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Subject[RFC v1 13/14] krsi: Provide an example to read and log environment variables
Date
From: KP Singh <kpsingh@google.com>

* The program takes the name of an environment variable as an
argument.
* An eBPF program is loaded and attached to the
process_execution hook.
* The name of the environment variable passed is updated in a
eBPF per-cpu map.
* The eBPF program uses the krsi_get_env_var helper to get the
the value of this variable and logs the result to the perf
events buffer.
* The user-space program listens to the perf events and prints
the values.

Example execution:

./krsi LD_PRELOAD

[p_pid=123] LD_PRELOAD is not set
[p_pid=456] LD_PRELOAD=/lib/bad.so
[p_pid=789] WARNING! LD_PRELOAD is set 2 times
[p_pid=789] LD_PRELOAD=/lib/decoy.so
[p_pid=789] LD_PRELOAD=/lib/bad.so

In a separate session the following [1, 2, 3] exec system calls
are made where:

[1, 2, 3] char *argv[] = {"/bin/ls", 0};

[1] char *envp = {0};
[2] char *envp = {"LD_PRELOAD=/lib/bad.so", 0};
[3] char *envp = {"LD_PRELOAD=/lib/decoy.so, "LD_PRELOAD=/lib/bad.so", 0};

This example demonstrates that user-space is free to choose the format
in which the data is logged and can use very specific helpers like
krsi_get_env_var to populate only the data that is required.

Signed-off-by: KP Singh <kpsingh@google.com>
---
MAINTAINERS | 3 +
samples/bpf/.gitignore | 1 +
samples/bpf/Makefile | 3 +
samples/bpf/krsi_helpers.h | 31 ++++++
samples/bpf/krsi_kern.c | 52 ++++++++++
samples/bpf/krsi_user.c | 202 +++++++++++++++++++++++++++++++++++++
6 files changed, 292 insertions(+)
create mode 100644 samples/bpf/krsi_helpers.h
create mode 100644 samples/bpf/krsi_kern.c
create mode 100644 samples/bpf/krsi_user.c

diff --git a/MAINTAINERS b/MAINTAINERS
index 8e0364391d8b..ec378abb4c23 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -9005,6 +9005,9 @@ F: kernel/kprobes.c
KRSI SECURITY MODULE
M: KP Singh <kpsingh@chromium.org>
S: Supported
+F: samples/bpf/krsi_helpers.h
+F: samples/bpf/krsi_kern.c
+F: samples/bpf/krsi_user.c
F: security/krsi/

KS0108 LCD CONTROLLER DRIVER
diff --git a/samples/bpf/.gitignore b/samples/bpf/.gitignore
index 74d31fd3c99c..6bbf5a04877f 100644
--- a/samples/bpf/.gitignore
+++ b/samples/bpf/.gitignore
@@ -2,6 +2,7 @@ cpustat
fds_example
hbm
ibumad
+krsi
lathist
lwt_len_hist
map_perf_test
diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
index 1d9be26b4edd..33d3bef17549 100644
--- a/samples/bpf/Makefile
+++ b/samples/bpf/Makefile
@@ -8,6 +8,7 @@ hostprogs-y := test_lru_dist
hostprogs-y += sock_example
hostprogs-y += fds_example
hostprogs-y += sockex1
+hostprogs-y += krsi
hostprogs-y += sockex2
hostprogs-y += sockex3
hostprogs-y += tracex1
@@ -62,6 +63,7 @@ TRACE_HELPERS := ../../tools/testing/selftests/bpf/trace_helpers.o

