[Qemu-devel] [PATCH 06/13] target/arm: Add v8M stack checks on exception entry

[Peter Maydell <peter.maydell@linaro.org>]

Add checks for breaches of the v8M stack limit when the
stack pointer is decremented to push the exception frame
for exception entry.

Note that the exception-entry case is unique in that the
stack pointer is updated to be the limit value if the limit
is hit (per rule R_ZLZG).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
 target/arm/helper.c | 54 ++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 46 insertions(+), 8 deletions(-)

diff --git a/target/arm/helper.c b/target/arm/helper.c
index ef8c244fb84..a10dff01a90 100644
--- a/target/arm/helper.c
+++ b/target/arm/helper.c
@@ -6839,6 +6839,8 @@ static bool v7m_push_callee_stack(ARMCPU *cpu, uint32_t lr, bool dotailchain,
     uint32_t frameptr;
     ARMMMUIdx mmu_idx;
     bool stacked_ok;
+    uint32_t limit;
+    bool want_psp;
 
     if (dotailchain) {
         bool mode = lr & R_V7M_EXCRET_MODE_MASK;
@@ -6848,12 +6850,34 @@ static bool v7m_push_callee_stack(ARMCPU *cpu, uint32_t lr, bool dotailchain,
         mmu_idx = arm_v7m_mmu_idx_for_secstate_and_priv(env, M_REG_S, priv);
         frame_sp_p = get_v7m_sp_ptr(env, M_REG_S, mode,
                                     lr & R_V7M_EXCRET_SPSEL_MASK);
+        want_psp = mode && (lr & R_V7M_EXCRET_SPSEL_MASK);
+        if (want_psp) {
+            limit = env->v7m.psplim[M_REG_S];
+        } else {
+            limit = env->v7m.msplim[M_REG_S];
+        }
     } else {
         mmu_idx = core_to_arm_mmu_idx(env, cpu_mmu_index(env, false));
         frame_sp_p = &env->regs[13];
+        limit = v7m_sp_limit(env);
     }
 
     frameptr = *frame_sp_p - 0x28;
+    if (frameptr < limit) {
+        /*
+         * Stack limit failure: set SP to the limit value, and generate
+         * STKOF UsageFault. Stack pushes below the limit must not be
+         * performed. It is IMPDEF whether pushes above the limit are
+         * performed; we choose not to.
+         */
+        qemu_log_mask(CPU_LOG_INT,
+                      "...STKOF during callee-saves register stacking\n");
+        env->v7m.cfsr[env->v7m.secure] |= R_V7M_CFSR_STKOF_MASK;
+        armv7m_nvic_set_pending(env->nvic, ARMV7M_EXCP_USAGE,
+                                env->v7m.secure);
+        *frame_sp_p = limit;
+        return true;
+    }
 
     /* Write as much of the stack frame as we can. A write failure may
      * cause us to pend a derived exception.
@@ -6877,10 +6901,7 @@ static bool v7m_push_callee_stack(ARMCPU *cpu, uint32_t lr, bool dotailchain,
         v7m_stack_write(cpu, frameptr + 0x24, env->regs[11], mmu_idx,
                         ignore_faults);
 
-    /* Update SP regardless of whether any of the stack accesses failed.
-     * When we implement v8M stack limit checking then this attempt to
-     * update SP might also fail and result in a derived exception.
-     */
+    /* Update SP regardless of whether any of the stack accesses failed. */
     *frame_sp_p = frameptr;
 
     return !stacked_ok;
@@ -7028,6 +7049,26 @@ static bool v7m_push_stack(ARMCPU *cpu)
 
     frameptr -= 0x20;
 
+    if (arm_feature(env, ARM_FEATURE_V8)) {
+        uint32_t limit = v7m_sp_limit(env);
+
+        if (frameptr < limit) {
+            /*
+             * Stack limit failure: set SP to the limit value, and generate
+             * STKOF UsageFault. Stack pushes below the limit must not be
+             * performed. It is IMPDEF whether pushes above the limit are
+             * performed; we choose not to.
+             */
+            qemu_log_mask(CPU_LOG_INT,
+                          "...STKOF during stacking\n");
+            env->v7m.cfsr[env->v7m.secure] |= R_V7M_CFSR_STKOF_MASK;
+            armv7m_nvic_set_pending(env->nvic, ARMV7M_EXCP_USAGE,
+                                    env->v7m.secure);
+            env->regs[13] = limit;
+            return true;
+        }
+    }
+
     /* Write as much of the stack frame as we can. If we fail a stack
      * write this will result in a derived exception being pended
      * (which may be taken in preference to the one we started with
@@ -7043,10 +7084,7 @@ static bool v7m_push_stack(ARMCPU *cpu)
         v7m_stack_write(cpu, frameptr + 24, env->regs[15], mmu_idx, false) &&
         v7m_stack_write(cpu, frameptr + 28, xpsr, mmu_idx, false);
 
-    /* Update SP regardless of whether any of the stack accesses failed.
-     * When we implement v8M stack limit checking then this attempt to
-     * update SP might also fail and result in a derived exception.
-     */
+    /* Update SP regardless of whether any of the stack accesses failed. */
     env->regs[13] = frameptr;
 
     return !stacked_ok;
-- 
2.19.0