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    SubjectRe: [PATCH bpf-next 3/8] bpf: Disallow NULL PTR_TO_MEM for trusted kfuncs
    On Fri, Jan 20, 2023 at 05:28:28AM IST, David Vernet wrote:
    > KF_TRUSTED_ARGS kfuncs currently have a subtle and insidious bug in
    > validating pointers to scalars. Say that you have a kfunc like the
    > following, which takes an array as the first argument:
    >
    > bool bpf_cpumask_empty(const struct cpumask *cpumask)
    > {
    > return cpumask_empty(cpumask);
    > }
    >
    > ...
    > BTF_ID_FLAGS(func, bpf_cpumask_empty, KF_TRUSTED_ARGS)
    > ...
    >

    This is known and expected.

    > If a BPF program were to invoke the kfunc with a NULL argument, it would
    > crash the kernel. The reason is that struct cpumask is defined as a
    > bitmap, which is itself defined as an array, and is accessed as a memory
    > address memory by bitmap operations. So when the verifier analyzes the
    > register, it interprets it as a pointer to a scalar struct, which is an
    > array of size 8. check_mem_reg() then sees that the register is NULL,
    > and returns 0, and the kfunc crashes when it passes it down to the
    > cpumask wrappers.
    >
    > To fix this, this patch adds a check for KF_ARG_PTR_TO_MEM which
    > verifies that the register doesn't contain a NULL pointer if the kfunc
    > is KF_TRUSTED_ARGS.
    >
    > This may or may not be desired behavior. Some kfuncs may want to
    > allow callers to pass NULL-able pointers. An alternative would be adding
    > a KF_NOT_NULL flag and leaving KF_TRUSTED_ARGS alone, though given that
    > a kfunc is saying it wants to "trust" an argument, it seems reasonable
    > to prevent NULL.
    >
    > Signed-off-by: David Vernet <void@manifault.com>
    > ---
    > kernel/bpf/verifier.c | 5 +++++
    > 1 file changed, 5 insertions(+)
    >
    > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
    > index 9fa101420046..28ccb92ebe65 100644
    > --- a/kernel/bpf/verifier.c
    > +++ b/kernel/bpf/verifier.c
    > @@ -9092,6 +9092,11 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
    > i, btf_type_str(ref_t), ref_tname, PTR_ERR(resolve_ret));
    > return -EINVAL;
    > }
    > + if (is_kfunc_trusted_args(meta) && register_is_null(reg)) {
    > + verbose(env, "NULL pointer passed to trusted arg%d\n", i);
    > + return -EACCES;
    > + }
    > +

    Current patch looks like a stop gap solution. Just checking for register_is_null
    is not enough, what about PTR_MAYBE_NULL? That can also be passed. Some
    arguments can be both PTR_TO_BTF_ID and PTR_TO_MEM, so it will be bypassed in
    the other case because this check is limited to KF_ARG_PTR_TO_MEM. It would
    probably be better to disallow NULL by default and explicitly tag the argument
    with __or_null to indicate that NULL is accepted. Seems like a much better
    default to me.

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