[PATCH bpf-next 3/8] bpf: Disallow NULL PTR_TO_MEM for trusted kfuncs

[David Vernet <void@manifault.com>]

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)
...

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;
+			}
+
 			ret = check_mem_reg(env, reg, regno, type_size);
 			if (ret < 0)
 				return ret;
-- 
2.39.0