core: riscv: Support Privileged Access Never by CSR status.SUM bit

The SUM (Supervisor User Memory access) bit modifies the privilege with
which S-mode loads and stores the user virtual memory. When SUM bit is
0, S-mode accesses to pages whose U bit of corresponding PTE is set will
fault. When SUM bit is 1, these accesses are permitted.

When CFG_PAN is disabled in RISC-V architecture, the status.SUM bit is
initialized as 1 by default. Therefore all accesses to user pages will
succeed. When CFG_PAN is enabled, the status.SUM bit is initialized as
0, and only set to 1 when kernel needs to access user pages.

Signed-off-by: Alvin Chang <alvinga@andestech.com>
Tested-by: Marouene Boubakri <marouene.boubakri@nxp.com>

core/arch/riscv/include/kernel/user_access_arch.h

@@ -1,15 +1,29 @@
 /* SPDX-License-Identifier: BSD-2-Clause */
 /*
+ * Copyright (c) 2023 Andes Technology Corporation
  * Copyright (c) 2023, Amazon.com Inc. or its affiliates. All rights Reserved.
  */
 
 #ifndef __KERNEL_USER_ACCESS_ARCH_H
 #define __KERNEL_USER_ACCESS_ARCH_H
 
+#include <riscv.h>
+
+#ifdef CFG_PAN
 /* Enter a section where user mode access is temporarily enabled. */
-static inline void enter_user_access(void) {}
+static inline void enter_user_access(void)
+{
+	set_csr(CSR_XSTATUS, CSR_XSTATUS_SUM);
+}
 
 /* Exit from the section where user mode access was temporarily enabled. */
+static inline void exit_user_access(void)
+{
+	clear_csr(CSR_XSTATUS, CSR_XSTATUS_SUM);
+}
+#else
+static inline void enter_user_access(void) {}
 static inline void exit_user_access(void) {}
+#endif /* CFG_PAN */
 
 #endif /* __KERNEL_USER_ACCESS_ARCH_H */

core/arch/riscv/kernel/arch_scall.c

@@ -10,6 +10,7 @@
 #include <kernel/scall.h>
 #include <kernel/thread.h>
 #include <kernel/trace_ta.h>
+#include <kernel/user_access.h>
 #include <kernel/user_ta.h>
 #include <mm/vm.h>
 #include <riscv.h>
@@ -23,15 +24,29 @@
 static void save_panic_regs_rv_ta(struct thread_specific_data *tsd,
 				  unsigned long *pushed)
 {
-	tsd->abort_regs = (struct thread_abort_regs){
-		.sp = (unsigned long)pushed,
+	TEE_Result res = TEE_SUCCESS;
+	unsigned long s0 = 0;
+	unsigned long epc = 0;
 #if defined(RV32)
-		.s0 = pushed[2],
-		.epc = pushed[3],
+	unsigned long *stack_s0 = &pushed[2];
+	unsigned long *stack_epc = &pushed[3];
 #elif defined(RV64)
-		.s0 = pushed[0],
-		.epc = pushed[1],
+	unsigned long *stack_s0 = &pushed[0];
+	unsigned long *stack_epc = &pushed[1];
 #endif
+
+	res = GET_USER_SCALAR(s0, stack_s0);
+	if (res)
+		s0 = 0;
+
+	res = GET_USER_SCALAR(epc, stack_epc);
+	if (res)
+		epc = 0;
+
+	tsd->abort_regs = (struct thread_abort_regs){
+		.sp = (unsigned long)pushed,
+		.s0 = s0,
+		.epc = epc,
 	};
 }
 

core/arch/riscv/kernel/thread_arch.c

@@ -565,8 +565,13 @@ void thread_init_per_cpu(void)
 	 * and kernel traps.
 	 */
 	write_csr(CSR_XSCRATCH, 0);
-	/* Allow access to user pages */
+#ifndef CFG_PAN
+	/*
+	 * Allow access to user pages. When CFG_PAN is enabled, the SUM bit will
+	 * be set and clear at runtime when necessary.
+	 */
 	set_csr(CSR_XSTATUS, CSR_XSTATUS_SUM);
+#endif
 }
 
 static void set_ctx_regs(struct thread_ctx_regs *regs, unsigned long a0,

mk/config.mk

@@ -929,9 +929,10 @@ endif
 
 # Privileged Access Never (PAN, part of the ARMv8.1 Extensions) can be
 # used to restrict accesses to unprivileged memory from privileged mode.
+# For RISC-V architecture, CSR {m|s}status.SUM bit is used to implement PAN.
 CFG_PAN ?= n
 
-$(eval $(call cfg-depends-all,CFG_PAN,CFG_ARM64_core))
+$(eval $(call cfg-depends-one,CFG_PAN,CFG_ARM64_core CFG_RV64_core CFG_RV32_core))
 
 # CFG_CORE_ASYNC_NOTIF is defined by the platform to enable support
 # for sending asynchronous notifications to normal world. Note that an