Date:   Mon, 24 Sep 2018 14:24:51 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Joe Stringer <joe@...d.net.nz>, ast@...nel.org
Cc:     netdev@...r.kernel.org, john.fastabend@...il.com, tgraf@...g.ch,
        kafai@...com, nitin.hande@...il.com, mauricio.vasquez@...ito.it
Subject: Re: [PATCHv2 bpf-next 04/11] bpf: Add PTR_TO_SOCKET verifier type

On 09/21/2018 07:10 PM, Joe Stringer wrote:
> Teach the verifier a little bit about a new type of pointer, a
> PTR_TO_SOCKET. This pointer type is accessed from BPF through the
> 'struct bpf_sock' structure.
> 
> Signed-off-by: Joe Stringer <joe@...d.net.nz>
> 
> ---
> 
> v2: Reuse reg_type_mismatch() in more places
>     Reduce the number of passes at convert_ctx_access()
> ---
>  include/linux/bpf.h          |  17 +++++
>  include/linux/bpf_verifier.h |   2 +
>  kernel/bpf/verifier.c        | 120 +++++++++++++++++++++++++++++++----
>  net/core/filter.c            |  30 +++++----
>  4 files changed, 143 insertions(+), 26 deletions(-)
> 
> diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> index 988a00797bcd..daeb0d343d9c 100644
> --- a/include/linux/bpf.h
> +++ b/include/linux/bpf.h
> @@ -154,6 +154,7 @@ enum bpf_arg_type {
>  
>  	ARG_PTR_TO_CTX,		/* pointer to context */
>  	ARG_ANYTHING,		/* any (initialized) argument is ok */
> +	ARG_PTR_TO_SOCKET,	/* pointer to bpf_sock */
>  };
>  
>  /* type of values returned from helper functions */
> @@ -162,6 +163,7 @@ enum bpf_return_type {
>  	RET_VOID,			/* function doesn't return anything */
>  	RET_PTR_TO_MAP_VALUE,		/* returns a pointer to map elem value */
>  	RET_PTR_TO_MAP_VALUE_OR_NULL,	/* returns a pointer to map elem value or NULL */
> +	RET_PTR_TO_SOCKET_OR_NULL,	/* returns a pointer to a socket or NULL */
>  };
>  
>  /* eBPF function prototype used by verifier to allow BPF_CALLs from eBPF programs
> @@ -213,6 +215,8 @@ enum bpf_reg_type {
>  	PTR_TO_PACKET,		 /* reg points to skb->data */
>  	PTR_TO_PACKET_END,	 /* skb->data + headlen */
>  	PTR_TO_FLOW_KEYS,	 /* reg points to bpf_flow_keys */
> +	PTR_TO_SOCKET,		 /* reg points to struct bpf_sock */
> +	PTR_TO_SOCKET_OR_NULL,	 /* reg points to struct bpf_sock or NULL */
>  };
>  
>  /* The information passed from prog-specific *_is_valid_access
> @@ -335,6 +339,11 @@ const struct bpf_func_proto *bpf_get_trace_printk_proto(void);
>  
>  typedef unsigned long (*bpf_ctx_copy_t)(void *dst, const void *src,
>  					unsigned long off, unsigned long len);
> +typedef u32 (*bpf_convert_ctx_access_t)(enum bpf_access_type type,
> +					const struct bpf_insn *src,
> +					struct bpf_insn *dst,
> +					struct bpf_prog *prog,
> +					u32 *target_size);
>  
>  u64 bpf_event_output(struct bpf_map *map, u64 flags, void *meta, u64 meta_size,
>  		     void *ctx, u64 ctx_size, bpf_ctx_copy_t ctx_copy);
> @@ -828,4 +837,12 @@ extern const struct bpf_func_proto bpf_get_local_storage_proto;
>  void bpf_user_rnd_init_once(void);
>  u64 bpf_user_rnd_u32(u64 r1, u64 r2, u64 r3, u64 r4, u64 r5);
>  
> +bool bpf_sock_is_valid_access(int off, int size, enum bpf_access_type type,
> +			      struct bpf_insn_access_aux *info);
> +u32 bpf_sock_convert_ctx_access(enum bpf_access_type type,
> +			        const struct bpf_insn *si,
> +			        struct bpf_insn *insn_buf,
> +			        struct bpf_prog *prog,
> +			        u32 *target_size);
> +
>  #endif /* _LINUX_BPF_H */
> diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
> index af262b97f586..23a2b17bfd75 100644
> --- a/include/linux/bpf_verifier.h
> +++ b/include/linux/bpf_verifier.h
> @@ -58,6 +58,8 @@ struct bpf_reg_state {
>  	 * offset, so they can share range knowledge.
