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bpf: rdonly_untrusted_mem for btf id walk pointer leafs

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When processing a load from a PTR_TO_BTF_ID, the verifier calculates
the type of the loaded structure field based on the load offset.
For example, given the following types:

  struct foo {
    struct foo *a;
    int *b;
  } *p;

The verifier would calculate the type of `p->a` as a pointer to
`struct foo`. However, the type of `p->b` is currently calculated as a
SCALAR_VALUE.

This commit updates the logic for processing PTR_TO_BTF_ID to instead
calculate the type of p->b as PTR_TO_MEM|MEM_RDONLY|PTR_UNTRUSTED.
This change allows further dereferencing of such pointers (using probe
memory instructions).

Suggested-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250704230354.1323244-3-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
This commit is contained in:
Eduard Zingerman 2025-07-04 16:03:48 -07:00 committed by Alexei Starovoitov
parent b9d44bc9fd
commit 2d5c91e1cc
3 changed files with 12 additions and 1 deletions
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6
kernel/bpf/btf.c
View File

@ -6915,6 +6915,7 @@ enum bpf_struct_walk_result {
/* < 0 error */
WALK_SCALAR = 0,
WALK_PTR,
WALK_PTR_UNTRUSTED,
WALK_STRUCT,
};
@ -7156,6 +7157,8 @@ static int btf_struct_walk(struct bpf_verifier_log *log, const struct btf *btf,
*field_name = mname;
return WALK_PTR;
}
return WALK_PTR_UNTRUSTED;
}
/* Allow more flexible access within an int as long as
@ -7228,6 +7231,9 @@ int btf_struct_access(struct bpf_verifier_log *log,
*next_btf_id = id;
*flag = tmp_flag;
return PTR_TO_BTF_ID;
case WALK_PTR_UNTRUSTED:
*flag = MEM_RDONLY | PTR_UNTRUSTED;
return PTR_TO_MEM;
case WALK_SCALAR:
return SCALAR_VALUE;
case WALK_STRUCT:

5
kernel/bpf/verifier.c
View File

@ -2814,6 +2814,11 @@ static int mark_btf_ld_reg(struct bpf_verifier_env *env,
if (type_may_be_null(flag))
regs[regno].id = ++env->id_gen;
return 0;
case PTR_TO_MEM:
mark_reg_known_zero(env, regs, regno);
regs[regno].type = PTR_TO_MEM | flag;
regs[regno].mem_size = 0;
return 0;
default:
verifier_bug(env, "unexpected reg_type %d in %s\n", reg_type, __func__);
return -EFAULT;

2
tools/testing/selftests/bpf/prog_tests/linked_list.c
View File

@ -72,7 +72,7 @@ static struct {
{ "new_null_ret", "R0 invalid mem access 'ptr_or_null_'" },
{ "obj_new_acq", "Unreleased reference id=" },
{ "use_after_drop", "invalid mem access 'scalar'" },
{ "ptr_walk_scalar", "type=scalar expected=percpu_ptr_" },
{ "ptr_walk_scalar", "type=rdonly_untrusted_mem expected=percpu_ptr_" },
{ "direct_read_lock", "direct access to bpf_spin_lock is disallowed" },
{ "direct_write_lock", "direct access to bpf_spin_lock is disallowed" },
{ "direct_read_head", "direct access to bpf_list_head is disallowed" },