arch: riscv: reset global pointer on exception

arch/riscv/core/isr.S

@@ -163,6 +163,14 @@ SECTION_FUNC(exception.entry, _isr_wrapper)
 	lr t0, ___cpu_t_current_OFFSET(s0)
 	lr tp, _thread_offset_to_tls(t0)
 
+	/* Make sure global pointer is sane */
+#ifdef CONFIG_RISCV_GP
+	.option push
+	.option norelax
+	la gp, __global_pointer$
+	.option pop
+#endif /* CONFIG_RISCV_GP */
+
 	/* Clear our per-thread usermode flag */
 	lui t0, %tprel_hi(is_user_mode)
 	add t0, t0, tp, %tprel_add(is_user_mode)

include/zephyr/arch/riscv/reg.h

@@ -0,0 +1,19 @@
+/*
+ * Copyright (c) 2024 Meta Platforms
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#ifndef ZEPHYR_INCLUDE_ZEPHYR_ARCH_RISCV_REG_H_
+#define ZEPHYR_INCLUDE_ZEPHYR_ARCH_RISCV_REG_H_
+
+#define reg_read(reg)                                                                              \
+	({                                                                                         \
+		register unsigned long __rv;                                                       \
+		__asm__ volatile("mv %0, " STRINGIFY(reg) : "=r"(__rv));                           \
+		__rv;                                                                              \
+	})
+
+#define reg_write(reg, val) ({ __asm__("mv " STRINGIFY(reg) ", %0" : : "r"(val)); })
+
+#endif /* ZEPHYR_INCLUDE_ZEPHYR_ARCH_RISCV_REG_H_ */

tests/arch/riscv/userspace/riscv_gp/CMakeLists.txt

@@ -0,0 +1,8 @@
+# SPDX-License-Identifier: Apache-2.0
+
+cmake_minimum_required(VERSION 3.20.0)
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
+project(riscv_gp)
+
+FILE(GLOB app_sources src/*.c)
+target_sources(app PRIVATE ${app_sources})

tests/arch/riscv/userspace/riscv_gp/prj.conf

@@ -0,0 +1,3 @@
+CONFIG_ZTEST=y
+CONFIG_RISCV_GP=y
+CONFIG_TEST_USERSPACE=y

tests/arch/riscv/userspace/riscv_gp/src/main.c

@@ -0,0 +1,47 @@
+/*
+ * Copyright (c) 2024 Meta Platforms
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+#include <stdio.h>
+
+#include <zephyr/arch/riscv/reg.h>
+#include <zephyr/kernel.h>
+#include <zephyr/ztest.h>
+
+#define ROGUE_USER_STACK_SZ 2048
+
+static struct k_thread rogue_user_thread;
+static K_THREAD_STACK_DEFINE(rogue_user_stack, ROGUE_USER_STACK_SZ);
+
+static void rogue_user_fn(void *p1, void *p2, void *p3)
+{
+	zassert_true(k_is_user_context());
+
+	reg_write(gp, 0xbad);
+	zassert_equal(reg_read(gp), 0xbad);
+}
+
+ZTEST_USER(riscv_gp, test_gp_value)
+{
+	uintptr_t gp_val = reg_read(gp);
+	k_tid_t th;
+
+	zassert_not_equal(gp_val, 0);
+
+	th = k_thread_create(&rogue_user_thread, rogue_user_stack, ROGUE_USER_STACK_SZ,
+			     rogue_user_fn, NULL, NULL, NULL, -1, K_USER, K_NO_WAIT);
+	zassert_ok(k_thread_join(th, K_FOREVER));
+
+	zassert_equal(reg_read(gp), gp_val, "`gp` corrupted by user thread");
+}
+
+static void *userspace_setup(void)
+{
+	k_thread_access_grant(k_current_get(), &rogue_user_thread, &rogue_user_stack);
+
+	return NULL;
+}
+
+ZTEST_SUITE(riscv_gp, NULL, userspace_setup, NULL, NULL, NULL);

tests/arch/riscv/userspace/riscv_gp/testcase.yaml

@@ -0,0 +1,8 @@
+common:
+  ignore_faults: true
+  ignore_qemu_crash: true
+  tags: kernel riscv
+  platform_allow:
+    - qemu_riscv64
+tests:
+  arch.riscv64.riscv_gp: {}