   %r8d,%r8d
     2f12: 74 1c                 je     2f30 
<nft_validate_register_store+0x60>
     2f14: 41 8d 04 b0           lea    (%r8,%rsi,4),%eax
     2f18: 83 f8 50             cmp    $0x50,%eax
     2f1b: 77 1b                 ja     2f38 
<nft_validate_register_store+0x68>
     2f1d: 48 85 d2             test   %rdx,%rdx
     2f20: 74 04                 je     2f26 
<nft_validate_register_store+0x56>
     2f22: 85 c9                 test   %ecx,%ecx
     2f24: 75 0a                 jne    2f30 
<nft_validate_register_store+0x60>
     2f26: 45 31 e4             xor    %r12d,%r12d
     2f29: 44 89 e0             mov    %r12d,%eax
     2f2c: 41 5c                 pop    %r12
     2f2e: 5d                   pop    %rbp
     2f2f: c3                   retq
     2f30: 41 bc ea ff ff ff     mov    $0xffffffea,%r12d
     2f36: eb f1                 jmp    2f29 
<nft_validate_register_store+0x59>
     2f38: 41 bc de ff ff ff     mov    $0xffffffde,%r12d
     2f3e: eb e9                 jmp    2f29 
<nft_validate_register_store+0x59>
```

the `lea` instruction at `2f14` will multiply `%rsi` (reg) by 4 and add 
`%r8` len to it.

I created a working local privilege escalation exploit by using such an 
out of bounds index to copy stack data to the actual register area 
(declared in nf_tables_core.c:nft_do_chain). Then, I wrote a a few nft 
rules that drop or accept packets depending on whether the targeted byte 
is greater than the constant comparand in the rule or not. This way I 
could create a binary search procedure that could determine the value of 
the leaked byte by registering whether the packet was dropped or not. 
This results in a kernel address leak.

Finally, I used a nft payload expression to write my arbitrary data 
supplied in a packet to the stack in order to overwrite a return address 
and execute a ROP chain.

An alternative exploitation strategy would be to overwrite to verdict 
register (including its chain pointer) to arbitrary values, as you can 
now get an register index of 0 in the same manner.

------------------------------

David Bouman

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