selftests/fpu: Add an FPU selftest

Add a selftest for the usage of FPU code in kernel mode.

Currently only implemented for x86. In the future, kernel FPU testing
could be unified between the different architectures supporting it.

 [ bp:

  - Split out from a conglomerate patch, put comments over statements.
  - run the test only on debugfs write.
  - Add bare-minimum run_test_fpu.sh, run 1000 iterations on all CPUs
    by default.
  - Add conditionally -msse2 so that clang doesn't generate library
    calls.
  - Use cc-option to detect gcc 7.1 not supporting -mpreferred-stack-boundary=3 (amluto).
  - Document stuff so that we don't forget.
  - Fix:
     ld: lib/test_fpu.o: in function `test_fpu_get':
     >> test_fpu.c:(.text+0x16e): undefined reference to `__sanitizer_cov_trace_cmpd'
     >> ld: test_fpu.c:(.text+0x1a7): undefined reference to `__sanitizer_cov_trace_cmpd'
     ld: test_fpu.c:(.text+0x1e0): undefined reference to `__sanitizer_cov_trace_cmpd'
  ]

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Petteri Aimonen <jpa@git.mail.kapsi.fi>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Link: https://lkml.kernel.org/r/20200624114646.28953-3-bp@alien8.de

lib/Kconfig.debug

@@ -2307,6 +2307,17 @@ config TEST_HMM
 
 	  If unsure, say N.
 
+config TEST_FPU
+	tristate "Test floating point operations in kernel space"
+	depends on X86 && !KCOV_INSTRUMENT_ALL
+	help
+	  Enable this option to add /sys/kernel/debug/selftest_helpers/test_fpu
+	  which will trigger a sequence of floating point operations. This is used
+	  for self-testing floating point control register setting in
+	  kernel_fpu_begin().
+
+	  If unsure, say N.
+
 endif # RUNTIME_TESTING_MENU
 
 config MEMTEST

lib/Makefile

@@ -99,6 +99,30 @@ obj-$(CONFIG_TEST_MEMINIT) += test_meminit.o
 obj-$(CONFIG_TEST_LOCKUP) += test_lockup.o
 obj-$(CONFIG_TEST_HMM) += test_hmm.o
 
+#
+# CFLAGS for compiling floating point code inside the kernel. x86/Makefile turns
+# off the generation of FPU/SSE* instructions for kernel proper but FPU_FLAGS
+# get appended last to CFLAGS and thus override those previous compiler options.
+#
+FPU_CFLAGS := -mhard-float -msse -msse2
+ifdef CONFIG_CC_IS_GCC
+# Stack alignment mismatch, proceed with caution.
+# GCC < 7.1 cannot compile code using `double` and -mpreferred-stack-boundary=3
+# (8B stack alignment).
+# See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53383
+#
+# The "-msse" in the first argument is there so that the
+# -mpreferred-stack-boundary=3 build error:
+#
+#  -mpreferred-stack-boundary=3 is not between 4 and 12
+#
+# can be triggered. Otherwise gcc doesn't complain.
+FPU_CFLAGS += $(call cc-option,-msse -mpreferred-stack-boundary=3,-mpreferred-stack-boundary=4)
+endif
+
+obj-$(CONFIG_TEST_FPU) += test_fpu.o
+CFLAGS_test_fpu.o += $(FPU_CFLAGS)
+
 obj-$(CONFIG_TEST_LIVEPATCH) += livepatch/
 
 obj-$(CONFIG_KUNIT) += kunit/

lib/test_fpu.c

@@ -0,0 +1,89 @@
+// SPDX-License-Identifier: GPL-2.0+
+/*
+ * Test cases for using floating point operations inside a kernel module.
+ *
+ * This tests kernel_fpu_begin() and kernel_fpu_end() functions, especially
+ * when userland has modified the floating point control registers. The kernel
+ * state might depend on the state set by the userland thread that was active
+ * before a syscall.
+ *
+ * To facilitate the test, this module registers file
+ * /sys/kernel/debug/selftest_helpers/test_fpu, which when read causes a
+ * sequence of floating point operations. If the operations fail, either the
+ * read returns error status or the kernel crashes.
+ * If the operations succeed, the read returns "1\n".
+ */
+
+#include <linux/module.h>
+#include <linux/kernel.h>
+#include <linux/debugfs.h>
+#include <asm/fpu/api.h>
+
+static int test_fpu(void)
+{
+	/*
+	 * This sequence of operations tests that rounding mode is
+	 * to nearest and that denormal numbers are supported.
+	 * Volatile variables are used to avoid compiler optimizing
+	 * the calculations away.
+	 */
+	volatile double a, b, c, d, e, f, g;
+
+	a = 4.0;
+	b = 1e-15;
+	c = 1e-310;
+
+	/* Sets precision flag */
+	d = a + b;
+
+	/* Result depends on rounding mode */
+	e = a + b / 2;
+
+	/* Denormal and very large values */
+	f = b / c;
+
+	/* Depends on denormal support */
+	g = a + c * f;
+
+	if (d > a && e > a && g > a)
+		return 0;
+	else
+		return -EINVAL;
+}
+
+static int test_fpu_get(void *data, u64 *val)
+{
+	int status = -EINVAL;
+
+	kernel_fpu_begin();
+	status = test_fpu();
+	kernel_fpu_end();
+
+	*val = 1;
+	return status;
+}
+
+DEFINE_SIMPLE_ATTRIBUTE(test_fpu_fops, test_fpu_get, NULL, "%lld\n");
+static struct dentry *selftest_dir;
+
+static int __init test_fpu_init(void)
+{
+	selftest_dir = debugfs_create_dir("selftest_helpers", NULL);
+	if (!selftest_dir)
+		return -ENOMEM;
+
+	debugfs_create_file("test_fpu", 0444, selftest_dir, NULL,
+			    &test_fpu_fops);
+
+	return 0;
+}
+
+static void __exit test_fpu_exit(void)
+{
+	debugfs_remove(selftest_dir);
+}
+
+module_init(test_fpu_init);
+module_exit(test_fpu_exit);
+
+MODULE_LICENSE("GPL");

