selftests/powerpc: Add test for pkey siginfo verification

Commit c46241a ("powerpc/pkeys: Check vma before
returning key fault error to the user") fixes a bug which
causes the kernel to set the wrong pkey in siginfo when a
pkey fault occurs after two competing threads that have
allocated different pkeys, one fully permissive and the
other restrictive, attempt to protect a common page at the
same time. This adds a test to detect the bug.

Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/ce40b6ee270bda52e8f4088578ed2faf7d1d509a.1595821792.git.sandipan@linux.ibm.com

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

@@ -8,3 +8,4 @@ large_vm_fork_separation
 bad_accesses
 tlbie_test
 pkey_exec_prot
+pkey_siginfo

tools/testing/selftests/powerpc/mm/Makefile

@@ -3,7 +3,8 @@ noarg:
 	$(MAKE) -C ../
 
 TEST_GEN_PROGS := hugetlb_vs_thp_test subpage_prot segv_errors wild_bctr \
-		  large_vm_fork_separation bad_accesses pkey_exec_prot
+		  large_vm_fork_separation bad_accesses pkey_exec_prot \
+		  pkey_siginfo
 TEST_GEN_PROGS_EXTENDED := tlbie_test
 TEST_GEN_FILES := tempfile
 
@@ -16,8 +17,10 @@ $(OUTPUT)/wild_bctr: CFLAGS += -m64
 $(OUTPUT)/large_vm_fork_separation: CFLAGS += -m64
 $(OUTPUT)/bad_accesses: CFLAGS += -m64
 $(OUTPUT)/pkey_exec_prot: CFLAGS += -m64
+$(OUTPUT)/pkey_siginfo: CFLAGS += -m64
 
 $(OUTPUT)/tempfile:
 	dd if=/dev/zero of=$@ bs=64k count=1
 
 $(OUTPUT)/tlbie_test: LDLIBS += -lpthread
+$(OUTPUT)/pkey_siginfo: LDLIBS += -lpthread