>  	 * For PTR_TO_MAP_VALUE_OR_NULL this is used to share which map value we
>  	 * came from, when one is tested for != NULL.
> +	 * For PTR_TO_SOCKET this is used to share which pointers retain the
> +	 * same reference to the socket, to determine proper reference freeing.
>  	 */
>  	u32 id;
>  	/* For scalar types (SCALAR_VALUE), this represents our knowledge of
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 7dccb18ede03..1fee63d82290 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -80,8 +80,8 @@ static const struct bpf_verifier_ops * const bpf_verifier_ops[] = {
>   * (like pointer plus pointer becomes SCALAR_VALUE type)
>   *
>   * When verifier sees load or store instructions the type of base register
> - * can be: PTR_TO_MAP_VALUE, PTR_TO_CTX, PTR_TO_STACK. These are three pointer
> - * types recognized by check_mem_access() function.
> + * can be: PTR_TO_MAP_VALUE, PTR_TO_CTX, PTR_TO_STACK, PTR_TO_SOCKET. These are
> + * four pointer types recognized by check_mem_access() function.
>   *
>   * PTR_TO_MAP_VALUE means that this register is pointing to 'map element value'
>   * and the range of [ptr, ptr + map's value_size) is accessible.
> @@ -267,6 +267,8 @@ static const char * const reg_type_str[] = {
>  	[PTR_TO_PACKET_META]	= "pkt_meta",
>  	[PTR_TO_PACKET_END]	= "pkt_end",
>  	[PTR_TO_FLOW_KEYS]	= "flow_keys",
> +	[PTR_TO_SOCKET]		= "sock",
> +	[PTR_TO_SOCKET_OR_NULL] = "sock_or_null",
>  };
>  
>  static char slot_type_char[] = {
> @@ -973,6 +975,8 @@ static bool is_spillable_regtype(enum bpf_reg_type type)
>  	case PTR_TO_PACKET_END:
>  	case PTR_TO_FLOW_KEYS:
>  	case CONST_PTR_TO_MAP:
> +	case PTR_TO_SOCKET:
> +	case PTR_TO_SOCKET_OR_NULL:
>  		return true;
>  	default:
>  		return false;
> @@ -1341,6 +1345,28 @@ static int check_flow_keys_access(struct bpf_verifier_env *env, int off,
>  	return 0;
>  }
>  
> +static int check_sock_access(struct bpf_verifier_env *env, u32 regno, int off,
> +			     int size, enum bpf_access_type t)
> +{
> +	struct bpf_reg_state *regs = cur_regs(env);
> +	struct bpf_reg_state *reg = &regs[regno];
> +	struct bpf_insn_access_aux info;
> +
> +	if (reg->smin_value < 0) {
> +		verbose(env, "R%d min value is negative, either use unsigned index or do a if (index >=0) check.\n",
> +			regno);
> +		return -EACCES;
> +	}
> +
> +	if (!bpf_sock_is_valid_access(off, size, t, &info)) {
> +		verbose(env, "invalid bpf_sock_ops access off=%d size=%d\n",
> +			off, size);
> +		return -EACCES;
> +	}
> +
> +	return 0;
> +}
> +
>  static bool __is_pointer_value(bool allow_ptr_leaks,
>  			       const struct bpf_reg_state *reg)
>  {
> @@ -1459,6 +1485,9 @@ static int check_ptr_alignment(struct bpf_verifier_env *env,
>  		 */
>  		strict = true;
>  		break;
> +	case PTR_TO_SOCKET:
> +		pointer_desc = "sock ";
> +		break;
>  	default:
>  		break;
>  	}
> @@ -1726,6 +1755,14 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
>  		err = check_flow_keys_access(env, off, size);
>  		if (!err && t == BPF_READ && value_regno >= 0)
>  			mark_reg_unknown(env, regs, value_regno);
> +	} else if (reg->type == PTR_TO_SOCKET) {
