[PATCH bpf v2 0/2] bpf: Require referenced or trusted pointer for __arg_trusted arg

[Yiyang Chen]

A global subprogram parameter tagged __arg_trusted is specified to accept
only the PTR_TRUSTED flavor of PTR_TO_BTF_ID, but the call-site check in
btf_check_func_arg_match() also accepts a bare PTR_TO_BTF_ID and an MEM_RCU
one.

check_reg_type() resolves the accepted set from the base argument type alone
and compares only MEM_RDONLY and PTR_MAYBE_NULL, so the PTR_TRUSTED bit of
arg_type is never consulted. btf_ptr_types lists bare PTR_TO_BTF_ID and
PTR_TO_BTF_ID | MEM_RCU alongside the trusted flavor, and nothing else on the
subprogram call path requires the register to be referenced or trusted.

The callee is then validated with PTR_TRUSTED set on the register while the
caller passed a pointer that is neither referenced nor trusted.
bpf_may_fault_on_deref() is false for PTR_TRUSTED, so the dereference becomes
a raw load instead of a BPF_PROBE_MEM probe and the callee can pass the
pointer on to a kfunc that would have rejected it at the original call site.

Reject a PTR_TO_BTF_ID that is neither referenced nor trusted when the
argument is marked PTR_TRUSTED. A referenced register is accepted, as in
is_trusted_reg(). PTR_MAYBE_NULL is not counted as unsafe when __arg_nullable
declares it, so trusted-and-nullable arguments keep working. The kfunc path
is unchanged.

Changes in v2:
- Retarget the fix to btf_check_func_arg_match(), where the subprogram
  argument check lives in this tree, instead of check_func_arg().
- Run the check after check_reg_type() and check_func_arg_reg_off() so type
  and offset diagnostics keep their wording.

v1: https://lore.kernel.org/bpf/20261005-a3-arg-trusted-v4-v1-0-50ee0268fd39@xxxxxxxxxxxxxxxxxxxxx/

Yiyang Chen (2):
      bpf: Require referenced or trusted pointer for __arg_trusted arg
      selftests/bpf: Cover non-trusted pointer to __arg_trusted subprog arg

 kernel/bpf/verifier.c                              | 24 +++++++++++++
 .../selftests/bpf/progs/verifier_global_ptr_args.c | 40 ++++++++++++++++++++++
 2 files changed, 64 insertions(+)

base-commit: ff47652a4b66c067c765a7ad464d930b5a9367cc

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2.34.0