Subject: Re: [PATCH v3 net-next 1/2] tools: psock_lib: tighten conditions checked in sock_setfilter

From: Daniel Borkmann <[email-masked]>
Date: Wed, 04 Jan 2017 23:16:23 +0100

On 01/04/2017 07:45 PM, Sowmini Varadhan wrote:
> The bpf_prog used in sock_setfilter() only attempts to check for
> ip pktlen, and verifies that the contents of the 80'th packet in
> the ethernet frame is 'a' or 'b'.  Thus many non-udp packets
> could incorrectly pass through this filter and cause incorrect
> test results.
>
> This commit hardens the conditions checked by the filter so
> that only UDP/IPv4 packets with the matching length and test-character
> will be permitted by the filter. The filter has been cleaned up
> to explicitly use the BPF macros to make it more readable.
>
> Signed-off-by: Sowmini Varadhan <[email-masked]>
> Acked-by: Willem de Bruijn <[email-masked]>
> ---
> v2: commit comment edited based on Willem de Bruijn review
> v3: Shuah Khan nit.
>
>   tools/testing/selftests/net/psock_lib.h |   29 ++++++++++++++++++++++-------
>   1 files changed, 22 insertions(+), 7 deletions(-)
>
> diff --git a/tools/testing/selftests/net/psock_lib.h b/tools/testing/selftests/net/psock_lib.h
> index 24bc7ec..9e5553a 100644
> --- a/tools/testing/selftests/net/psock_lib.h
> +++ b/tools/testing/selftests/net/psock_lib.h
> @@ -27,6 +27,7 @@
>   #include <string.h>
>   #include <arpa/inet.h>
>   #include <unistd.h>
> +#include <netinet/udp.h>
>
>   #define DATA_LEN			100
>   #define DATA_CHAR			'a'
> @@ -40,14 +41,28 @@
>
>   static __maybe_unused void sock_setfilter(int fd, int lvl, int optnum)
>   {
> +	uint16_t ip_len = DATA_LEN +
> +			  sizeof(struct iphdr) +
> +			  sizeof(struct udphdr);
> +	/* the filter below checks for all of the following conditions that
> +	 * are based on the contents of create_payload()
> +	 *  ether type 0x800 and
> +	 *  ip proto udp     and
> +	 *  ip len == ip_len and
> +	 *  udp[38] == 'a' or udp[38] == 'b'
> +	 */
>   	struct sock_filter bpf_filter[] = {
> -		{ 0x80, 0, 0, 0x00000000 },  /* LD  pktlen		      */
> -		{ 0x35, 0, 4, DATA_LEN   },  /* JGE DATA_LEN  [f goto nomatch]*/
> -		{ 0x30, 0, 0, 0x00000050 },  /* LD  ip[80]		      */
> -		{ 0x15, 1, 0, DATA_CHAR  },  /* JEQ DATA_CHAR   [t goto match]*/
> -		{ 0x15, 0, 1, DATA_CHAR_1},  /* JEQ DATA_CHAR_1 [t goto match]*/
> -		{ 0x06, 0, 0, 0x00000060 },  /* RET match	              */
> -		{ 0x06, 0, 0, 0x00000000 },  /* RET no match		      */
> +		BPF_STMT(BPF_LD | BPF_H | BPF_ABS, 12),	/* LD ethertype */
> +		BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, ETH_P_IP, 0, 8),
> +		BPF_STMT(BPF_LD|BPF_B|BPF_ABS, 23),	/* LD ip_proto */
> +		BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, IPPROTO_UDP, 0, 6),
> +		BPF_STMT(BPF_LD|BPF_H|BPF_ABS, 16),	/* LD ip_len */
> +		BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, ip_len, 0, 4),
> +		BPF_STMT(BPF_LD|BPF_B|BPF_ABS, 80),	/* LD udp[38] */
> +		BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, DATA_CHAR, 1, 0),
> +		BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, DATA_CHAR_1, 0, 1),
> +		BPF_STMT(BPF_RET | BPF_K, ~0),		/* match */
> +		BPF_STMT(BPF_RET | BPF_K, 0)		/* no match */

Just reading up on the thread, sorry to jump in late. Can't you just
use the generated code from bpf_asm (tools/net/) and add the asm program
as a comment above? Something like we do in net/core/ptp_classifier.c +13.
As it stands it makes it a bit harder to parse / less readable with macros
actually. Rest seems fine, thanks.

>   	};
>   	struct sock_fprog bpf_prog;
>
>