selftests/powerpc: Add a test of sigreturning to the kernel

We have a general signal fuzzer, sigfuz, which can modify the MSR & NIP
before sigreturn. But the chance of it hitting a kernel address and also
clearing MSR_PR is fairly slim.

So add a specific test of sigreturn to a kernel address, both with and
without attempting to clear MSR_PR (which the kernel must block).

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211209115944.4062384-1-mpe@ellerman.id.au

tools/testing/selftests/powerpc/signal/.gitignore

@@ -4,3 +4,4 @@ signal_tm
 sigfuz
 sigreturn_vdso
 sig_sc_double_restart
+sigreturn_kernel

tools/testing/selftests/powerpc/signal/Makefile

@@ -1,5 +1,6 @@
 # SPDX-License-Identifier: GPL-2.0
 TEST_GEN_PROGS := signal signal_tm sigfuz sigreturn_vdso sig_sc_double_restart
+TEST_GEN_PROGS += sigreturn_kernel
 
 CFLAGS += -maltivec
 $(OUTPUT)/signal_tm: CFLAGS += -mhtm

tools/testing/selftests/powerpc/signal/sigreturn_kernel.c

@@ -0,0 +1,132 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * Test that we can't sigreturn to kernel addresses, or to kernel mode.
+ */
+
+#define _GNU_SOURCE
+
+#include <stdio.h>
+#include <signal.h>
+#include <stdlib.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <unistd.h>
+
+#include "utils.h"
+
+#define MSR_PR (1ul << 14)
+
+static volatile unsigned long long sigreturn_addr;
+static volatile unsigned long long sigreturn_msr_mask;
+
+static void sigusr1_handler(int signo, siginfo_t *si, void *uc_ptr)
+{
+	ucontext_t *uc = (ucontext_t *)uc_ptr;
+
+	if (sigreturn_addr)
+		UCONTEXT_NIA(uc) = sigreturn_addr;
+
+	if (sigreturn_msr_mask)
+		UCONTEXT_MSR(uc) &= sigreturn_msr_mask;
+}
+
+static pid_t fork_child(void)
+{
+	pid_t pid;
+
+	pid = fork();
+	if (pid == 0) {
+		raise(SIGUSR1);
+		exit(0);
+	}
+
+	return pid;
+}
+
+static int expect_segv(pid_t pid)
+{
+	int child_ret;
+
+	waitpid(pid, &child_ret, 0);
+	FAIL_IF(WIFEXITED(child_ret));
+	FAIL_IF(!WIFSIGNALED(child_ret));
+	FAIL_IF(WTERMSIG(child_ret) != 11);
+
+	return 0;
+}
+
+int test_sigreturn_kernel(void)
+{
+	struct sigaction act;
+	int child_ret, i;
+	pid_t pid;
+
+	act.sa_sigaction = sigusr1_handler;
+	act.sa_flags = SA_SIGINFO;
+	sigemptyset(&act.sa_mask);
+
+	FAIL_IF(sigaction(SIGUSR1, &act, NULL));
+
+	for (i = 0; i < 2; i++) {
+		// Return to kernel
+		sigreturn_addr = 0xcull << 60;
+		pid = fork_child();
+		expect_segv(pid);
+
+		// Return to kernel virtual
+		sigreturn_addr = 0xc008ull << 48;
+		pid = fork_child();
+		expect_segv(pid);
+
+		// Return out of range
+		sigreturn_addr = 0xc010ull << 48;
+		pid = fork_child();
+		expect_segv(pid);
+
+		// Return to no-man's land, just below PAGE_OFFSET
+		sigreturn_addr = (0xcull << 60) - (64 * 1024);
+		pid = fork_child();
+		expect_segv(pid);
+
+		// Return to no-man's land, above TASK_SIZE_4PB
+		sigreturn_addr = 0x1ull << 52;
+		pid = fork_child();
+		expect_segv(pid);
+
+		// Return to 0xd space
+		sigreturn_addr = 0xdull << 60;
+		pid = fork_child();
+		expect_segv(pid);
+
+		// Return to 0xe space
+		sigreturn_addr = 0xeull << 60;
+		pid = fork_child();
+		expect_segv(pid);
+
+		// Return to 0xf space
+		sigreturn_addr = 0xfull << 60;
+		pid = fork_child();
+		expect_segv(pid);
+
+		// Attempt to set PR=0 for 2nd loop (should be blocked by kernel)
+		sigreturn_msr_mask = ~MSR_PR;
+	}
+
+	printf("All children killed as expected\n");
+
+	// Don't change address, just MSR, should return to user as normal
+	sigreturn_addr = 0;
+	sigreturn_msr_mask = ~MSR_PR;
+	pid = fork_child();
+	waitpid(pid, &child_ret, 0);
+	FAIL_IF(!WIFEXITED(child_ret));
+	FAIL_IF(WIFSIGNALED(child_ret));
+	FAIL_IF(WEXITSTATUS(child_ret) != 0);
+
+	return 0;
+}
+
+int main(void)
+{
+	return test_harness(test_sigreturn_kernel, "sigreturn_kernel");
+}