semihosting: Allow optional use of semihosting from userspace

Currently our semihosting implementations generally prohibit use of
semihosting calls in system emulation from the guest userspace.  This
is a very long standing behaviour justified originally "to provide
some semblance of security" (since code with access to the
semihosting ABI can do things like read and write arbitrary files on
the host system).  However, it is sometimes useful to be able to run
trusted guest code which performs semihosting calls from guest
userspace, notably for test code.  Add a command line suboption to
the existing semihosting-config option group so that you can
explicitly opt in to semihosting from guest userspace with
 -semihosting-config userspace=on

(There is no equivalent option for the user-mode emulator, because
there by definition all code runs in userspace and has access to
semihosting already.)

This commit adds the infrastructure for the command line option and
adds a bool 'is_user' parameter to the function
semihosting_userspace_enabled() that target code can use to check
whether it should be permitting the semihosting call for userspace.
It mechanically makes all the callsites pass 'false', so they
continue checking "is semihosting enabled in general".  Subsequent
commits will make each target that implements semihosting honour the
userspace=on option by passing the correct value and removing
whatever "don't do this for userspace" checking they were doing by
hand.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220822141230.3658237-2-peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

include/semihosting/semihost.h

@@ -27,7 +27,7 @@ typedef enum SemihostingTarget {
 } SemihostingTarget;
 
 #ifdef CONFIG_USER_ONLY
-static inline bool semihosting_enabled(void)
+static inline bool semihosting_enabled(bool is_user)
 {
     return true;
 }
@@ -52,7 +52,13 @@ static inline const char *semihosting_get_cmdline(void)
     return NULL;
 }
 #else /* !CONFIG_USER_ONLY */
-bool semihosting_enabled(void);
+/**
+ * semihosting_enabled:
+ * @is_user: true if guest code is in usermode (i.e. not privileged)
+ *
+ * Return true if guest code is allowed to make semihosting calls.
+ */
+bool semihosting_enabled(bool is_user);
 SemihostingTarget semihosting_get_target(void);
 const char *semihosting_get_arg(int i);
 int semihosting_get_argc(void);

qemu-options.hx

@@ -4627,12 +4627,12 @@ SRST
     information about the facilities this enables.
 ERST
 DEF("semihosting-config", HAS_ARG, QEMU_OPTION_semihosting_config,
-    "-semihosting-config [enable=on|off][,target=native|gdb|auto][,chardev=id][,arg=str[,...]]\n" \
+    "-semihosting-config [enable=on|off][,target=native|gdb|auto][,chardev=id][,userspace=on|off][,arg=str[,...]]\n" \
     "                semihosting configuration\n",
 QEMU_ARCH_ARM | QEMU_ARCH_M68K | QEMU_ARCH_XTENSA |
 QEMU_ARCH_MIPS | QEMU_ARCH_NIOS2 | QEMU_ARCH_RISCV)
 SRST
-``-semihosting-config [enable=on|off][,target=native|gdb|auto][,chardev=id][,arg=str[,...]]``
+``-semihosting-config [enable=on|off][,target=native|gdb|auto][,chardev=id][,userspace=on|off][,arg=str[,...]]``
     Enable and configure semihosting (ARM, M68K, Xtensa, MIPS, Nios II, RISC-V
     only).
 
@@ -4659,6 +4659,13 @@ SRST
         Send the output to a chardev backend output for native or auto
         output when not in gdb
 
+    ``userspace=on|off``
+        Allows code running in guest userspace to access the semihosting
+        interface. The default is that only privileged guest code can
+        make semihosting calls. Note that setting ``userspace=on`` should
+        only be used if all guest code is trusted (for example, in
+        bare-metal test case code).
+
     ``arg=str1,arg=str2,...``
         Allows the user to pass input arguments, and can be used
         multiple times to build up a list. The old-style

semihosting/config.c

@@ -34,6 +34,9 @@ QemuOptsList qemu_semihosting_config_opts = {
         {
             .name = "enable",
             .type = QEMU_OPT_BOOL,
+        }, {
+            .name = "userspace",
+            .type = QEMU_OPT_BOOL,
         }, {
             .name = "target",
             .type = QEMU_OPT_STRING,
@@ -50,6 +53,7 @@ QemuOptsList qemu_semihosting_config_opts = {
 
 typedef struct SemihostingConfig {
     bool enabled;
+    bool userspace_enabled;
     SemihostingTarget target;
     char **argv;
     int argc;
@@ -59,9 +63,9 @@ typedef struct SemihostingConfig {
 static SemihostingConfig semihosting;
 static const char *semihost_chardev;
 
-bool semihosting_enabled(void)
+bool semihosting_enabled(bool is_user)
 {
-    return semihosting.enabled;
+    return semihosting.enabled && (!is_user || semihosting.userspace_enabled);
 }
 
