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[Qemu-devel] [PATCH v2 for-4.1] target/arm: NS BusFault on vector table

From: Peter Maydell
Subject: [Qemu-devel] [PATCH v2 for-4.1] target/arm: NS BusFault on vector table fetch escalates to NS HardFault
Date: Fri, 5 Jul 2019 10:48:23 +0100

In the M-profile architecture, when we do a vector table fetch and it
fails, we need to report a HardFault.  Whether this is a Secure HF or
a NonSecure HF depends on several things.  If AIRCR.BFHFNMINS is 0
then HF is always Secure, because there is no NonSecure HardFault.
Otherwise, the answer depends on whether the 'underlying exception'
(MemManage, BusFault, SecureFault) targets Secure or NonSecure.  (In
the pseudocode, this is handled in the Vector() function: the final
exc.isSecure is calculated by looking at the exc.isSecure from the
exception returned from the memory access, not the isSecure input

We weren't doing this correctly, because we were looking at
the target security domain of the exception we were trying to
load the vector table entry for. This produces errors of two kinds:
 * a load from the NS vector table which hits the "NS access
   to S memory" SecureFault should end up as a Secure HardFault,
   but we were raising an NS HardFault
 * a load from the S vector table which causes a BusFault
   should raise an NS HardFault if BFHFNMINS == 1 (because
   in that case all BusFaults are NonSecure), but we were raising
   a Secure HardFault

Correct the logic.

We also fix a comment error where we claimed that we might
be escalating MemManage to HardFault, and forgot about SecureFault.
(Vector loads can never hit MPU access faults, because they're
always aligned and always use the default address map.)

Signed-off-by: Peter Maydell <address@hidden>
This is the one remaining patch from my earlier 'minor M-profile
bugfixes' series; the rest are in master now.

Changes v1->v2:
 * rebased on master (function has moved to m_helper.c)
 * fixed logic bug pointed out by rth
 target/arm/m_helper.c | 21 +++++++++++++++++----
 1 file changed, 17 insertions(+), 4 deletions(-)

diff --git a/target/arm/m_helper.c b/target/arm/m_helper.c
index 1867435db7d..84609f446e6 100644
--- a/target/arm/m_helper.c
+++ b/target/arm/m_helper.c
@@ -624,7 +624,11 @@ static bool arm_v7m_load_vector(ARMCPU *cpu, int exc, bool 
         if (sattrs.ns) {
             attrs.secure = false;
         } else if (!targets_secure) {
-            /* NS access to S memory */
+            /*
+             * NS access to S memory: the underlying exception which we 
+             * to HardFault is SecureFault, which always targets Secure.
+             */
+            exc_secure = true;
             goto load_fail;
@@ -632,6 +636,11 @@ static bool arm_v7m_load_vector(ARMCPU *cpu, int exc, bool 
     vector_entry = address_space_ldl(arm_addressspace(cs, attrs), addr,
                                      attrs, &result);
     if (result != MEMTX_OK) {
+        /*
+         * Underlying exception is BusFault: its target security state
+         * depends on BFHFNMINS.
+         */
+        exc_secure = !(cpu->env.v7m.aircr & R_V7M_AIRCR_BFHFNMINS_MASK);
         goto load_fail;
     *pvec = vector_entry;
@@ -641,13 +650,17 @@ load_fail:
      * All vector table fetch fails are reported as HardFault, with
      * HFSR.VECTTBL and .FORCED set. (FORCED is set because
-     * technically the underlying exception is a MemManage or BusFault
+     * technically the underlying exception is a SecureFault or BusFault
      * that is escalated to HardFault.) This is a terminal exception,
      * so we will either take the HardFault immediately or else enter
      * lockup (the latter case is handled in 
+     * The HardFault is Secure if BFHFNMINS is 0 (meaning that all HFs are
+     * secure); otherwise it targets the same security state as the
+     * underlying exception.
-    exc_secure = targets_secure ||
-        !(cpu->env.v7m.aircr & R_V7M_AIRCR_BFHFNMINS_MASK);
+    if (!(cpu->env.v7m.aircr & R_V7M_AIRCR_BFHFNMINS_MASK)) {
+        exc_secure = true;
+    }
     env->v7m.hfsr |= R_V7M_HFSR_VECTTBL_MASK | R_V7M_HFSR_FORCED_MASK;
     armv7m_nvic_set_pending_derived(env->nvic, ARMV7M_EXCP_HARD, exc_secure);
     return false;

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