selftests/powerpc: Add test for execute-disabled pkeys

Apart from read and write access, memory protection keys can
also be used for restricting execute permission of pages on
powerpc. This adds a test to verify if the feature works as
expected.

Signed-off-by: Sandipan Das <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

tools/testing/selftests/powerpc/mm/.gitignore

···
 large_vm_fork_separation
 bad_accesses
 tlbie_test
+pkey_exec_prot

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
+		  large_vm_fork_separation bad_accesses pkey_exec_prot
 TEST_GEN_PROGS_EXTENDED := tlbie_test
 TEST_GEN_FILES := tempfile
 
···
 $(OUTPUT)/wild_bctr: CFLAGS += -m64
 $(OUTPUT)/large_vm_fork_separation: CFLAGS += -m64
 $(OUTPUT)/bad_accesses: CFLAGS += -m64
+$(OUTPUT)/pkey_exec_prot: CFLAGS += -m64
 
 $(OUTPUT)/tempfile:
 	dd if=/dev/zero of=$@ bs=64k count=1

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

···
+// SPDX-License-Identifier: GPL-2.0+
+
+/*
+ * Copyright 2020, Sandipan Das, IBM Corp.
+ *
+ * Test if applying execute protection on pages using memory
+ * protection keys 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 "reg.h"
+#include "utils.h"
+
+/*
+ * Older versions of libc use the Intel-specific access rights.
+ * Hence, override the definitions as they might be incorrect.
+ */
+#undef PKEY_DISABLE_ACCESS
+#define PKEY_DISABLE_ACCESS	0x3
+
+#undef PKEY_DISABLE_WRITE
+#define PKEY_DISABLE_WRITE	0x2
+
+#undef PKEY_DISABLE_EXECUTE
+#define PKEY_DISABLE_EXECUTE	0x4
+
+/* Older versions of libc do not not define this */
+#ifndef SEGV_PKUERR
+#define SEGV_PKUERR	4
+#endif
+
+#define SI_PKEY_OFFSET	0x20
+
+#define SYS_pkey_mprotect	386
+#define SYS_pkey_alloc		384
+#define SYS_pkey_free		385
+
+#define PKEY_BITS_PER_PKEY	2
+#define NR_PKEYS		32
+#define PKEY_BITS_MASK		((1UL << PKEY_BITS_PER_PKEY) - 1)
+
+#define PPC_INST_NOP	0x60000000
+#define PPC_INST_TRAP	0x7fe00008
+#define PPC_INST_BLR	0x4e800020
+
+#define sigsafe_err(msg)	({ \
+		ssize_t nbytes __attribute__((unused)); \
+		nbytes = write(STDERR_FILENO, msg, strlen(msg)); })
+
+static inline unsigned long pkeyreg_get(void)
+{
+	return mfspr(SPRN_AMR);
+}
+
+static inline void pkeyreg_set(unsigned long amr)
+{
+	set_amr(amr);
+}
+
+static void pkey_set_rights(int pkey, unsigned long rights)
+{
+	unsigned long amr, shift;
+
+	shift = (NR_PKEYS - pkey - 1) * PKEY_BITS_PER_PKEY;
+	amr = pkeyreg_get();
+	amr &= ~(PKEY_BITS_MASK << shift);
+	amr |= (rights & PKEY_BITS_MASK) << shift;
+	pkeyreg_set(amr);
+}
+
+static int sys_pkey_mprotect(void *addr, size_t len, int prot, int pkey)
+{
+	return syscall(SYS_pkey_mprotect, addr, len, prot, pkey);
+}
+
+static int sys_pkey_alloc(unsigned long flags, unsigned long rights)
+{
+	return syscall(SYS_pkey_alloc, flags, rights);
+}
+
+static int sys_pkey_free(int pkey)
+{
+	return syscall(SYS_pkey_free, pkey);
+}
+
+static volatile sig_atomic_t fault_pkey, fault_code, fault_type;
+static volatile sig_atomic_t remaining_faults;
+static volatile unsigned int *fault_addr;
+static unsigned long pgsize, numinsns;
+static unsigned int *insns;
+
+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)
+{
+	int signal_pkey;
+
+	/*