tools/testing/selftests/Makefile

@@ -15,6 +15,7 @@ TARGETS += filesystems
 TARGETS += filesystems/binderfs
 TARGETS += filesystems/epoll
 TARGETS += firmware
+TARGETS += fpu
 TARGETS += ftrace
 TARGETS += futex
 TARGETS += gpio

tools/testing/selftests/fpu/.gitignore

@@ -0,0 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0+
+test_fpu

tools/testing/selftests/fpu/Makefile

@@ -0,0 +1,9 @@
+# SPDX-License-Identifier: GPL-2.0+
+
+LDLIBS := -lm
+
+TEST_GEN_PROGS := test_fpu
+
+TEST_PROGS := run_test_fpu.sh
+
+include ../lib.mk

tools/testing/selftests/fpu/run_test_fpu.sh

@@ -0,0 +1,46 @@
+#!/bin/bash
+# SPDX-License-Identifier: GPL-2.0
+#
+# Load kernel module for FPU tests
+
+uid=$(id -u)
+if [ $uid -ne 0 ]; then
+	echo "$0: Must be run as root"
+	exit 1
+fi
+
+if ! which modprobe > /dev/null 2>&1; then
+	echo "$0: You need modprobe installed"
+        exit 4
+fi
+
+if ! modinfo test_fpu > /dev/null 2>&1; then
+	echo "$0: You must have the following enabled in your kernel:"
+	echo "CONFIG_TEST_FPU=m"
+	exit 4
+fi
+
+NR_CPUS=$(getconf _NPROCESSORS_ONLN)
+if [ ! $NR_CPUS ]; then
+	NR_CPUS=1
+fi
+
+modprobe test_fpu
+
+if [ ! -e /sys/kernel/debug/selftest_helpers/test_fpu ]; then
+	mount -t debugfs none /sys/kernel/debug
+
+	if [ ! -e /sys/kernel/debug/selftest_helpers/test_fpu ]; then
+		echo "$0: Error mounting debugfs"
+		exit 4
+	fi
+fi
+
+echo "Running 1000 iterations on all CPUs... "
+for i in $(seq 1 1000); do
+	for c in $(seq 1 $NR_CPUS); do
+		./test_fpu &
+	done
+done
+
+rmmod test_fpu

tools/testing/selftests/fpu/test_fpu.c

@@ -0,0 +1,61 @@
+// SPDX-License-Identifier: GPL-2.0+
+/* This testcase operates with the test_fpu kernel driver.
+ * It modifies the FPU control register in user mode and calls the kernel
+ * module to perform floating point operations in the kernel. The control
+ * register value should be independent between kernel and user mode.
+ */
+
+#define _GNU_SOURCE
+#include <stdio.h>
+#include <errno.h>
+#include <string.h>
+#include <fenv.h>
+#include <unistd.h>
+#include <fcntl.h>
+
+const char *test_fpu_path = "/sys/kernel/debug/selftest_helpers/test_fpu";
+
+int main(void)
+{
+	char dummy[1];
+	int fd = open(test_fpu_path, O_RDONLY);
+
+	if (fd < 0) {
+		printf("[SKIP]\tcan't access %s: %s\n",
+		       test_fpu_path, strerror(errno));
+		return 0;
+	}
+
+	if (read(fd, dummy, 1) < 0) {
+		printf("[FAIL]\taccess with default rounding mode failed\n");
+		return 1;
+	}
+
+	fesetround(FE_DOWNWARD);
+	if (read(fd, dummy, 1) < 0) {
+		printf("[FAIL]\taccess with downward rounding mode failed\n");
+		return 2;
+	}
+	if (fegetround() != FE_DOWNWARD) {
+		printf("[FAIL]\tusermode rounding mode clobbered\n");
+		return 3;
+	}
+
+	/* Note: the tests up to this point are quite safe and will only return
+	 * an error. But the exception mask setting can cause misbehaving kernel
+	 * to crash.
+	 */
+	feclearexcept(FE_ALL_EXCEPT);
+	feenableexcept(FE_ALL_EXCEPT);
+	if (read(fd, dummy, 1) < 0) {
+		printf("[FAIL]\taccess with fpu exceptions unmasked failed\n");
+		return 4;
+	}
+	if (fegetexcept() != FE_ALL_EXCEPT) {
+		printf("[FAIL]\tusermode fpu exception mask clobbered\n");
+		return 5;
+	}
+
+	printf("[OK]\ttest_fpu\n");
+	return 0;
+}