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@xxxxxxxxx>
Suggested-by: Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx>
Signed-off-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
---
kernel/bpf/verifier.c | 75 +++++++++++++++++++++++++++++++++++--------
1 file changed, 62 insertions(+), 13 deletions(-)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 71de4c9487d5..6b2c38b7a7b6 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -45,7 +45,8 @@ static const struct bpf_verifier_ops * const bpf_verifier_ops[] = {
};
enum bpf_features {
- __MAX_BPF_FEAT = 0,
+ BPF_FEAT_RDONLY_CAST_TO_VOID = 0,
+ __MAX_BPF_FEAT = 1,
};
struct bpf_mem_alloc bpf_global_percpu_ma;
@@ -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:
@@ -19571,10 +19593,27 @@ 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
@@ -19591,15 +19630,24 @@ 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;
@@ -21207,6 +21255,7 @@ 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 |
--
2.47.1
