selftests/seccomp: validate uretprobe syscall passes through seccomp

The uretprobe syscall is implemented as a performance enhancement on
x86_64 by having the kernel inject a call to it on function exit; User
programs cannot call this system call explicitly.

As such, this syscall is considered a kernel implementation detail and
should not be filtered by seccomp.

Enhance the seccomp bpf test suite to check that uretprobes can be
attached to processes without the killing the process regardless of
seccomp policy.

Signed-off-by: Eyal Birger <eyal.birger@gmail.com>
Link: https://lore.kernel.org/r/20250202162921.335813-3-eyal.birger@gmail.com
[kees: Skip archs without __NR_uretprobe]
Signed-off-by: Kees Cook <kees@kernel.org>

Author: ebirger

tools/testing/selftests/seccomp/seccomp_bpf.c

@@ -47,6 +47,7 @@
 #include <linux/kcmp.h>
 #include <sys/resource.h>
 #include <sys/capability.h>
+#include <linux/perf_event.h>
 
 #include <unistd.h>
 #include <sys/syscall.h>
@@ -68,6 +69,10 @@
 # define PR_SET_PTRACER 0x59616d61
 #endif
 
+#ifndef noinline
+#define noinline __attribute__((noinline))
+#endif
+
 #ifndef PR_SET_NO_NEW_PRIVS
 #define PR_SET_NO_NEW_PRIVS 38
 #define PR_GET_NO_NEW_PRIVS 39
@@ -4888,6 +4893,200 @@ TEST(tsync_vs_dead_thread_leader)
 	EXPECT_EQ(0, status);
 }
 
