From f2362a57aefff5816dc7cc078dab23de0a2918c4 Mon Sep 17 00:00:00 2001 From: Eduard Zingerman Date: Wed, 25 Jun 2025 11:24:13 -0700 Subject: [PATCH] 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: Alexei Starovoitov Suggested-by: Andrii Nakryiko Signed-off-by: Eduard Zingerman Link: https://lore.kernel.org/r/20250625182414.30659-3-eddyz87@gmail.com Signed-off-by: Alexei Starovoitov --- kernel/bpf/verifier.c | 73 ++++++++++++++++++++++++++++++++++++------- 1 file changed, 61 insertions(+), 12 deletions(-) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index 527fa80be4291..42e497a26d6f6 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -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, @@ -7558,8 +7560,13 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn return -EACCES; } - err = check_mem_region_access(env, regno, off, size, - reg->mem_size, false); + /* + * 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)) mark_reg_unknown(env, regs, value_regno); } else if (reg->type == PTR_TO_CTX) { @@ -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)) { + 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\n"); + "kfunc bpf_rdonly_cast type ID argument must be of a struct or void\n"); return -EINVAL; } - - 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 (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 | -- 2.47.2