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

@@ -0,0 +1,333 @@
+// SPDX-License-Identifier: GPL-2.0
+
+/*
+ * Copyright 2020, Sandipan Das, IBM Corp.
+ *
+ * Test if the signal information reports the correct memory protection
+ * key upon getting a key access violation fault for a page that was
+ * attempted to be protected by two different keys from two competing
+ * threads at the same time.
+ */
+
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <signal.h>
+
+#include <unistd.h>
+#include <pthread.h>
+#include <sys/mman.h>
+
+#include "pkeys.h"
+
+#define PPC_INST_NOP	0x60000000
+#define PPC_INST_BLR	0x4e800020
+#define PROT_RWX	(PROT_READ | PROT_WRITE | PROT_EXEC)
+
+#define NUM_ITERATIONS	1000000
+
+static volatile sig_atomic_t perm_pkey, rest_pkey;
+static volatile sig_atomic_t rights, fault_count;
+static volatile unsigned int *volatile fault_addr;
+static pthread_barrier_t iteration_barrier;
+
+static void segv_handler(int signum, siginfo_t *sinfo, void *ctx)
+{
+	void *pgstart;
+	size_t pgsize;
+	int pkey;
+
+	pkey = siginfo_pkey(sinfo);
+
+	/* Check if this fault originated from a pkey access violation */
+	if (sinfo->si_code != SEGV_PKUERR) {
+		sigsafe_err("got a fault for an unexpected reason\n");
+		_exit(1);
+	}
+
+	/* 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 this fault originated from the restrictive pkey */
+	if (pkey != rest_pkey) {
+		sigsafe_err("got a fault for an unexpected pkey\n");
+		_exit(1);
+	}
+
+	/* Check if too many faults have occurred for the same iteration */
+	if (fault_count > 0) {
+		sigsafe_err("got too many faults for the same address\n");
+		_exit(1);
+	}
+
+	pgsize = getpagesize();
+	pgstart = (void *) ((unsigned long) fault_addr & ~(pgsize - 1));
+
+	/*
+	 * If the current fault occurred due to lack of execute rights,
+	 * reassociate the page with the exec-only pkey since execute
+	 * rights cannot be changed directly for the faulting pkey as
+	 * IAMR is inaccessible from userspace.
+	 *
+	 * Otherwise, if the current fault occurred due to lack of
+	 * read-write rights, change the AMR permission bits for the
+	 * pkey.
+	 *
+	 * This will let the test continue.
+	 */
+	if (rights == PKEY_DISABLE_EXECUTE &&
+	    mprotect(pgstart, pgsize, PROT_EXEC))
+		_exit(1);
+	else
+		pkey_set_rights(pkey, 0);
+
+	fault_count++;
+}
+
+struct region {
+	unsigned long rights;
+	unsigned int *base;
+	size_t size;
+};
+
+static void *protect(void *p)
+{
+	unsigned long rights;
+	unsigned int *base;
+	size_t size;
+	int tid, i;
+
+	tid = gettid();
+	base = ((struct region *) p)->base;
+	size = ((struct region *) p)->size;
+	FAIL_IF_EXIT(!base);
+
+	/* No read, write and execute restrictions */
+	rights = 0;
+
+	printf("tid %d, pkey permissions are %s\n", tid, pkey_rights(rights));
+
+	/* Allocate the permissive pkey */
+	perm_pkey = sys_pkey_alloc(0, rights);
+	FAIL_IF_EXIT(perm_pkey < 0);
+
+	/*
+	 * Repeatedly try to protect the common region with a permissive
+	 * pkey
+	 */
+	for (i = 0; i < NUM_ITERATIONS; i++) {
+		/*
+		 * Wait until the other thread has finished allocating the
+		 * restrictive pkey or until the next iteration has begun
+		 */
+		pthread_barrier_wait(&iteration_barrier);
+
+		/* Try to associate the permissive pkey with the region */
+		FAIL_IF_EXIT(sys_pkey_mprotect(base, size, PROT_RWX,
+					       perm_pkey));
+	}
+
+	/* Free the permissive pkey */
+	sys_pkey_free(perm_pkey);
+
+	return NULL;
+}
+
+static void *protect_access(void *p)
+{
+	size_t size, numinsns;
+	unsigned int *base;
+	int tid, i;
+
+	tid = gettid();
+	base = ((struct region *) p)->base;
+	size = ((struct region *) p)->size;
+	rights = ((struct region *) p)->rights;
+	numinsns = size / sizeof(base[0]);
+	FAIL_IF_EXIT(!base);
+
+	/* Allocate the restrictive pkey */
+	rest_pkey = sys_pkey_alloc(0, rights);
+	FAIL_IF_EXIT(rest_pkey < 0);
+
+	printf("tid %d, pkey permissions are %s\n", tid, pkey_rights(rights));
+	printf("tid %d, %s randomly in range [%p, %p]\n", tid,
+	       (rights == PKEY_DISABLE_EXECUTE) ? "execute" :
+	       (rights == PKEY_DISABLE_WRITE)  ? "write" : "read",
+	       base, base + numinsns);
+
+	/*
+	 * Repeatedly try to protect the common region with a restrictive
+	 * pkey and read, write or execute from it
+	 */
+	for (i = 0; i < NUM_ITERATIONS; i++) {
+		/*
+		 * Wait until the other thread has finished allocating the
+		 * permissive pkey or until the next iteration has begun
+		 */
+		pthread_barrier_wait(&iteration_barrier);
+
+		/* Try to associate the restrictive pkey with the region */
+		FAIL_IF_EXIT(sys_pkey_mprotect(base, size, PROT_RWX,
+					       rest_pkey));
+
+		/* Choose a random instruction word address from the region */