> +		if (t == BPF_WRITE) {
> +			verbose(env, "cannot write into socket\n");
> +			return -EACCES;
> +		}
> +		err = check_sock_access(env, regno, off, size, t);
> +		if (!err && value_regno >= 0)
> +			mark_reg_unknown(env, regs, value_regno);
>  	} else {
>  		verbose(env, "R%d invalid mem access '%s'\n", regno,
>  			reg_type_str[reg->type]);
> @@ -1949,6 +1986,10 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 regno,
>  		err = check_ctx_reg(env, reg, regno);
>  		if (err < 0)
>  			return err;
> +	} else if (arg_type == ARG_PTR_TO_SOCKET) {
> +		expected_type = PTR_TO_SOCKET;
> +		if (type != expected_type)
> +			goto err_type;
>  	} else if (arg_type_is_mem_ptr(arg_type)) {
>  		expected_type = PTR_TO_STACK;
>  		/* One exception here. In case function allows for NULL to be
> @@ -2542,6 +2583,10 @@ static int check_helper_call(struct bpf_verifier_env *env, int func_id, int insn
>  		}
>  		regs[BPF_REG_0].map_ptr = meta.map_ptr;
>  		regs[BPF_REG_0].id = ++env->id_gen;
> +	} else if (fn->ret_type == RET_PTR_TO_SOCKET_OR_NULL) {
> +		mark_reg_known_zero(env, regs, BPF_REG_0);
> +		regs[BPF_REG_0].type = PTR_TO_SOCKET_OR_NULL;
> +		regs[BPF_REG_0].id = ++env->id_gen;
>  	} else {
>  		verbose(env, "unknown return type %d of func %s#%d\n",
>  			fn->ret_type, func_id_name(func_id), func_id);
> @@ -2679,6 +2724,8 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,
>  		return -EACCES;
>  	case CONST_PTR_TO_MAP:
>  	case PTR_TO_PACKET_END:
> +	case PTR_TO_SOCKET:
> +	case PTR_TO_SOCKET_OR_NULL:
>  		verbose(env, "R%d pointer arithmetic on %s prohibited\n",
>  			dst, reg_type_str[ptr_reg->type]);
>  		return -EACCES;
> @@ -3626,6 +3673,8 @@ static void mark_ptr_or_null_reg(struct bpf_reg_state *reg, u32 id,
>  			} else {
>  				reg->type = PTR_TO_MAP_VALUE;
>  			}
> +		} else if (reg->type == PTR_TO_SOCKET_OR_NULL) {
> +			reg->type = PTR_TO_SOCKET;
>  		}
>  		/* We don't need id from this point onwards anymore, thus we
>  		 * should better reset it, so that state pruning has chances
> @@ -4401,6 +4450,8 @@ static bool regsafe(struct bpf_reg_state *rold, struct bpf_reg_state *rcur,
>  	case CONST_PTR_TO_MAP:
>  	case PTR_TO_PACKET_END:
>  	case PTR_TO_FLOW_KEYS:
> +	case PTR_TO_SOCKET:
> +	case PTR_TO_SOCKET_OR_NULL:
>  		/* Only valid matches are exact, which memcmp() above
>  		 * would have accepted
>  		 */
> @@ -4678,6 +4729,37 @@ static int is_state_visited(struct bpf_verifier_env *env, int insn_idx)
>  	return 0;
>  }
>  
> +/* Return true if it's OK to have the same insn return a different type. */
> +static bool reg_type_mismatch_ok(enum bpf_reg_type type)
> +{
> +	switch (type) {
> +	case PTR_TO_CTX:
> +	case PTR_TO_SOCKET:
> +	case PTR_TO_SOCKET_OR_NULL:
> +		return false;
> +	default:
> +		return true;
> +	}
> +}
> +
> +/* If an instruction was previously used with particular pointer types, then we
> + * need to be careful to avoid cases such as the below, where it may be ok
> + * for one branch accessing the pointer, but not ok for the other branch:
> + *
> + * R1 = sock_ptr
> + * goto X;
> + * ...
