Commit f2362a57 authored by Eduard Zingerman's avatar Eduard Zingerman Committed by Alexei Starovoitov
Browse files

bpf: allow void* cast using bpf_rdonly_cast() 



Introduce support for `bpf_rdonly_cast(v, 0)`, which casts the value
`v` to an untyped, untrusted pointer, logically similar to a `void *`.
The memory pointed to by such a pointer is treated as read-only.
As with other untrusted pointers, memory access violations on loads
return zero instead of causing a fault.

Technically:
- The resulting pointer is represented as a register of type
  `PTR_TO_MEM | MEM_RDONLY | PTR_UNTRUSTED` with size zero.
- Offsets within such pointers are not tracked.
- Same load instructions are allowed to have both
  `PTR_TO_MEM | MEM_RDONLY | PTR_UNTRUSTED` and `PTR_TO_BTF_ID`
  as the base pointer types.
  In such cases, `bpf_insn_aux_data->ptr_type` is considered the
  weaker of the two: `PTR_TO_MEM | MEM_RDONLY | PTR_UNTRUSTED`.

The following constraints apply to the new pointer type:
- can be used as a base for LDX instructions;
- can't be used as a base for ST/STX or atomic instructions;
- can't be used as parameter for kfuncs or helpers.

These constraints are enforced by existing handling of `MEM_RDONLY`
flag and `PTR_TO_MEM` of size zero.

Suggested-by: default avatarAlexei Starovoitov <alexei.starovoitov@gmail.com>
Suggested-by: default avatarAndrii Nakryiko <andrii.nakryiko@gmail.com>
Signed-off-by: default avatarEduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250625182414.30659-3-eddyz87@gmail.com


Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
parent b23e97ff

kernel/bpf/verifier.c

+61 −12
Original line number Diff line number Diff line
@@ -45,6 +45,7 @@ static const struct bpf_verifier_ops * const bpf_verifier_ops[] = { 
};









enum bpf_features {









	BPF_FEAT_RDONLY_CAST_TO_VOID = 0,









	__MAX_BPF_FEAT,









};
















@@ -7539,6 +7540,7 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn







		}









	} else if (base_type(reg->type) == PTR_TO_MEM) {









		bool rdonly_mem = type_is_rdonly_mem(reg->type);









		bool rdonly_untrusted = rdonly_mem && (reg->type & PTR_UNTRUSTED);

















		if (type_may_be_null(reg->type)) {









			verbose(env, "R%d invalid mem access '%s'\n", regno,








@@ -7558,6 +7560,11 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn







			return -EACCES;









		}

















		/*









		 * Accesses to untrusted PTR_TO_MEM are done through probe









		 * instructions, hence no need to check bounds in that case.









		 */









		if (!rdonly_untrusted)









			err = check_mem_region_access(env, regno, off, size,









						      reg->mem_size, false);









		if (!err && value_regno >= 0 && (t == BPF_READ || rdonly_mem))








@@ -13606,16 +13613,24 @@ static int check_special_kfunc(struct bpf_verifier_env *env, struct bpf_kfunc_ca







		regs[BPF_REG_0].btf_id = meta->ret_btf_id;









	} else if (meta->func_id == special_kfunc_list[KF_bpf_rdonly_cast]) {









		ret_t = btf_type_by_id(desc_btf, meta->arg_constant.value);









		if (!ret_t || !btf_type_is_struct(ret_t)) {









			verbose(env,









				"kfunc bpf_rdonly_cast type ID argument must be of a struct\n");









		if (!ret_t) {









			verbose(env, "Unknown type ID %lld passed to kfunc bpf_rdonly_cast\n",









				meta->arg_constant.value);









			return -EINVAL;









		}

















		} else if (btf_type_is_struct(ret_t)) {









			mark_reg_known_zero(env, regs, BPF_REG_0);









			regs[BPF_REG_0].type = PTR_TO_BTF_ID | PTR_UNTRUSTED;









			regs[BPF_REG_0].btf = desc_btf;









			regs[BPF_REG_0].btf_id = meta->arg_constant.value;









