Commit 282de143 authored by Kumar Kartikeya Dwivedi's avatar Kumar Kartikeya Dwivedi Committed by Alexei Starovoitov
Browse files

bpf: Introduce allocated objects support 



Introduce support for representing pointers to objects allocated by the
BPF program, i.e. PTR_TO_BTF_ID that point to a type in program BTF.
This is indicated by the presence of MEM_ALLOC type flag in reg->type to
avoid having to check btf_is_kernel when trying to match argument types
in helpers.

Whenever walking such types, any pointers being walked will always yield
a SCALAR instead of pointer. In the future we might permit kptr inside
such allocated objects (either kernel or program allocated), and it will
then form a PTR_TO_BTF_ID of the respective type.

For now, such allocated objects will always be referenced in verifier
context, hence ref_obj_id == 0 for them is a bug. It is allowed to write
to such objects, as long fields that are special are not touched
(support for which will be added in subsequent patches). Note that once
such a pointer is marked PTR_UNTRUSTED, it is no longer allowed to write
to it.

No PROBE_MEM handling is therefore done for loads into this type unless
PTR_UNTRUSTED is part of the register type, since they can never be in
an undefined state, and their lifetime will always be valid.

Signed-off-by: default avatarKumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20221118015614.2013203-6-memxor@gmail.com


Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parent f73e601a

include/linux/bpf.h

+11 −0
Original line number Diff line number Diff line
@@ -525,6 +525,11 @@ enum bpf_type_flag { 
	/* Size is known at compile time. */

	MEM_FIXED_SIZE		= BIT(10 + BPF_BASE_TYPE_BITS),



	/* MEM is of an allocated object of type in program BTF. This is used to

	 * tag PTR_TO_BTF_ID allocated using bpf_obj_new.

	 */

	MEM_ALLOC		= BIT(11 + BPF_BASE_TYPE_BITS),



	__BPF_TYPE_FLAG_MAX,

	__BPF_TYPE_LAST_FLAG	= __BPF_TYPE_FLAG_MAX - 1,

};
@@ -2792,4 +2797,10 @@ struct bpf_key { 
	bool has_ref;

};

#endif /* CONFIG_KEYS */



static inline bool type_is_alloc(u32 type)

{

	return type & MEM_ALLOC;

}



#endif /* _LINUX_BPF_H */

kernel/bpf/btf.c

+5 −0
Original line number Diff line number Diff line
@@ -6034,6 +6034,11 @@ int btf_struct_access(struct bpf_verifier_log *log, 


		switch (err) {

		case WALK_PTR:

			/* For local types, the destination register cannot

			 * become a pointer again.

			 */

			if (type_is_alloc(reg->type))

				return SCALAR_VALUE;

			/* If we found the pointer or scalar on t+off,

			 * we're done.

			 */

kernel/bpf/verifier.c

+23 −2
Original line number Diff line number Diff line
@@ -4687,14 +4687,27 @@ static int check_ptr_to_btf_access(struct bpf_verifier_env *env, 
		return -EACCES;

	}



	if (env->ops->btf_struct_access) {

	if (env->ops->btf_struct_access && !type_is_alloc(reg->type)) {

		if (!btf_is_kernel(reg->btf)) {

			verbose(env, "verifier internal error: reg->btf must be kernel btf\n");

			return -EFAULT;

		}

		ret = env->ops->btf_struct_access(&env->log, reg, off, size, atype, &btf_id, &flag);

	} else {

		if (atype != BPF_READ) {

		/* Writes are permitted with default btf_struct_access for

		 * program allocated objects (which always have ref_obj_id > 0),

		 * but not for untrusted PTR_TO_BTF_ID | MEM_ALLOC.

		 */

		if (atype != BPF_READ && reg->type != (PTR_TO_BTF_ID | MEM_ALLOC)) {

			verbose(env, "only read is supported\n");

			return -EACCES;

		}



		if (type_is_alloc(reg->type) && !reg->ref_obj_id) {

			verbose(env, "verifier internal error: ref_obj_id for allocated object must be non-zero\n");

			return -EFAULT;

		}



		ret = btf_struct_access(&env->log, reg, off, size, atype, &btf_id, &flag);

	}
@@ -5973,6 +5986,7 @@ int check_func_arg_reg_off(struct bpf_verifier_env *env, 
	 * fixed offset.

	 */

	case PTR_TO_BTF_ID:

	case PTR_TO_BTF_ID | MEM_ALLOC:

		/* When referenced PTR_TO_BTF_ID is passed to release function,

		 * it's fixed offset must be 0.	In the other cases, fixed offset

		 * can be non-zero.
@@ -13690,6 +13704,13 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env) 
			break;

		case PTR_TO_BTF_ID:

		case PTR_TO_BTF_ID | PTR_UNTRUSTED:

		/* PTR_TO_BTF_ID | MEM_ALLOC always has a valid lifetime, unlike

		 * PTR_TO_BTF_ID, and an active ref_obj_id, but the same cannot

		 * be said once it is marked PTR_UNTRUSTED, hence we must handle

		 * any faults for loads into such types. BPF_WRITE is disallowed

		 * for this case.

		 */

		case PTR_TO_BTF_ID | MEM_ALLOC | PTR_UNTRUSTED:

			if (type == BPF_READ) {

				insn->code = BPF_LDX | BPF_PROBE_MEM |

					BPF_SIZE((insn)->code);