hw/timer/renesas_tmr: Fix use of uninitialized data in read_tcnt()

The read_tcnt() function calculates the TCNT register values for the
two channels of the timer module; it sets these up in the local
tcnt[] array, and eventually returns either one or both of them,
depending on whether the access is 8 or 16 bits.  However, not all of
the code paths through this function set both elements of this array:
if the guest has programmed the TCCR.CSS register fields to values
which are either documented as not to be used or which QEMU does not
implement, then the function will return uninitialized data.  (This
was spotted by Coverity.)

Add the missing CSS cases to this code, so that we return a
consistent value instead of uninitialized data, and so the code
structure indicates what's happening.

Fixes: CID 1429976
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20210219223241.16344-3-peter.maydell@linaro.org

diff --git a/hw/timer/renesas_tmr.c b/hw/timer/renesas_tmr.c
index 22260aa..eed3991 100644 (file)
--- a/hw/timer/renesas_tmr.c
+++ b/hw/timer/renesas_tmr.c
@@ -46,7 +46,9 @@ REG8(TCCR, 10)
   FIELD(TCCR, CSS,   3, 2)
   FIELD(TCCR, TMRIS, 7, 1)
 
+#define CSS_EXTERNAL  0x00
 #define CSS_INTERNAL  0x01
+#define CSS_INVALID   0x02
 #define CSS_CASCADING 0x03
 #define CCLR_A    0x01
 #define CCLR_B    0x02
@@ -130,13 +132,20 @@ static uint16_t read_tcnt(RTMRState *tmr, unsigned size, int ch)
     if (delta > 0) {
         tmr->tick = now;
 
-        if (FIELD_EX8(tmr->tccr[1], TCCR, CSS) == CSS_INTERNAL) {
+        switch (FIELD_EX8(tmr->tccr[1], TCCR, CSS)) {
+        case CSS_INTERNAL:
             /* timer1 count update */
             elapsed = elapsed_time(tmr, 1, delta);
             if (elapsed >= 0x100) {
                 ovf = elapsed >> 8;
             }
             tcnt[1] = tmr->tcnt[1] + (elapsed & 0xff);
+            break;
+        case CSS_INVALID: /* guest error to have set this */
+        case CSS_EXTERNAL: /* QEMU doesn't implement these */
+        case CSS_CASCADING:
+            tcnt[1] = tmr->tcnt[1];
+            break;
         }
         switch (FIELD_EX8(tmr->tccr[0], TCCR, CSS)) {
         case CSS_INTERNAL:
@@ -144,9 +153,11 @@ static uint16_t read_tcnt(RTMRState *tmr, unsigned size, int ch)
             tcnt[0] = tmr->tcnt[0] + elapsed;
             break;
         case CSS_CASCADING:
-            if (ovf > 0) {
-                tcnt[0] = tmr->tcnt[0] + ovf;
-            }
+            tcnt[0] = tmr->tcnt[0] + ovf;
+            break;
+        case CSS_INVALID: /* guest error to have set this */
+        case CSS_EXTERNAL: /* QEMU doesn't implement this */
+            tcnt[0] = tmr->tcnt[0];
             break;
         }
     } else {