+		fault_addr = base + (rand() % numinsns);
+		fault_count = 0;
+
+		switch (rights) {
+		/* Read protection test */
+		case PKEY_DISABLE_ACCESS:
+			/*
+			 * Read an instruction word from the region and
+			 * verify if it has not been overwritten to
+			 * something unexpected
+			 */
+			FAIL_IF_EXIT(*fault_addr != PPC_INST_NOP &&
+				     *fault_addr != PPC_INST_BLR);
+			break;
+
+		/* Write protection test */
+		case PKEY_DISABLE_WRITE:
+			/*
+			 * Write an instruction word to the region and
+			 * verify if the overwrite has succeeded
+			 */
+			*fault_addr = PPC_INST_BLR;
+			FAIL_IF_EXIT(*fault_addr != PPC_INST_BLR);
+			break;
+
+		/* Execute protection test */
+		case PKEY_DISABLE_EXECUTE:
+			/* Jump to the region and execute instructions */
+			asm volatile(
+				"mtctr	%0; bctrl"
+				: : "r"(fault_addr) : "ctr", "lr");
+			break;
+		}
+
+		/*
+		 * Restore the restrictions originally imposed by the
+		 * restrictive pkey as the signal handler would have
+		 * cleared out the corresponding AMR bits
+		 */
+		pkey_set_rights(rest_pkey, rights);
+	}
+
+	/* Free restrictive pkey */
+	sys_pkey_free(rest_pkey);
+
+	return NULL;
+}
+
+static void reset_pkeys(unsigned long rights)
+{
+	int pkeys[NR_PKEYS], i;
+
+	/* Exhaustively allocate all available pkeys */
+	for (i = 0; i < NR_PKEYS; i++)
+		pkeys[i] = sys_pkey_alloc(0, rights);
+
+	/* Free all allocated pkeys */
+	for (i = 0; i < NR_PKEYS; i++)
+		sys_pkey_free(pkeys[i]);
+}
+
+static int test(void)
+{
+	pthread_t prot_thread, pacc_thread;
+	struct sigaction act;
+	pthread_attr_t attr;
+	size_t numinsns;
+	struct region r;
+	int ret, i;
+
+	srand(time(NULL));
+	ret = pkeys_unsupported();
+	if (ret)
+		return ret;
+
+	/* Allocate the region */
+	r.size = getpagesize();
+	r.base = mmap(NULL, r.size, PROT_RWX,
+		      MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+	FAIL_IF(r.base == MAP_FAILED);
+
+	/*
+	 * Fill the region with no-ops with a branch at the end
+	 * for returning to the caller
+	 */
+	numinsns = r.size / sizeof(r.base[0]);
+	for (i = 0; i < numinsns - 1; i++)
+		r.base[i] = PPC_INST_NOP;
+	r.base[i] = PPC_INST_BLR;
+
+	/* Setup SIGSEGV handler */
+	act.sa_handler = 0;
+	act.sa_sigaction = segv_handler;
+	FAIL_IF(sigprocmask(SIG_SETMASK, 0, &act.sa_mask) != 0);
+	act.sa_flags = SA_SIGINFO;
+	act.sa_restorer = 0;
+	FAIL_IF(sigaction(SIGSEGV, &act, NULL) != 0);
+
+	/*
+	 * For these tests, the parent process should clear all bits of
+	 * AMR and IAMR, i.e. impose no restrictions, for all available
+	 * pkeys. This will be the base for the initial AMR and IAMR
+	 * values for all the test thread pairs.
+	 *
+	 * If the AMR and IAMR bits of all available pkeys are cleared
+	 * before running the tests and a fault is generated when
+	 * attempting to read, write or execute instructions from a
+	 * pkey protected region, the pkey responsible for this must be
+	 * the one from the protect-and-access thread since the other
+	 * one is fully permissive. Despite that, if the pkey reported
+	 * by siginfo is not the restrictive pkey, then there must be a
+	 * kernel bug.
+	 */
+	reset_pkeys(0);
+
+	/* Setup barrier for protect and protect-and-access threads */
+	FAIL_IF(pthread_attr_init(&attr) != 0);
+	FAIL_IF(pthread_barrier_init(&iteration_barrier, NULL, 2) != 0);
+
+	/* Setup and start protect and protect-and-read threads */
+	puts("starting thread pair (protect, protect-and-read)");
+	r.rights = PKEY_DISABLE_ACCESS;
+	FAIL_IF(pthread_create(&prot_thread, &attr, &protect, &r) != 0);
+	FAIL_IF(pthread_create(&pacc_thread, &attr, &protect_access, &r) != 0);
+	FAIL_IF(pthread_join(prot_thread, NULL) != 0);
+	FAIL_IF(pthread_join(pacc_thread, NULL) != 0);
+
+	/* Setup and start protect and protect-and-write threads */
+	puts("starting thread pair (protect, protect-and-write)");
+	r.rights = PKEY_DISABLE_WRITE;
+	FAIL_IF(pthread_create(&prot_thread, &attr, &protect, &r) != 0);
+	FAIL_IF(pthread_create(&pacc_thread, &attr, &protect_access, &r) != 0);
+	FAIL_IF(pthread_join(prot_thread, NULL) != 0);
+	FAIL_IF(pthread_join(pacc_thread, NULL) != 0);
+
+	/* Setup and start protect and protect-and-execute threads */
+	puts("starting thread pair (protect, protect-and-execute)");
+	r.rights = PKEY_DISABLE_EXECUTE;
+	FAIL_IF(pthread_create(&prot_thread, &attr, &protect, &r) != 0);
+	FAIL_IF(pthread_create(&pacc_thread, &attr, &protect_access, &r) != 0);
+	FAIL_IF(pthread_join(prot_thread, NULL) != 0);
+	FAIL_IF(pthread_join(pacc_thread, NULL) != 0);
+
+	/* Cleanup */
+	FAIL_IF(pthread_attr_destroy(&attr) != 0);
+	FAIL_IF(pthread_barrier_destroy(&iteration_barrier) != 0);
+	munmap(r.base, r.size);
+
+	return 0;
+}
+
+int main(void)
+{
+	test_harness(test, "pkey_siginfo");
+}