		} else if (btf_type_is_void(ret_t)) {









			mark_reg_known_zero(env, regs, BPF_REG_0);









			regs[BPF_REG_0].type = PTR_TO_MEM | MEM_RDONLY | PTR_UNTRUSTED;









			regs[BPF_REG_0].mem_size = 0;









		} else {









			verbose(env,









				"kfunc bpf_rdonly_cast type ID argument must be of a struct or void\n");









			return -EINVAL;









		}









	} else if (meta->func_id == special_kfunc_list[KF_bpf_dynptr_slice] ||









		   meta->func_id == special_kfunc_list[KF_bpf_dynptr_slice_rdwr]) {









		enum bpf_type_flag type_flag = get_dynptr_type_flag(meta->initialized_dynptr.type);








@@ -14414,6 +14429,13 @@ static int adjust_ptr_min_max_vals(struct bpf_verifier_env *env,







		return -EACCES;









	}

















	/*









	 * Accesses to untrusted PTR_TO_MEM are done through probe









	 * instructions, hence no need to track offsets.









	 */









	if (base_type(ptr_reg->type) == PTR_TO_MEM && (ptr_reg->type & PTR_UNTRUSTED))









		return 0;

















	switch (base_type(ptr_reg->type)) {









	case PTR_TO_CTX:









	case PTR_TO_MAP_VALUE:








@@ -19622,10 +19644,27 @@ static bool reg_type_mismatch(enum bpf_reg_type src, enum bpf_reg_type prev)







			       !reg_type_mismatch_ok(prev));









}

















static bool is_ptr_to_mem_or_btf_id(enum bpf_reg_type type)









{









	switch (base_type(type)) {









	case PTR_TO_MEM:









	case PTR_TO_BTF_ID:









		return true;









	default:









		return false;









	}









}

















static bool is_ptr_to_mem(enum bpf_reg_type type)









{









	return base_type(type) == PTR_TO_MEM;









}

















static int save_aux_ptr_type(struct bpf_verifier_env *env, enum bpf_reg_type type,









			     bool allow_trust_mismatch)









{









	enum bpf_reg_type *prev_type = &env->insn_aux_data[env->insn_idx].ptr_type;









	enum bpf_reg_type merged_type;

















	if (*prev_type == NOT_INIT) {









		/* Saw a valid insn








@@ -19642,15 +19681,24 @@ static int save_aux_ptr_type(struct bpf_verifier_env *env, enum bpf_reg_type typ







		 * Reject it.









		 */









		if (allow_trust_mismatch &&









		    base_type(type) == PTR_TO_BTF_ID &&









		    base_type(*prev_type) == PTR_TO_BTF_ID) {









		    is_ptr_to_mem_or_btf_id(type) &&









		    is_ptr_to_mem_or_btf_id(*prev_type)) {









			/*









			 * Have to support a use case when one path through









			 * the program yields TRUSTED pointer while another









			 * is UNTRUSTED. Fallback to UNTRUSTED to generate









			 * BPF_PROBE_MEM/BPF_PROBE_MEMSX.









			 * Same behavior of MEM_RDONLY flag.









			 */









			*prev_type = PTR_TO_BTF_ID | PTR_UNTRUSTED;









			if (is_ptr_to_mem(type) || is_ptr_to_mem(*prev_type))









				merged_type = PTR_TO_MEM;









			else









				merged_type = PTR_TO_BTF_ID;









			if ((type & PTR_UNTRUSTED) || (*prev_type & PTR_UNTRUSTED))









				merged_type |= PTR_UNTRUSTED;









			if ((type & MEM_RDONLY) || (*prev_type & MEM_RDONLY))









				merged_type |= MEM_RDONLY;









			*prev_type = merged_type;









		} else {









			verbose(env, "same insn cannot be used with different pointers\n");









			return -EINVAL;








@@ -21258,6 +21306,7 @@ static int convert_ctx_accesses(struct bpf_verifier_env *env)







		 * for this case.









		 */









		case PTR_TO_BTF_ID | MEM_ALLOC | PTR_UNTRUSTED:









		case PTR_TO_MEM | MEM_RDONLY | PTR_UNTRUSTED:









			if (type == BPF_READ) {









				if (BPF_MODE(insn->code) == BPF_MEM)









					insn->code = BPF_LDX | BPF_PROBE_MEM |