selftests/powerpc: Add a test for execute-only memory

This selftest is designed to cover execute-only protections
on the Radix MMU but will also work with Hash.

The tests are based on those found in pkey_exec_test with modifications
to use the generic mprotect() instead of the pkey variants.

Signed-off-by: Nicholas Miehlbradt <[e-mail]>
Signed-off-by: Russell Currey <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

tools/testing/selftests/powerpc/mm/Makefile

···
 	$(MAKE) -C ../
 
 TEST_GEN_PROGS := hugetlb_vs_thp_test subpage_prot prot_sao segv_errors wild_bctr \
-		  large_vm_fork_separation bad_accesses pkey_exec_prot \
+		  large_vm_fork_separation bad_accesses exec_prot pkey_exec_prot \
 		  pkey_siginfo stack_expansion_signal stack_expansion_ldst \
 		  large_vm_gpr_corruption
 TEST_PROGS := stress_code_patching.sh
···
 $(OUTPUT)/large_vm_fork_separation: CFLAGS += -m64
 $(OUTPUT)/large_vm_gpr_corruption: CFLAGS += -m64
 $(OUTPUT)/bad_accesses: CFLAGS += -m64
+$(OUTPUT)/exec_prot: CFLAGS += -m64
 $(OUTPUT)/pkey_exec_prot: CFLAGS += -m64
 $(OUTPUT)/pkey_siginfo: CFLAGS += -m64
 

