[WIP 3/4] bpf: Add a way to mark functions as requiring privilege

From: Andy Lutomirski
Date: Mon Aug 05 2019 - 17:29:39 EST


This is horribly incomplete:

- I only marked one function as requiring privilege, and there are
surely more.

- Checking is_priv is probably not the right thing to do. This should
probably do something more clever. At the very lease, it needs to
integrate with the upcoming lockdown LSM infrastructure.

- The seen_privileged_funcs mechanism is probably not a good solution.
Instead we should check something while we still have enough context
to give a good error message. But we *don't* want to check for
capabilities up front before even seeing a function call, since we
don't want to inadvertently generate audit events for privileges that
are never used.

So it's the idea that counts :)

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
---
include/linux/bpf.h | 15 +++++++++++++++
include/linux/bpf_verifier.h | 1 +
kernel/bpf/verifier.c | 8 ++++++++
kernel/trace/bpf_trace.c | 1 +
4 files changed, 25 insertions(+)

diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index 2d5e1a4dff6c..de31b9888b6c 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -229,6 +229,7 @@ struct bpf_func_proto {
u64 (*func)(u64 r1, u64 r2, u64 r3, u64 r4, u64 r5);
bool gpl_only;
bool pkt_access;
+ u16 privilege;
enum bpf_return_type ret_type;
enum bpf_arg_type arg1_type;
enum bpf_arg_type arg2_type;
@@ -237,6 +238,20 @@ struct bpf_func_proto {
enum bpf_arg_type arg5_type;
};

+/*
+ * Some functions should require privilege to call at all, even in a test
+ * run. These flags indicate why privilege is required. The core BPF
+ * code will verify that the creator of such a program has the requisite
+ * privilege.
+ *
+ * NB: This means that anyone who creates a privileged program (due to
+ * such a call or due to a privilege-requiring pointer-to-integer conversion)
+ * is responsible for restricting access to the program in an appropriate
+ * manner.
+ */
+#define BPF_FUNC_PRIV_READ_KERNEL_MEMORY BIT(0)
+#define BPT_FUNC_PRIV_WRITE_GLOBAL_LOGS BIT(1)
+
/* bpf_context is intentionally undefined structure. Pointer to bpf_context is
* the first argument to eBPF programs.
* For socket filters: 'struct bpf_context *' == 'struct sk_buff *'
diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 5fe99f322b1c..9877f5753cf4 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -363,6 +363,7 @@ struct bpf_verifier_env {
u32 id_gen; /* used to generate unique reg IDs */
bool allow_ptr_leaks;
bool seen_direct_write;
+ u16 seen_privileged_funcs;
struct bpf_insn_aux_data *insn_aux_data; /* array of per-insn state */
const struct bpf_line_info *prev_linfo;
struct bpf_verifier_log log;
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 5900cbb966b1..5e048688fd8d 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4129,6 +4129,9 @@ static int check_helper_call(struct bpf_verifier_env *env, int func_id, int insn

if (changes_data)
clear_all_pkt_pointers(env);
+
+ env->seen_privileged_funcs |= fn->privilege;
+
return 0;
}

@@ -9371,6 +9374,11 @@ int bpf_check(struct bpf_prog **prog, union bpf_attr *attr,
if (ret == 0)
adjust_btf_func(env);

+ if (env->seen_privileged_funcs && !is_priv) {
+ ret = -EPERM;
+ goto err_release_maps;
+ }
+
err_release_maps:
if (!env->prog->aux->used_maps)
/* if we didn't copy map pointers into bpf_prog_info, release
diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
index ca1255d14576..d9454588d9e8 100644
--- a/kernel/trace/bpf_trace.c
+++ b/kernel/trace/bpf_trace.c
@@ -152,6 +152,7 @@ BPF_CALL_3(bpf_probe_read, void *, dst, u32, size, const void *, unsafe_ptr)
static const struct bpf_func_proto bpf_probe_read_proto = {
.func = bpf_probe_read,
.gpl_only = true,
+ .privilege = BPF_FUNC_PRIV_READ_KERNEL_MEMORY,
.ret_type = RET_INTEGER,
.arg1_type = ARG_PTR_TO_UNINIT_MEM,
.arg2_type = ARG_CONST_SIZE_OR_ZERO,
--
2.21.0