 SemihostingTarget semihosting_get_target(void)
@@ -137,6 +141,8 @@ int qemu_semihosting_config_options(const char *optarg)
     if (opts != NULL) {
         semihosting.enabled = qemu_opt_get_bool(opts, "enable",
                                                 true);
+        semihosting.userspace_enabled = qemu_opt_get_bool(opts, "userspace",
+                                                          false);
         const char *target = qemu_opt_get(opts, "target");
         /* setup of chardev is deferred until they are initialised */
         semihost_chardev = qemu_opt_get(opts, "chardev");

softmmu/vl.c

@@ -1822,7 +1822,7 @@ static void qemu_apply_machine_options(QDict *qdict)
 {
     object_set_properties_from_keyval(OBJECT(current_machine), qdict, false, &error_fatal);
 
-    if (semihosting_enabled() && !semihosting_get_argc()) {
+    if (semihosting_enabled(false) && !semihosting_get_argc()) {
         /* fall back to the -kernel/-append */
         semihosting_arg_fallback(current_machine->kernel_filename, current_machine->kernel_cmdline);
     }

stubs/semihost.c

@@ -23,7 +23,7 @@ QemuOptsList qemu_semihosting_config_opts = {
 };
 
 /* Queries to config status default to off */
-bool semihosting_enabled(void)
+bool semihosting_enabled(bool is_user)
 {
     return false;
 }

target/arm/translate-a64.c

@@ -2219,7 +2219,7 @@ static void disas_exc(DisasContext *s, uint32_t insn)
          * it is required for halting debug disabled: it will UNDEF.
          * Secondly, "HLT 0xf000" is the A64 semihosting syscall instruction.
          */
-        if (semihosting_enabled() && imm16 == 0xf000) {
+        if (semihosting_enabled(false) && imm16 == 0xf000) {
 #ifndef CONFIG_USER_ONLY
             /* In system mode, don't allow userspace access to semihosting,
              * to provide some semblance of security (and for consistency

target/arm/translate.c

@@ -1169,7 +1169,7 @@ static inline void gen_hlt(DisasContext *s, int imm)
      * semihosting, to provide some semblance of security
      * (and for consistency with our 32-bit semihosting).
      */
-    if (semihosting_enabled() &&
+    if (semihosting_enabled(false) &&
 #ifndef CONFIG_USER_ONLY
         s->current_el != 0 &&
 #endif
@@ -6556,7 +6556,7 @@ static bool trans_BKPT(DisasContext *s, arg_BKPT *a)
     /* BKPT is OK with ECI set and leaves it untouched */
     s->eci_handled = true;
     if (arm_dc_feature(s, ARM_FEATURE_M) &&
-        semihosting_enabled() &&
+        semihosting_enabled(false) &&
 #ifndef CONFIG_USER_ONLY
         !IS_USER(s) &&
 #endif
@@ -8764,7 +8764,7 @@ static bool trans_SVC(DisasContext *s, arg_SVC *a)
 {
     const uint32_t semihost_imm = s->thumb ? 0xab : 0x123456;
 
-    if (!arm_dc_feature(s, ARM_FEATURE_M) && semihosting_enabled() &&
+    if (!arm_dc_feature(s, ARM_FEATURE_M) && semihosting_enabled(false) &&
 #ifndef CONFIG_USER_ONLY
         !IS_USER(s) &&
 #endif

target/m68k/op_helper.c

@@ -203,7 +203,7 @@ static void cf_interrupt_all(CPUM68KState *env, int is_hw)
             cf_rte(env);
             return;
         case EXCP_HALT_INSN:
-            if (semihosting_enabled()
+            if (semihosting_enabled(false)
                     && (env->sr & SR_S) != 0
                     && (env->pc & 3) == 0
                     && cpu_lduw_code(env, env->pc - 4) == 0x4e71

target/nios2/translate.c

@@ -818,7 +818,7 @@ static void gen_break(DisasContext *dc, uint32_t code, uint32_t flags)
 #ifndef CONFIG_USER_ONLY
     /* The semihosting instruction is "break 1".  */
     R_TYPE(instr, code);
-    if (semihosting_enabled() && instr.imm5 == 1) {
+    if (semihosting_enabled(false) && instr.imm5 == 1) {
         t_gen_helper_raise_exception(dc, EXCP_SEMIHOST);
         return;
     }

target/xtensa/translate.c

@@ -2366,9 +2366,9 @@ static uint32_t test_exceptions_simcall(DisasContext *dc,
     bool ill = true;
 #else
     /* Between RE.2 and RE.3 simcall opcode's become nop for the hardware. */
-    bool ill = dc->config->hw_version <= 250002 && !semihosting_enabled();
+    bool ill = dc->config->hw_version <= 250002 && !semihosting_enabled(false);
 #endif
-    if (ill || !semihosting_enabled()) {
+    if (ill || !semihosting_enabled(false)) {
         qemu_log_mask(LOG_GUEST_ERROR, "SIMCALL but semihosting is disabled\n");
     }
     return ill ? XTENSA_OP_ILL : 0;
@@ -2378,7 +2378,7 @@ static void translate_simcall(DisasContext *dc, const OpcodeArg arg[],
                               const uint32_t par[])
 {
 #ifndef CONFIG_USER_ONLY
-    if (semihosting_enabled()) {
+    if (semihosting_enabled(false)) {
         gen_helper_simcall(cpu_env);
     }
 #endif