Received-SPF: pass (google.com: domain of linux-kernel+bounces-21102-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45e3:2400::1 as permitted sender) client-ip=2604:1380:45e3:2400::1;
Message-ID: <f84ffb6623d2901624337e88daf73ac639b37a2c.camel@gmail.com>
Subject: Re: [PATCH] bpf: Reject variable offset alu on PTR_TO_FLOW_KEYS
From: Eduard Zingerman <eddyz87@gmail.com>
To: Hao Sun <sunhao.th@gmail.com>, bpf@vger.kernel.org
Cc: ppenkov@google.com, willemb@google.com, ast@kernel.org, 
	linux-kernel@vger.kernel.org
Date: Tue, 09 Jan 2024 18:21:01 +0200
In-Reply-To: <20240109153609.10185-1-sunhao.th@gmail.com>
References: <20240109153609.10185-1-sunhao.th@gmail.com>
Autocrypt: addr=eddyz87@gmail.com; prefer-encrypt=mutual; keydata=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
 nYzhLWGcczc6J71q1Dje0l5vIPaSFOgwmWD4DA+WvuxM/shH4rtWeodbv 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
 t1iq+gsfnXbPz5AnS598ScZI1oP7OrPSFJkt/z4acEbOQDQs8aUqrd46PV jsdqGvKnXZxzylux29UTNby4jTlz9pNJM+wPrDRmGfchLDUmf6CffaUYCbu4FiId+9+dcTCDvxbABRy1C3OJ8QY7cxfJ+pEZW18fRJ0XCl/fiV/ecAOfB3HsqgTzAn555h0rkFgay0hAvMU/mAW/CFNSIxV397zm749ZNLA0L2dMy1AKuOqH+/B+/ImBfJMDjmdyJQ8WU/OFRuGLdqOd2oZrA1iuPIa+yUYyZkaZfz/emQwpIL1+Q4p1R/OplA4yc301AqruXXUcVDbEB+joHW3hy5FwK5t5OwTKatrSJBkydSF9zdXy98fYzGniRyRA65P0Ix/8J3BYB4edY2/w0Ip/mdYsYQljBY0A==
Content-Type: text/plain; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable
User-Agent: Evolution 3.50.2 
Precedence: bulk
MIME-Version: 1.0

On Tue, 2024-01-09 at 16:36 +0100, Hao Sun wrote:
> For PTR_TO_FLOW_KEYS, check_flow_keys_access() only uses fixed off
> for validation. However, variable offset ptr alu is not prohibited
> for this ptr kind. So the variable offset is not checked.
>=20
[...]
>=20
> Fixes: d58e468b1112 ("flow_dissector: implements flow dissector BPF hook"=
)
> Signed-off-by: Hao Sun <sunhao.th@gmail.com>
> ---
>  kernel/bpf/verifier.c | 4 ++++
>  1 file changed, 4 insertions(+)
>=20
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index adbf330d364b..65f598694d55 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -12826,6 +12826,10 @@ static int adjust_ptr_min_max_vals(struct bpf_ve=
rifier_env *env,
>  	}
> =20
>  	switch (base_type(ptr_reg->type)) {
> +	case PTR_TO_FLOW_KEYS:
> +		if (known)
> +			break;
> +		fallthrough;
>  	case CONST_PTR_TO_MAP:
>  		/* smin_val represents the known value */
>  		if (known && smin_val =3D=3D 0 && opcode =3D=3D BPF_ADD)

This change makes sense, could you please add a testcase?

Also, this switch is written to explicitly disallow and implicitly allow
pointer arithmetics, which might be a bit unsafe when new ptr types are add=
ed.
Would it make more sense to instead rewrite it to explicitly allow?
E.g. here is what it currently allows / disallows:

| Pointer type        | Arithmetics allowed |
|---------------------+---------------------|
| PTR_TO_CTX          | yes                 |
| CONST_PTR_TO_MAP    | conditionally       |
| PTR_TO_MAP_VALUE    | yes                 |
| PTR_TO_MAP_KEY      | yes                 |
| PTR_TO_STACK        | yes                 |
| PTR_TO_PACKET_META  | yes                 |
| PTR_TO_PACKET       | yes                 |
| PTR_TO_PACKET_END   | no                  |
| PTR_TO_FLOW_KEYS    | yes                 |
| PTR_TO_SOCKET       | no                  |
| PTR_TO_SOCK_COMMON  | no                  |
| PTR_TO_TCP_SOCK     | no                  |
| PTR_TO_TP_BUFFER    | yes                 |
| PTR_TO_XDP_SOCK     | no                  |
| PTR_TO_BTF_ID       | yes                 |
| PTR_TO_MEM          | yes                 |
| PTR_TO_BUF          | yes                 |
| PTR_TO_FUNC         | yes                 |
| CONST_PTR_TO_DYNPTR | yes                 |

Of these PTR_TO_FUNC and CONST_PTR_TO_DYNPTR (?) should not be allowed
as well, probably (not sure if that could be exploited).