+	 * In older versions of libc, siginfo_t does not have si_pkey as
+	 * a member.
+	 */
+#ifdef si_pkey
+	signal_pkey = sinfo->si_pkey;
+#else
+	signal_pkey = *((int *)(((char *) sinfo) + SI_PKEY_OFFSET));
+#endif
+
+	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 */
+	switch (fault_code) {
+	case SEGV_ACCERR:
+		if (mprotect(insns, pgsize, PROT_READ | PROT_WRITE)) {
+			sigsafe_err("failed to set access permissions\n");
+			_exit(1);
+		}
+		break;
+	case SEGV_PKUERR:
+		if (signal_pkey != fault_pkey) {
+			sigsafe_err("got a fault for an unexpected pkey\n");
+			_exit(1);
+		}
+
+		switch (fault_type) {
+		case PKEY_DISABLE_ACCESS:
+			pkey_set_rights(fault_pkey, 0);
+			break;
+		case PKEY_DISABLE_EXECUTE:
+			/*
+			 * Reassociate the exec-only pkey with the region
+			 * to be able to continue. Unlike AMR, we cannot
+			 * set IAMR directly from userspace to restore the
+			 * permissions.
+			 */
+			if (mprotect(insns, pgsize, PROT_EXEC)) {
+				sigsafe_err("failed to set execute permissions\n");
+				_exit(1);
+			}
+			break;
+		default:
+			sigsafe_err("got a fault with an unexpected type\n");
+			_exit(1);
+		}
+		break;
+	default:
+		sigsafe_err("got a fault with an unexpected code\n");
+		_exit(1);
+	}
+
+	remaining_faults--;
+}
+
+static int pkeys_unsupported(void)
+{
+	bool hash_mmu = false;
+	int pkey;
+
+	/* Protection keys are currently supported on Hash MMU only */
+	FAIL_IF(using_hash_mmu(&hash_mmu));
+	SKIP_IF(!hash_mmu);
+
+	/* Check if the system call is supported */
+	pkey = sys_pkey_alloc(0, 0);
+	SKIP_IF(pkey < 0);
+	sys_pkey_free(pkey);
+
+	return 0;
+}
+
+static int test(void)
+{
+	struct sigaction segv_act, trap_act;
+	int pkey, ret, i;
+
+	ret = pkeys_unsupported();
+	if (ret)
+		return ret;
+
+	/* 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;
+
+	/* Allocate a pkey that restricts execution */
+	pkey = sys_pkey_alloc(0, PKEY_DISABLE_EXECUTE);
+	FAIL_IF(pkey < 0);
+
+	/*
+	 * Pick the first instruction's address from the executable
+	 * region.
+	 */
+	fault_addr = insns;
+
+	/* The following two cases will avoid SEGV_PKUERR */
+	fault_type = -1;
+	fault_pkey = -1;
+
+	/*
+	 * Read an instruction word from the address when AMR bits
+	 * are not set i.e. the pkey permits both read and write
+	 * access.
+	 *
+	 * This should not generate a fault as having PROT_EXEC
+	 * implies PROT_READ on GNU systems. The pkey currently
+	 * restricts execution only based on the IAMR bits. The
+	 * AMR bits are cleared.
+	 */
+	remaining_faults = 0;
+	FAIL_IF(sys_pkey_mprotect(insns, pgsize, PROT_EXEC, pkey) != 0);
+	printf("read from %p, pkey is execute-disabled, access-enabled\n",
+	       (void *) fault_addr);
+	i = *fault_addr;
+	FAIL_IF(remaining_faults != 0);
+
+	/*
+	 * Write an instruction word to the address when AMR bits
+	 * are not set i.e. the pkey permits both read and write
+	 * access.
+	 *
+	 * This should generate an access fault as having just
+	 * PROT_EXEC also restricts writes. The pkey currently
+	 * restricts execution only based on the IAMR bits. The
+	 * AMR bits are cleared.
+	 */
+	remaining_faults = 1;
+	FAIL_IF(sys_pkey_mprotect(insns, pgsize, PROT_EXEC, pkey) != 0);