+noinline int probed(void)
+{
+	return 1;
+}
+
+static int parse_uint_from_file(const char *file, const char *fmt)
+{
+	int err = -1, ret;
+	FILE *f;
+
+	f = fopen(file, "re");
+	if (f) {
+		err = fscanf(f, fmt, &ret);
+		fclose(f);
+	}
+	return err == 1 ? ret : err;
+}
+
+static int determine_uprobe_perf_type(void)
+{
+	const char *file = "/sys/bus/event_source/devices/uprobe/type";
+
+	return parse_uint_from_file(file, "%d\n");
+}
+
+static int determine_uprobe_retprobe_bit(void)
+{
+	const char *file = "/sys/bus/event_source/devices/uprobe/format/retprobe";
+
+	return parse_uint_from_file(file, "config:%d\n");
+}
+
+static ssize_t get_uprobe_offset(const void *addr)
+{
+	size_t start, base, end;
+	bool found = false;
+	char buf[256];
+	FILE *f;
+
+	f = fopen("/proc/self/maps", "r");
+	if (!f)
+		return -1;
+
+	while (fscanf(f, "%zx-%zx %s %zx %*[^\n]\n", &start, &end, buf, &base) == 4) {
+		if (buf[2] == 'x' && (uintptr_t)addr >= start && (uintptr_t)addr < end) {
+			found = true;
+			break;
+		}
+	}
+	fclose(f);
+	return found ? (uintptr_t)addr - start + base : -1;
+}
+
+FIXTURE(URETPROBE) {
+	int fd;
+};
+
+FIXTURE_VARIANT(URETPROBE) {
+	/*
+	 * All of the URETPROBE behaviors can be tested with either
+	 * uretprobe attached or not
+	 */
+	bool attach;
+};
+
+FIXTURE_VARIANT_ADD(URETPROBE, attached) {
+	.attach = true,
+};
+
+FIXTURE_VARIANT_ADD(URETPROBE, not_attached) {
+	.attach = false,
+};
+
+FIXTURE_SETUP(URETPROBE)
+{
+	const size_t attr_sz = sizeof(struct perf_event_attr);
+	struct perf_event_attr attr;
+	ssize_t offset;
+	int type, bit;
+
+#ifndef __NR_uretprobe
+	SKIP(return, "__NR_uretprobe syscall not defined");
+#endif
+
+	if (!variant->attach)
+		return;
+
+	memset(&attr, 0, attr_sz);
+
+	type = determine_uprobe_perf_type();
+	ASSERT_GE(type, 0);
+	bit = determine_uprobe_retprobe_bit();
+	ASSERT_GE(bit, 0);
+	offset = get_uprobe_offset(probed);
+	ASSERT_GE(offset, 0);
+
+	attr.config |= 1 << bit;
+	attr.size = attr_sz;
+	attr.type = type;
+	attr.config1 = ptr_to_u64("/proc/self/exe");
+	attr.config2 = offset;
+
+	self->fd = syscall(__NR_perf_event_open, &attr,
+			   getpid() /* pid */, -1 /* cpu */, -1 /* group_fd */,
+			   PERF_FLAG_FD_CLOEXEC);
+}
+
+FIXTURE_TEARDOWN(URETPROBE)
+{
+	/* we could call close(self->fd), but we'd need extra filter for
+	 * that and since we are calling _exit right away..
+	 */
+}
+
+static int run_probed_with_filter(struct sock_fprog *prog)
+{
+	if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) ||
+	    seccomp(SECCOMP_SET_MODE_FILTER, 0, prog)) {
+		return -1;
+	}
+
+	probed();
+	return 0;
+}
+
+TEST_F(URETPROBE, uretprobe_default_allow)
+{
+	struct sock_filter filter[] = {
+		BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_ALLOW),
+	};
+	struct sock_fprog prog = {
+		.len = (unsigned short)ARRAY_SIZE(filter),
+		.filter = filter,
+	};
+
+	ASSERT_EQ(0, run_probed_with_filter(&prog));
+}
+
+TEST_F(URETPROBE, uretprobe_default_block)
+{
+	struct sock_filter filter[] = {
+		BPF_STMT(BPF_LD|BPF_W|BPF_ABS,
+			offsetof(struct seccomp_data, nr)),
+		BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, __NR_exit_group, 1, 0),
+		BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_KILL),
+		BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_ALLOW),
+	};
+	struct sock_fprog prog = {
+		.len = (unsigned short)ARRAY_SIZE(filter),
+		.filter = filter,
+	};
+
+	ASSERT_EQ(0, run_probed_with_filter(&prog));
+}
+
+TEST_F(URETPROBE, uretprobe_block_uretprobe_syscall)
+{
+	struct sock_filter filter[] = {
+		BPF_STMT(BPF_LD|BPF_W|BPF_ABS,
+			offsetof(struct seccomp_data, nr)),
+#ifdef __NR_uretprobe
+		BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, __NR_uretprobe, 0, 1),
+#endif
+		BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_KILL),
+		BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_ALLOW),
+	};
+	struct sock_fprog prog = {
+		.len = (unsigned short)ARRAY_SIZE(filter),
+		.filter = filter,
+	};
+
+	ASSERT_EQ(0, run_probed_with_filter(&prog));
+}
+
+TEST_F(URETPROBE, uretprobe_default_block_with_uretprobe_syscall)
+{
+	struct sock_filter filter[] = {
+		BPF_STMT(BPF_LD|BPF_W|BPF_ABS,
+			offsetof(struct seccomp_data, nr)),
+#ifdef __NR_uretprobe
+		BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, __NR_uretprobe, 2, 0),
+#endif
+		BPF_JUMP(BPF_JMP|BPF_JEQ|BPF_K, __NR_exit_group, 1, 0),
+		BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_KILL),
+		BPF_STMT(BPF_RET|BPF_K, SECCOMP_RET_ALLOW),
+	};
+	struct sock_fprog prog = {
+		.len = (unsigned short)ARRAY_SIZE(filter),
+		.filter = filter,
+	};
+
+	ASSERT_EQ(0, run_probed_with_filter(&prog));
+}
+
 /*
  * TODO:
  * - expand NNP testing