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authorPeter Maydell <peter.maydell@linaro.org>2017-02-28 12:08:19 +0000
committerPeter Maydell <peter.maydell@linaro.org>2017-02-28 12:08:19 +0000
commitaa488fe3bb5460c6675800ccd80f6dccbbd70159 (patch)
tree7bdaadadae7c270049e14f8fdca1a1abcdfba756 /target/arm/helper.c
parent39ae2474e337247e5930e8be783b689adc9f6215 (diff)
downloadqemu-arm-aa488fe3bb5460c6675800ccd80f6dccbbd70159.tar.gz
armv7m: Check exception return consistency
Implement the exception return consistency checks described in the v7M pseudocode ExceptionReturn(). Inspired by a patch from Michael Davidsaver's series, but this is a reimplementation from scratch based on the ARM ARM pseudocode. Signed-off-by: Peter Maydell <peter.maydell@linaro.org> Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Diffstat (limited to 'target/arm/helper.c')
-rw-r--r--target/arm/helper.c112
1 files changed, 101 insertions, 11 deletions
diff --git a/target/arm/helper.c b/target/arm/helper.c
index be731dce54..9081771656 100644
--- a/target/arm/helper.c
+++ b/target/arm/helper.c
@@ -6068,22 +6068,99 @@ static void v7m_push_stack(ARMCPU *cpu)
v7m_push(env, env->regs[0]);
}
-static void do_v7m_exception_exit(CPUARMState *env)
+static void do_v7m_exception_exit(ARMCPU *cpu)
{
+ CPUARMState *env = &cpu->env;
uint32_t type;
uint32_t xpsr;
-
+ bool ufault = false;
+ bool return_to_sp_process = false;
+ bool return_to_handler = false;
+ bool rettobase = false;
+
+ /* We can only get here from an EXCP_EXCEPTION_EXIT, and
+ * arm_v7m_do_unassigned_access() enforces the architectural rule
+ * that jumps to magic addresses don't have magic behaviour unless
+ * we're in Handler mode (compare pseudocode BXWritePC()).
+ */
+ assert(env->v7m.exception != 0);
+
+ /* In the spec pseudocode ExceptionReturn() is called directly
+ * from BXWritePC() and gets the full target PC value including
+ * bit zero. In QEMU's implementation we treat it as a normal
+ * jump-to-register (which is then caught later on), and so split
+ * the target value up between env->regs[15] and env->thumb in
+ * gen_bx(). Reconstitute it.
+ */
type = env->regs[15];
+ if (env->thumb) {
+ type |= 1;
+ }
+
+ qemu_log_mask(CPU_LOG_INT, "Exception return: magic PC %" PRIx32
+ " previous exception %d\n",
+ type, env->v7m.exception);
+
+ if (extract32(type, 5, 23) != extract32(-1, 5, 23)) {
+ qemu_log_mask(LOG_GUEST_ERROR, "M profile: zero high bits in exception "
+ "exit PC value 0x%" PRIx32 " are UNPREDICTABLE\n", type);
+ }
+
if (env->v7m.exception != ARMV7M_EXCP_NMI) {
/* Auto-clear FAULTMASK on return from other than NMI */
env->daif &= ~PSTATE_F;
}
- if (env->v7m.exception != 0) {
- armv7m_nvic_complete_irq(env->nvic, env->v7m.exception);
+
+ switch (armv7m_nvic_complete_irq(env->nvic, env->v7m.exception)) {
+ case -1:
+ /* attempt to exit an exception that isn't active */
+ ufault = true;
+ break;
+ case 0:
+ /* still an irq active now */
+ break;
+ case 1:
+ /* we returned to base exception level, no nesting.
+ * (In the pseudocode this is written using "NestedActivation != 1"
+ * where we have 'rettobase == false'.)
+ */
+ rettobase = true;
+ break;
+ default:
+ g_assert_not_reached();
+ }
+
+ switch (type & 0xf) {
+ case 1: /* Return to Handler */
+ return_to_handler = true;
+ break;
+ case 13: /* Return to Thread using Process stack */
+ return_to_sp_process = true;
+ /* fall through */
+ case 9: /* Return to Thread using Main stack */
+ if (!rettobase &&
+ !(env->v7m.ccr & R_V7M_CCR_NONBASETHRDENA_MASK)) {
+ ufault = true;
+ }
+ break;
+ default:
+ ufault = true;
+ }
+
+ if (ufault) {
+ /* Bad exception return: instead of popping the exception
+ * stack, directly take a usage fault on the current stack.
+ */
+ env->v7m.cfsr |= R_V7M_CFSR_INVPC_MASK;
+ armv7m_nvic_set_pending(env->nvic, ARMV7M_EXCP_USAGE);
+ v7m_exception_taken(cpu, type | 0xf0000000);
+ qemu_log_mask(CPU_LOG_INT, "...taking UsageFault on existing "
+ "stackframe: failed exception return integrity check\n");
+ return;
}
/* Switch to the target stack. */
- switch_v7m_sp(env, (type & 4) != 0);
+ switch_v7m_sp(env, return_to_sp_process);
/* Pop registers. */
env->regs[0] = v7m_pop(env);
env->regs[1] = v7m_pop(env);
@@ -6107,11 +6184,24 @@ static void do_v7m_exception_exit(CPUARMState *env)
/* Undo stack alignment. */
if (xpsr & 0x200)
env->regs[13] |= 4;
- /* ??? The exception return type specifies Thread/Handler mode. However
- this is also implied by the xPSR value. Not sure what to do
- if there is a mismatch. */
- /* ??? Likewise for mismatches between the CONTROL register and the stack
- pointer. */
+
+ /* The restored xPSR exception field will be zero if we're
+ * resuming in Thread mode. If that doesn't match what the
+ * exception return type specified then this is a UsageFault.
+ */
+ if (return_to_handler == (env->v7m.exception == 0)) {
+ /* Take an INVPC UsageFault by pushing the stack again. */
+ armv7m_nvic_set_pending(env->nvic, ARMV7M_EXCP_USAGE);
+ env->v7m.cfsr |= R_V7M_CFSR_INVPC_MASK;
+ v7m_push_stack(cpu);
+ v7m_exception_taken(cpu, type | 0xf0000000);
+ qemu_log_mask(CPU_LOG_INT, "...taking UsageFault on new stackframe: "
+ "failed exception return integrity check\n");
+ return;
+ }
+
+ /* Otherwise, we have a successful exception exit. */
+ qemu_log_mask(CPU_LOG_INT, "...successful exception return\n");
}
static void arm_log_exception(int idx)
@@ -6184,7 +6274,7 @@ void arm_v7m_cpu_do_interrupt(CPUState *cs)
case EXCP_IRQ:
break;
case EXCP_EXCEPTION_EXIT:
- do_v7m_exception_exit(env);
+ do_v7m_exception_exit(cpu);
return;
default:
cpu_abort(cs, "Unhandled exception 0x%x\n", cs->exception_index);