+	printf("write to %p, pkey is execute-disabled, access-enabled\n",
+	       (void *) fault_addr);
+	*fault_addr = PPC_INST_TRAP;
+	FAIL_IF(remaining_faults != 0 || fault_code != SEGV_ACCERR);
+
+	/* The following three cases will generate SEGV_PKUERR */
+	fault_type = PKEY_DISABLE_ACCESS;
+	fault_pkey = pkey;
+
+	/*
+	 * Read an instruction word from the address when AMR bits
+	 * are set i.e. the pkey permits neither read nor write
+	 * access.
+	 *
+	 * This should generate a pkey fault based on AMR bits only
+	 * as having PROT_EXEC implicitly allows reads.
+	 */
+	remaining_faults = 1;
+	FAIL_IF(sys_pkey_mprotect(insns, pgsize, PROT_EXEC, pkey) != 0);
+	printf("read from %p, pkey is execute-disabled, access-disabled\n",
+	       (void *) fault_addr);
+	pkey_set_rights(pkey, PKEY_DISABLE_ACCESS);
+	i = *fault_addr;
+	FAIL_IF(remaining_faults != 0 || fault_code != SEGV_PKUERR);
+
+	/*
+	 * Write an instruction word to the address when AMR bits
+	 * are set i.e. the pkey permits neither read nor write
+	 * access.
+	 *
+	 * This should generate two faults. First, a pkey fault
+	 * based on AMR bits and then an access fault since
+	 * PROT_EXEC does not allow writes.
+	 */
+	remaining_faults = 2;
+	FAIL_IF(sys_pkey_mprotect(insns, pgsize, PROT_EXEC, pkey) != 0);
+	printf("write to %p, pkey is execute-disabled, access-disabled\n",
+	       (void *) fault_addr);
+	pkey_set_rights(pkey, PKEY_DISABLE_ACCESS);
+	*fault_addr = PPC_INST_NOP;
+	FAIL_IF(remaining_faults != 0 || fault_code != SEGV_ACCERR);
+
+	/*
+	 * Jump to the executable region when AMR bits are set i.e.
+	 * the pkey permits neither read nor write access.
+	 *
+	 * This should generate a pkey fault based on IAMR bits which
+	 * are set to not permit execution. AMR bits should not affect
+	 * execution.
+	 *
+	 * This also checks if the overwrite of the first instruction
+	 * word from a trap to a no-op succeeded.
+	 */
+	fault_addr = insns;
+	fault_type = PKEY_DISABLE_EXECUTE;
+	fault_pkey = pkey;
+	remaining_faults = 1;
+	FAIL_IF(sys_pkey_mprotect(insns, pgsize, PROT_EXEC, pkey) != 0);
+	pkey_set_rights(pkey, PKEY_DISABLE_ACCESS);
+	printf("execute at %p, pkey is execute-disabled, access-disabled\n",
+	       (void *) fault_addr);
+	asm volatile("mtctr	%0; bctrl" : : "r"(insns));
+	FAIL_IF(remaining_faults != 0 || fault_code != SEGV_PKUERR);
+
+	/*
+	 * Free the current pkey and allocate a new one that is
+	 * fully permissive.
+	 */
+	sys_pkey_free(pkey);
+	pkey = sys_pkey_alloc(0, 0);
+
+	/*
+	 * Jump to the executable region when AMR bits are not set
+	 * i.e. the pkey permits read and write access.
+	 *
+	 * This should not generate any faults as the IAMR bits are
+	 * also not set and hence will the pkey will not restrict
+	 * execution.
+	 */
+	fault_pkey = pkey;
+	remaining_faults = 0;
+	FAIL_IF(sys_pkey_mprotect(insns, pgsize, PROT_EXEC, pkey) != 0);
+	printf("execute at %p, pkey is execute-enabled, access-enabled\n",
+	       (void *) fault_addr);
+	asm volatile("mtctr	%0; bctrl" : : "r"(insns));
+	FAIL_IF(remaining_faults != 0);
+
+	/* Cleanup */
+	munmap((void *) insns, pgsize);
+	sys_pkey_free(pkey);
+
+	return 0;
+}
+
+int main(void)
+{
+	test_harness(test, "pkey_exec_prot");
+}