> + * R1 = some_other_valid_ptr;
> + * goto X;
> + * ...
> + * R2 = *(u32 *)(R1 + 0);
> + */
> +static bool reg_type_mismatch(enum bpf_reg_type src, enum bpf_reg_type prev)
> +{
> +	return src != prev && (!reg_type_mismatch_ok(src) ||
> +			       !reg_type_mismatch_ok(prev));
> +}
> +
>  static int do_check(struct bpf_verifier_env *env)
>  {
>  	struct bpf_verifier_state *state;
> @@ -4810,9 +4892,7 @@ static int do_check(struct bpf_verifier_env *env)
>  				 */
>  				*prev_src_type = src_reg_type;
>  
> -			} else if (src_reg_type != *prev_src_type &&
> -				   (src_reg_type == PTR_TO_CTX ||
> -				    *prev_src_type == PTR_TO_CTX)) {
> +			} else if (reg_type_mismatch(src_reg_type, *prev_src_type)) {
>  				/* ABuser program is trying to use the same insn
>  				 * dst_reg = *(u32*) (src_reg + off)
>  				 * with different pointer types:
> @@ -4857,9 +4937,7 @@ static int do_check(struct bpf_verifier_env *env)
>  
>  			if (*prev_dst_type == NOT_INIT) {
>  				*prev_dst_type = dst_reg_type;
> -			} else if (dst_reg_type != *prev_dst_type &&
> -				   (dst_reg_type == PTR_TO_CTX ||
> -				    *prev_dst_type == PTR_TO_CTX)) {
> +			} else if (reg_type_mismatch(dst_reg_type, *prev_dst_type)) {
>  				verbose(env, "same insn cannot be used with different pointers\n");
>  				return -EINVAL;
>  			}
> @@ -5276,8 +5354,10 @@ static void sanitize_dead_code(struct bpf_verifier_env *env)
>  	}
>  }
>  
> -/* convert load instructions that access fields of 'struct __sk_buff'
> - * into sequence of instructions that access fields of 'struct sk_buff'
> +/* convert load instructions that access fields of a context type into a
> + * sequence of instructions that access fields of the underlying structure:
> + *     struct __sk_buff    -> struct sk_buff
> + *     struct bpf_sock_ops -> struct sock
>   */
>  static int convert_ctx_accesses(struct bpf_verifier_env *env)
>  {
> @@ -5306,12 +5386,14 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
>  		}
>  	}
>  
> -	if (!ops->convert_ctx_access || bpf_prog_is_dev_bound(env->prog->aux))
> +	if (bpf_prog_is_dev_bound(env->prog->aux))
>  		return 0;
>  
>  	insn = env->prog->insnsi + delta;
>  
>  	for (i = 0; i < insn_cnt; i++, insn++) {
> +		bpf_convert_ctx_access_t convert_ctx_access;
> +
>  		if (insn->code == (BPF_LDX | BPF_MEM | BPF_B) ||
>  		    insn->code == (BPF_LDX | BPF_MEM | BPF_H) ||
>  		    insn->code == (BPF_LDX | BPF_MEM | BPF_W) ||
> @@ -5353,8 +5435,18 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)
>  			continue;
>  		}
>  
> -		if (env->insn_aux_data[i + delta].ptr_type != PTR_TO_CTX)
> +		switch (env->insn_aux_data[i + delta].ptr_type) {
> +		case PTR_TO_CTX:
> +			if (!ops->convert_ctx_access)
> +				continue;
> +			convert_ctx_access = ops->convert_ctx_access;
> +			break;
> +		case PTR_TO_SOCKET:
> +			convert_ctx_access = bpf_sock_convert_ctx_access;
> +			break;
> +		default:
>  			continue;
> +		}
>  
>  		ctx_field_size = env->insn_aux_data[i + delta].ctx_field_size;
>  		size = BPF_LDST_BYTES(insn);

When you compile without NET, this would need to have a stub implementation as
otherwise it might break build. We recently tripped over it in bpf flow dissector
as well.

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