tools/testing/selftests/powerpc/mm/exec_prot.c

···
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * Copyright 2022, Nicholas Miehlbradt, IBM Corporation
+ * based on pkey_exec_prot.c
+ *
+ * Test if applying execute protection on pages works as expected.
+ */
+
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <signal.h>
+
+#include <unistd.h>
+#include <sys/mman.h>
+
+#include "pkeys.h"
+
+
+#define PPC_INST_NOP	0x60000000
+#define PPC_INST_TRAP	0x7fe00008
+#define PPC_INST_BLR	0x4e800020
+
+static volatile sig_atomic_t fault_code;
+static volatile sig_atomic_t remaining_faults;
+static volatile unsigned int *fault_addr;
+static unsigned long pgsize, numinsns;
+static unsigned int *insns;
+static bool pkeys_supported;
+
+static bool is_fault_expected(int fault_code)
+{
+	if (fault_code == SEGV_ACCERR)
+		return true;
+
+	/* Assume any pkey error is fine since pkey_exec_prot test covers them */
+	if (fault_code == SEGV_PKUERR && pkeys_supported)
+		return true;
+
+	return false;
+}
+
+static void trap_handler(int signum, siginfo_t *sinfo, void *ctx)
+{
+	/* Check if this fault originated from the expected address */
+	if (sinfo->si_addr != (void *)fault_addr)
+		sigsafe_err("got a fault for an unexpected address\n");
+
+	_exit(1);
+}
+
+static void segv_handler(int signum, siginfo_t *sinfo, void *ctx)
+{
+	fault_code = sinfo->si_code;
+
+	/* Check if this fault originated from the expected address */
+	if (sinfo->si_addr != (void *)fault_addr) {
+		sigsafe_err("got a fault for an unexpected address\n");
+		_exit(1);
+	}
+
+	/* Check if too many faults have occurred for a single test case */
+	if (!remaining_faults) {
+		sigsafe_err("got too many faults for the same address\n");
+		_exit(1);
+	}
+
+
+	/* Restore permissions in order to continue */
+	if (is_fault_expected(fault_code)) {
+		if (mprotect(insns, pgsize, PROT_READ | PROT_WRITE | PROT_EXEC)) {
+			sigsafe_err("failed to set access permissions\n");
+			_exit(1);
+		}
+	} else {
+		sigsafe_err("got a fault with an unexpected code\n");
+		_exit(1);
+	}
+
+	remaining_faults--;
+}
+
+static int check_exec_fault(int rights)
+{
+	/*
+	 * Jump to the executable region.
+	 *
+	 * The first iteration also checks if the overwrite of the
+	 * first instruction word from a trap to a no-op succeeded.
+	 */
+	fault_code = -1;
+	remaining_faults = 0;
+	if (!(rights & PROT_EXEC))
+		remaining_faults = 1;
+
+	FAIL_IF(mprotect(insns, pgsize, rights) != 0);
+	asm volatile("mtctr	%0; bctrl" : : "r"(insns));
+
+	FAIL_IF(remaining_faults != 0);
+	if (!(rights & PROT_EXEC))
+		FAIL_IF(!is_fault_expected(fault_code));
+
+	return 0;
+}
+
+static int test(void)
+{
+	struct sigaction segv_act, trap_act;
+	int i;
+
+	/* Skip the test if the CPU doesn't support Radix */
+	SKIP_IF(!have_hwcap2(PPC_FEATURE2_ARCH_3_00));
+
+	/* Check if pkeys are supported */
+	pkeys_supported = pkeys_unsupported() == 0;
+
+	/* Setup SIGSEGV handler */
+	segv_act.sa_handler = 0;
+	segv_act.sa_sigaction = segv_handler;
+	FAIL_IF(sigprocmask(SIG_SETMASK, 0, &segv_act.sa_mask) != 0);
+	segv_act.sa_flags = SA_SIGINFO;
+	segv_act.sa_restorer = 0;
+	FAIL_IF(sigaction(SIGSEGV, &segv_act, NULL) != 0);
+
+	/* Setup SIGTRAP handler */
+	trap_act.sa_handler = 0;
+	trap_act.sa_sigaction = trap_handler;
+	FAIL_IF(sigprocmask(SIG_SETMASK, 0, &trap_act.sa_mask) != 0);
+	trap_act.sa_flags = SA_SIGINFO;
+	trap_act.sa_restorer = 0;
+	FAIL_IF(sigaction(SIGTRAP, &trap_act, NULL) != 0);
+
+	/* Setup executable region */
+	pgsize = getpagesize();
+	numinsns = pgsize / sizeof(unsigned int);
+	insns = (unsigned int *)mmap(NULL, pgsize, PROT_READ | PROT_WRITE,
+				      MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+	FAIL_IF(insns == MAP_FAILED);
+
+	/* Write the instruction words */
+	for (i = 1; i < numinsns - 1; i++)
+		insns[i] = PPC_INST_NOP;
+
+	/*
+	 * Set the first instruction as an unconditional trap. If
+	 * the last write to this address succeeds, this should
+	 * get overwritten by a no-op.
+	 */
+	insns[0] = PPC_INST_TRAP;
+
+	/*
+	 * Later, to jump to the executable region, we use a branch
+	 * and link instruction (bctrl) which sets the return address
+	 * automatically in LR. Use that to return back.
+	 */
+	insns[numinsns - 1] = PPC_INST_BLR;
+
+	/*
+	 * Pick the first instruction's address from the executable
+	 * region.
+	 */
+	fault_addr = insns;
+
+	/*
+	 * Read an instruction word from the address when the page
+	 * is execute only. This should generate an access fault.
+	 */
+	fault_code = -1;
+	remaining_faults = 1;
+	printf("Testing read on --x, should fault...");
+	FAIL_IF(mprotect(insns, pgsize, PROT_EXEC) != 0);
+	i = *fault_addr;
+	FAIL_IF(remaining_faults != 0 || !is_fault_expected(fault_code));
+	printf("ok!\n");
+
+	/*
+	 * Write an instruction word to the address when the page
+	 * execute only. This should also generate an access fault.
+	 */
+	fault_code = -1;
+	remaining_faults = 1;
+	printf("Testing write on --x, should fault...");
+	FAIL_IF(mprotect(insns, pgsize, PROT_EXEC) != 0);
+	*fault_addr = PPC_INST_NOP;
+	FAIL_IF(remaining_faults != 0 || !is_fault_expected(fault_code));
+	printf("ok!\n");
+
+	printf("Testing exec on ---, should fault...");
+	FAIL_IF(check_exec_fault(PROT_NONE));
+	printf("ok!\n");
+
+	printf("Testing exec on r--, should fault...");
+	FAIL_IF(check_exec_fault(PROT_READ));
+	printf("ok!\n");
+
+	printf("Testing exec on -w-, should fault...");
+	FAIL_IF(check_exec_fault(PROT_WRITE));
+	printf("ok!\n");
+
+	printf("Testing exec on rw-, should fault...");
+	FAIL_IF(check_exec_fault(PROT_READ | PROT_WRITE));
+	printf("ok!\n");
+
+	printf("Testing exec on --x, should succeed...");
+	FAIL_IF(check_exec_fault(PROT_EXEC));
+	printf("ok!\n");
+
+	printf("Testing exec on r-x, should succeed...");
+	FAIL_IF(check_exec_fault(PROT_READ | PROT_EXEC));
+	printf("ok!\n");
+
+	printf("Testing exec on -wx, should succeed...");
+	FAIL_IF(check_exec_fault(PROT_WRITE | PROT_EXEC));
+	printf("ok!\n");
+
+	printf("Testing exec on rwx, should succeed...");
+	FAIL_IF(check_exec_fault(PROT_READ | PROT_WRITE | PROT_EXEC));
+	printf("ok!\n");
+
+	/* Cleanup */
+	FAIL_IF(munmap((void *)insns, pgsize));
+
+	return 0;
+}
+
+int main(void)
+{
+	return test_harness(test, "exec_prot");
+}