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Subject[PATCH bpf-next v4 4/7] bpf: Attachment verification for BPF_MODIFY_RETURN
Date
From: KP Singh <kpsingh@google.com>

- Allow BPF_MODIFY_RETURN attachment only to functions that are:

* Whitelisted for error injection by checking
within_error_injection_list. Similar discussions happened for the
bpf_override_return helper.

* security hooks, this is expected to be cleaned up with the LSM
changes after the KRSI patches introduce the LSM_HOOK macro:

https://lore.kernel.org/bpf/20200220175250.10795-1-kpsingh@chromium.org/

- The attachment is currently limited to functions that return an int.
This can be extended later other types (e.g. PTR).

Signed-off-by: KP Singh <kpsingh@google.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
---
kernel/bpf/btf.c | 28 ++++++++++++++++++++--------
kernel/bpf/verifier.c | 31 +++++++++++++++++++++++++++++++
2 files changed, 51 insertions(+), 8 deletions(-)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 30841fb8b3c0..50080add2ab9 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -3710,14 +3710,26 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
nr_args--;
}

- if ((prog->expected_attach_type == BPF_TRACE_FEXIT ||
- prog->expected_attach_type == BPF_MODIFY_RETURN) &&
- arg == nr_args) {
- if (!t)
- /* Default prog with 5 args. 6th arg is retval. */
- return true;
- /* function return type */
- t = btf_type_by_id(btf, t->type);
+ if (arg == nr_args) {
+ if (prog->expected_attach_type == BPF_TRACE_FEXIT) {
+ if (!t)
+ return true;
+ t = btf_type_by_id(btf, t->type);
+ } else if (prog->expected_attach_type == BPF_MODIFY_RETURN) {
+ /* For now the BPF_MODIFY_RETURN can only be attached to
+ * functions that return an int.
+ */
+ if (!t)
+ return false;
+
+ t = btf_type_skip_modifiers(btf, t->type, NULL);
+ if (!btf_type_is_int(t)) {
+ bpf_log(log,
+ "ret type %s not allowed for fmod_ret\n",
+ btf_kind_str[BTF_INFO_KIND(t->info)]);
+ return false;
+ }
+ }
} else if (arg >= nr_args) {
bpf_log(log, "func '%s' doesn't have %d-th argument\n",
tname, arg + 1);
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 2460c8e6b5be..ae32517d4ccd 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -19,6 +19,7 @@
#include <linux/sort.h>
#include <linux/perf_event.h>
#include <linux/ctype.h>
+#include <linux/error-injection.h>

#include "disasm.h"

@@ -9800,6 +9801,33 @@ static int check_struct_ops_btf_id(struct bpf_verifier_env *env)

return 0;
}
+#define SECURITY_PREFIX "security_"
+
+static int check_attach_modify_return(struct bpf_verifier_env *env)
+{
+ struct bpf_prog *prog = env->prog;
+ unsigned long addr = (unsigned long) prog->aux->trampoline->func.addr;
+
+ if (within_error_injection_list(addr))
+ return 0;
+
+ /* This is expected to be cleaned up in the future with the KRSI effort
+ * introducing the LSM_HOOK macro for cleaning up lsm_hooks.h.
+ */
+ if (!strncmp(SECURITY_PREFIX, prog->aux->attach_func_name,
+ sizeof(SECURITY_PREFIX) - 1)) {
+
+ if (!capable(CAP_MAC_ADMIN))
+ return -EPERM;
+
+ return 0;
+ }
+
+ verbose(env, "fmod_ret attach_btf_id %u (%s) is not modifiable\n",
+ prog->aux->attach_btf_id, prog->aux->attach_func_name);
+
+ return -EINVAL;
+}

static int check_attach_btf_id(struct bpf_verifier_env *env)
{
@@ -10000,6 +10028,9 @@ static int check_attach_btf_id(struct bpf_verifier_env *env)
}
tr->func.addr = (void *)addr;
prog->aux->trampoline = tr;
+
+ if (prog->expected_attach_type == BPF_MODIFY_RETURN)
+ ret = check_attach_modify_return(env);
out:
mutex_unlock(&tr->mutex);
if (ret)
--
2.20.1
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