From aa488fe3bb5460c6675800ccd80f6dccbbd70159 Mon Sep 17 00:00:00 2001 From: Peter Maydell Date: Tue, 28 Feb 2017 12:08:19 +0000 Subject: armv7m: Check exception return consistency MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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 Reviewed-by: Alex Bennée --- target/arm/cpu.h | 12 +++++- target/arm/helper.c | 112 ++++++++++++++++++++++++++++++++++++++++++++++------ 2 files changed, 112 insertions(+), 12 deletions(-) (limited to 'target/arm') diff --git a/target/arm/cpu.h b/target/arm/cpu.h index 68ad00c480..045830aeae 100644 --- a/target/arm/cpu.h +++ b/target/arm/cpu.h @@ -1366,7 +1366,17 @@ static inline bool armv7m_nvic_can_take_pending_exception(void *opaque) #endif void armv7m_nvic_set_pending(void *opaque, int irq); void armv7m_nvic_acknowledge_irq(void *opaque); -void armv7m_nvic_complete_irq(void *opaque, int irq); +/** + * armv7m_nvic_complete_irq: complete specified interrupt or exception + * @opaque: the NVIC + * @irq: the exception number to complete + * + * Returns: -1 if the irq was not active + * 1 if completing this irq brought us back to base (no active irqs) + * 0 if there is still an irq active after this one was completed + * (Ignoring -1, this is the same as the RETTOBASE value before completion.) + */ +int armv7m_nvic_complete_irq(void *opaque, int irq); /* Interface for defining coprocessor registers. * Registers are defined in tables of arm_cp_reginfo structs 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); -- cgit v1.2.3