[Qemu-devel] [PATCH v2] monitor: Fix crashes when using HMP commands without CPU

[Thomas Huth <thuth@redhat.com>]

When running certain HMP commands ("info registers", "info cpustats",
"nmi", "memsave" or dumping virtual memory) with the "none" machine,
QEMU crashes with a segmentation fault. This happens because the "none"
machine does not have any CPUs by default, but these HMP commands did
not check for a valid CPU pointer yet. Add such checks now, so we get
an error message about the missing CPU instead.

Signed-off-by: Thomas Huth <thuth@redhat.com>
---
 v2:
 - Added more checks to cover "nmi" and "memsave", too

 hmp.c     |  8 +++++++-
 monitor.c | 37 +++++++++++++++++++++++++++++++------
 2 files changed, 38 insertions(+), 7 deletions(-)

diff --git a/hmp.c b/hmp.c
index b869617..b1c503a 100644
--- a/hmp.c
+++ b/hmp.c
@@ -1013,8 +1013,14 @@ void hmp_memsave(Monitor *mon, const QDict *qdict)
     const char *filename = qdict_get_str(qdict, "filename");
     uint64_t addr = qdict_get_int(qdict, "val");
     Error *err = NULL;
+    int cpu_index = monitor_get_cpu_index();
 
-    qmp_memsave(addr, size, filename, true, monitor_get_cpu_index(), &err);
+    if (cpu_index < 0) {
+        monitor_printf(mon, "No CPU available\n");
+        return;
+    }
+
+    qmp_memsave(addr, size, filename, true, cpu_index, &err);
     hmp_handle_error(mon, &err);
 }
 
diff --git a/monitor.c b/monitor.c
index 0841d43..74843eb 100644
--- a/monitor.c
+++ b/monitor.c
@@ -1025,6 +1025,9 @@ int monitor_set_cpu(int cpu_index)
 CPUState *mon_get_cpu(void)
 {
     if (!cur_mon->mon_cpu) {
+        if (!first_cpu) {
+            return NULL;
+        }
         monitor_set_cpu(first_cpu->cpu_index);
     }
     cpu_synchronize_state(cur_mon->mon_cpu);
@@ -1033,17 +1036,27 @@ CPUState *mon_get_cpu(void)
 
 CPUArchState *mon_get_cpu_env(void)
 {
-    return mon_get_cpu()->env_ptr;
+    CPUState *cs = mon_get_cpu();
+
+    return cs ? cs->env_ptr : NULL;
 }
 
 int monitor_get_cpu_index(void)
 {
-    return mon_get_cpu()->cpu_index;
+    CPUState *cs = mon_get_cpu();
+
+    return cs ? cs->cpu_index : -1;
 }
 
 static void hmp_info_registers(Monitor *mon, const QDict *qdict)
 {
-    cpu_dump_state(mon_get_cpu(), (FILE *)mon, monitor_fprintf, CPU_DUMP_FPU);
+    CPUState *cs = mon_get_cpu();
+
+    if (!cs) {
+        monitor_printf(mon, "No CPU available\n");
+        return;
+    }
+    cpu_dump_state(cs, (FILE *)mon, monitor_fprintf, CPU_DUMP_FPU);
 }
 
 static void hmp_info_jit(Monitor *mon, const QDict *qdict)
@@ -1076,7 +1089,13 @@ static void hmp_info_history(Monitor *mon, const QDict *qdict)
 
 static void hmp_info_cpustats(Monitor *mon, const QDict *qdict)
 {
-    cpu_dump_statistics(mon_get_cpu(), (FILE *)mon, &monitor_fprintf, 0);
+    CPUState *cs = mon_get_cpu();
+
+    if (!cs) {
+        monitor_printf(mon, "No CPU available\n");
+        return;
+    }
+    cpu_dump_statistics(cs, (FILE *)mon, &monitor_fprintf, 0);
 }
 
 static void hmp_info_trace_events(Monitor *mon, const QDict *qdict)
@@ -1235,6 +1254,12 @@ static void memory_dump(Monitor *mon, int count, int format, int wsize,
     int l, line_size, i, max_digits, len;
     uint8_t buf[16];
     uint64_t v;
+    CPUState *cs = mon_get_cpu();
+
+    if (!cs && (format == 'i' || !is_physical)) {
+        monitor_printf(mon, "Can not dump without CPU\n");
+        return;
+    }
 
     if (format == 'i') {
         int flags = 0;
@@ -1264,7 +1289,7 @@ static void memory_dump(Monitor *mon, int count, int format, int wsize,
         flags = msr_le << 16;
         flags |= env->bfd_mach;
 #endif
-        monitor_disas(mon, mon_get_cpu(), addr, count, is_physical, flags);
+        monitor_disas(mon, cs, addr, count, is_physical, flags);
         return;
     }
 
@@ -1303,7 +1328,7 @@ static void memory_dump(Monitor *mon, int count, int format, int wsize,
         if (is_physical) {
             cpu_physical_memory_read(addr, buf, l);
         } else {
-            if (cpu_memory_rw_debug(mon_get_cpu(), addr, buf, l, 0) < 0) {
+            if (cpu_memory_rw_debug(cs, addr, buf, l, 0) < 0) {
                 monitor_printf(mon, " Cannot access memory\n");
                 break;
             }
-- 
1.8.3.1