fds_example-objs := fds_example.o
sockex1-objs := sockex1_user.o
+krsi-objs := krsi_user.o $(TRACE_HELPERS)
sockex2-objs := sockex2_user.o
sockex3-objs := bpf_load.o sockex3_user.o
tracex1-objs := bpf_load.o tracex1_user.o
@@ -113,6 +115,7 @@ hbm-objs := bpf_load.o hbm.o $(CGROUP_HELPERS)
# Tell kbuild to always build the programs
always := $(hostprogs-y)
always += sockex1_kern.o
+always += krsi_kern.o
always += sockex2_kern.o
always += sockex3_kern.o
always += tracex1_kern.o
diff --git a/samples/bpf/krsi_helpers.h b/samples/bpf/krsi_helpers.h
new file mode 100644
index 000000000000..3007bfd6212e
--- /dev/null
+++ b/samples/bpf/krsi_helpers.h
@@ -0,0 +1,31 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef _KRSI_HELPERS_H
+#define _KRSI_HELPERS_H
+
+#define __bpf_percpu_val_align __aligned(8)
+
+#define ENV_VAR_NAME_MAX_LEN 48
+#define ENV_VAR_VAL_MAX_LEN 4096
+
+#define MAX_CPUS 128
+
+#define __LOWER(x) (x & 0xffffffff)
+#define __UPPER(x) (x >> 32)
+
+struct krsi_env_value {
+ // The name of the environment variable.
+ char name[ENV_VAR_NAME_MAX_LEN];
+ // The value of the environment variable (if set).
+ char value[ENV_VAR_VAL_MAX_LEN];
+ // Indicates if an overflow occurred while reading the value of the
+ // of the environment variable. This means that an -E2BIG was received
+ // from the krsi_get_env_var helper.
+ bool overflow;
+ // The number of times the environment variable was set.
+ __u32 times;
+ // The PID of the parent process.
+ __u32 p_pid;
+} __bpf_percpu_val_align;
+
+#endif // _KRSI_HELPERS_H
diff --git a/samples/bpf/krsi_kern.c b/samples/bpf/krsi_kern.c
new file mode 100644
index 000000000000..087a6f0cc81d
--- /dev/null
+++ b/samples/bpf/krsi_kern.c
@@ -0,0 +1,52 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/ptrace.h>
+#include <uapi/linux/bpf.h>
+#include <uapi/linux/ip.h>
+#include "bpf_helpers.h"
+#include "krsi_helpers.h"
+
+#define MAX_CPUS 128
+
+struct bpf_map_def SEC("maps") env_map = {
+ .type = BPF_MAP_TYPE_PERCPU_ARRAY,
+ .key_size = sizeof(u32),
+ .value_size = sizeof(struct krsi_env_value),
+ .max_entries = 1,
+};
+
+struct bpf_map_def SEC("maps") perf_map = {
+ .type = BPF_MAP_TYPE_PERF_EVENT_ARRAY,
+ .key_size = sizeof(int),
+ .value_size = sizeof(u32),
+ .max_entries = MAX_CPUS,
+};
+
+SEC("krsi")
+int env_dumper(void *ctx)
+{
+ u64 times_ret;
+ s32 ret;
+ u32 map_id = 0;
+ char *map_value;
+ struct krsi_env_value *env;
+
+ env = bpf_map_lookup_elem(&env_map, &map_id);
+ if (!env)
+ return -ENOMEM;
+ times_ret = krsi_get_env_var(ctx, env->name, ENV_VAR_NAME_MAX_LEN,
+ env->value, ENV_VAR_VAL_MAX_LEN);
+ ret = __LOWER(times_ret);
+ if (ret == -E2BIG)
+ env->overflow = true;
+ else if (ret < 0)
+ return ret;
+
+ env->times = __UPPER(times_ret);
+ env->p_pid = bpf_get_current_pid_tgid();
+ bpf_perf_event_output(ctx, &perf_map, BPF_F_CURRENT_CPU, env,
+ sizeof(struct krsi_env_value));
+
+ return 0;
+}
+char _license[] SEC("license") = "GPL";
diff --git a/samples/bpf/krsi_user.c b/samples/bpf/krsi_user.c
new file mode 100644
index 000000000000..1fad29bf017a
--- /dev/null
+++ b/samples/bpf/krsi_user.c
@@ -0,0 +1,202 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <errno.h>
+#include <err.h>
+#include <assert.h>
+#include <linux/limits.h>
+#include <linux/bpf.h>
+#include <bpf/bpf.h>
+#include "bpf/libbpf.h"
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/sysinfo.h>
+#include <sys/ioctl.h>
+#include <fcntl.h>
+#include <sys/resource.h>
+
+#include "perf-sys.h"
+#include "trace_helpers.h"
+#include "krsi_helpers.h"
+
+#define LSM_HOOK_PATH "/sys/kernel/security/krsi/process_execution"
+
+static int pmu_fds[MAX_CPUS];
+static struct perf_event_mmap_page *headers[MAX_CPUS];
+
+static int print_env(void *d, int size)
+{
+ struct krsi_env_value *env = d;
+ int times = env->times;
+ char *next = env->value;
+ size_t total = 0;
+
+ if (env->times > 1)
+ printf("[p_pid=%u] WARNING! %s is set %u times\n",
+ env->p_pid, env->name, env->times);
+
+ /*
+ * krsi_get_env_var ensures that even overflows
