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bpf: allow void* cast using bpf_rdonly_cast()

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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 <alexei.starovoitov@gmail.com>
Suggested-by: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250625182414.30659-3-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Eduard Zingerman 2025-06-25 11:24:13 -07:00 • committed by Alexei Starovoitov
parent b23e97ffc2
commit f2362a57ae
1 changed files with 61 additions and 12 deletions
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73
kernel/bpf/verifier.c
View file

@ -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 |