[PATCH bpf-next] selftests/bpf: more precise cpu_mitigations state detection

[Eduard Zingerman <eddyz87@gmail.com>]

test_progs and test_verifier binaries execute unpriv tests under the
following conditions:
- unpriv BPF is enabled;
- CPU mitigations are enabled (see [1] for details).

The detection of the "mitigations enabled" state is performed by
unpriv_helpers.c:get_mitigations_off() via inspecting kernel boot
command line, looking for a parameter "mitigations=off".

Such detection scheme won't work for certain configurations,
e.g. when CONFIG_CPU_MIGITGATIONS is disabled and boot parameter is
not supplied.

Miss-detection leads to test_progs executing tests meant to be run
only with mitigations enabled, e.g.
verifier_and.c:known_subreg_with_unknown_reg(), and reporting false
failures.

Internally, verifier sets bpf_verifier_env->bypass_spec_{v1,v4}
basing on the value returned by kernel/cpu.c:cpu_mitigations_off().
This function is backed by a variable kernel/cpu.c:cpu_mitigations.

This state is not fully introspect-able via sysfs. The closest proxy
is /sys/devices/system/cpu/vulnerabilities/spectre_v1, but it reports
"vulnerable" state only if mitigations are disabled *and* current cpu
is vulnerable, while verifier does not check cpu state.

There are only two ways the kernel/cpu.c:cpu_mitigations can be set:
- via boot parameter;
- via CONFIG_CPU_MIGITGATIONS option.

This commit updates unpriv_helpers.c:get_mitigations_off() to scan
/proc/config.gz for CONFIG_CPU_MIGITGATIONS value in addition to boot
command line check.

Tested using the following configurations:
- mitigations enabled (unpriv tests are enabled)
- mitigations disabled via boot cmdline (unpriv tests skipped)
- mitigations disabled via CONFIG_CPU_MIGITGATIONS
  (unpriv tests skipped)

[1] https://lore.kernel.org/bpf/20231025031144.5508-1-laoar.shao@gmail.com/

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
---
 tools/testing/selftests/bpf/unpriv_helpers.c | 45 +++++++++++++++++++-
 1 file changed, 43 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/bpf/unpriv_helpers.c b/tools/testing/selftests/bpf/unpriv_helpers.c
index 220f6a963813..1dec3c6b3d70 100644
--- a/tools/testing/selftests/bpf/unpriv_helpers.c
+++ b/tools/testing/selftests/bpf/unpriv_helpers.c
@@ -6,10 +6,46 @@
 #include <string.h>
 #include <unistd.h>
 #include <fcntl.h>
+#include <zlib.h>
 
 #include "unpriv_helpers.h"
 
-static bool get_mitigations_off(void)
+static bool scan_config(const char *pat)
+{
+	bool ret = false;
+	const char *msg;
+	char buf[1024];
+	gzFile config;
+	int n, err;
+
+	config = gzopen("/proc/config.gz", "rb");
+	if (!config) {
+		perror("gzopen /proc/config.gz");
+		goto out;
+	}
+	for (;;) {
+		if (!gzgets(config, buf, sizeof(buf))) {
+			msg = gzerror(config, &err);
+			if (err == Z_ERRNO)
+				perror("gzgets /proc/config.gz");
+			else if (err != Z_OK)
+				fprintf(stderr, "gzgets /proc/config.gz: %s", msg);
+			goto out;
+		}
+		n = strlen(buf);
+		if (buf[n - 1] == '\n')
+			buf[n - 1] = 0;
+		if (strcmp(buf, pat) == 0) {
+			ret = true;
+			goto out;
+		}
+	}
+out:
+	gzclose(config);
+	return ret;
+}
+
+static bool scan_cmdline(const char *pat)
 {
 	char cmdline[4096], *c;
 	int fd, ret = false;
@@ -27,7 +63,7 @@ static bool get_mitigations_off(void)
 
 	cmdline[sizeof(cmdline) - 1] = '\0';
 	for (c = strtok(cmdline, " \n"); c; c = strtok(NULL, " \n")) {
-		if (strncmp(c, "mitigations=off", strlen(c)))
+		if (strncmp(c, pat, strlen(c)))
 			continue;
 		ret = true;
 		break;
@@ -37,6 +73,11 @@ static bool get_mitigations_off(void)
 	return ret;
 }
 
+static bool get_mitigations_off(void)
+{
+	return scan_cmdline("mitigations=off") || !scan_config("CONFIG_CPU_MITIGATIONS=y");
+}
+
 bool get_unpriv_disabled(void)
 {
 	bool disabled;
-- 
2.47.1