+ * are null terminated. Incase of an overflow,
+ * this logic tries to print as much information
+ * that was gathered.
+ */
+ while (times && total < ENV_VAR_NAME_MAX_LEN) {
+ next += total;
+ if (env->overflow)
+ printf("[p_pid=%u] OVERFLOW! %s=%s\n",
+ env->p_pid, env->name, next);
+ else
+ printf("[p_pid=%u] %s=%s\n",
+ env->p_pid, env->name, next);
+ times--;
+ total += strlen(next) + 1;
+ }
+
+ if (!env->times)
+ printf("p_pid=%u] %s is not set\n",
+ env->p_pid, env->name);
+
+ return LIBBPF_PERF_EVENT_CONT;
+}
+
+static int open_perf_events(int map_fd, int num)
+{
+ int i;
+ struct perf_event_attr attr = {
+ .sample_type = PERF_SAMPLE_RAW,
+ .type = PERF_TYPE_SOFTWARE,
+ .config = PERF_COUNT_SW_BPF_OUTPUT,
+ .wakeup_events = 1, /* get an fd notification for every event */
+ };
+
+ for (i = 0; i < num; i++) {
+ int key = i;
+ int ret;
+
+ ret = sys_perf_event_open(&attr, -1 /*pid*/, i/*cpu*/,
+ -1/*group_fd*/, 0);
+ if (ret < 0)
+ return ret;
+ pmu_fds[i] = ret;
+ ret = bpf_map_update_elem(map_fd, &key, &pmu_fds[i], BPF_ANY);
+ if (ret < 0)
+ return ret;
+ ioctl(pmu_fds[i], PERF_EVENT_IOC_ENABLE, 0);
+ }
+ return 0;
+}
+
+static int update_env_map(struct bpf_object *prog_obj, const char *env_var_name,
+ int numcpus)
+{
+ struct bpf_map *map;
+ struct krsi_env_value *env;
+ int map_fd;
+ int key = 0, ret = 0, i;
+
+ map = bpf_object__find_map_by_name(prog_obj, "env_map");
+ if (!map)
+ return -EINVAL;
+
+ map_fd = bpf_map__fd(map);
+ if (map_fd < 0)
+ return map_fd;
+
+ env = malloc(numcpus * sizeof(struct krsi_env_value));
+ if (!env) {
+ ret = -ENOMEM;
+ goto out;
+ }
+
+ for (i = 0; i < numcpus; i++)
+ strcpy(env[i].name, env_var_name);
+
+ ret = bpf_map_update_elem(map_fd, &key, env, BPF_ANY);
+ if (ret < 0)
+ goto out;
+
+out:
+ free(env);
+ return ret;
+}
+
+int main(int argc, char **argv)
+{
+ struct bpf_object *prog_obj;
+ const char *env_var_name;
+ struct bpf_prog_load_attr attr;
+ int prog_fd, target_fd, map_fd;
+ int ret, i, numcpus;
+ struct bpf_map *map;
+ char filename[PATH_MAX];
+ struct rlimit r = {RLIM_INFINITY, RLIM_INFINITY};
+
+
+ if (argc != 2)
+ errx(EXIT_FAILURE, "Usage %s env_var_name\n", argv[0]);
+
+ env_var_name = argv[1];
+ if (strlen(env_var_name) > ENV_VAR_NAME_MAX_LEN - 1)
+ errx(EXIT_FAILURE,
+ "<env_var_name> cannot be more than %d in length",
+ ENV_VAR_NAME_MAX_LEN - 1);
+
+
+ setrlimit(RLIMIT_MEMLOCK, &r);
+ snprintf(filename, sizeof(filename), "%s_kern.o", argv[0]);
+
+ memset(&attr, 0, sizeof(struct bpf_prog_load_attr));
+ attr.prog_type = BPF_PROG_TYPE_KRSI;
+ attr.expected_attach_type = BPF_KRSI;
+ attr.file = filename;
+
+ /* Attach the BPF program to the given hook */
+ target_fd = open(LSM_HOOK_PATH, O_RDWR);
+ if (target_fd < 0)
+ err(EXIT_FAILURE, "Failed to open target file");
+
+ if (bpf_prog_load_xattr(&attr, &prog_obj, &prog_fd))
+ err(EXIT_FAILURE, "Failed to load eBPF program");
+
+ numcpus = get_nprocs();
+ if (numcpus > MAX_CPUS)
+ numcpus = MAX_CPUS;
+
+ ret = update_env_map(prog_obj, env_var_name, numcpus);
+ if (ret < 0)
+ err(EXIT_FAILURE, "Failed to update env map");
+
+ map = bpf_object__find_map_by_name(prog_obj, "perf_map");
+ if (!map)
+ err(EXIT_FAILURE,
+ "Finding the perf event map in obj file failed");
+
+ map_fd = bpf_map__fd(map);
+ if (map_fd < 0)
+ err(EXIT_FAILURE, "Failed to get fd for perf events map");
+
+ ret = bpf_prog_attach(prog_fd, target_fd, BPF_KRSI,
+ BPF_F_ALLOW_OVERRIDE);
+ if (ret < 0)
+ err(EXIT_FAILURE, "Failed to attach prog to LSM hook");
+
+ ret = open_perf_events(map_fd, numcpus);
+ if (ret < 0)
+ err(EXIT_FAILURE, "Failed to open perf events handler");
+
+ for (i = 0; i < numcpus; i++) {
+ ret = perf_event_mmap_header(pmu_fds[i], &headers[i]);
+ if (ret < 0)
+ err(EXIT_FAILURE, "perf_event_mmap_header");
+ }
+
+ ret = perf_event_poller_multi(pmu_fds, headers, numcpus, print_env);
+ if (ret < 0)
+ err(EXIT_FAILURE, "Failed to poll perf events");
+
+ return EXIT_SUCCESS;
+}
--
2.20.1
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