[PATCH] xen/sched: fix scheduler callback verification on init

[Ruslan Ruslichenko <ruslichenko.r@xxxxxxxxx>]

From: Ruslan Ruslichenko <Ruslan_Ruslichenko@xxxxxxxx>

During core scheduler initialization, each registered scheduler
is sanity tested in two steps:

- it must provide required callbacks (e.g. init, do_schedule).
- if global_init callback is present, it must succeed.

If any of the steps fail, scheduler entry is cleared in global
'schedulers' array.

However, in the current implementation, if verification fails during
the first step, the scheduler entry is cleared but verification
sequence is not interrupted. This lead to NULL pointer dereference
when subsequent required callbacks verified, and possible during
the second step.

The patch fixes the crashes by adding check inside sched_test_func
macro and skipping the call to a global_init if first step did not pass.

Signed-off-by: Ruslan Ruslichenko <Ruslan_Ruslichenko@xxxxxxxx>
---
 xen/common/sched/core.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/xen/common/sched/core.c b/xen/common/sched/core.c
index a57d5dd929..4270c89491 100644
--- a/xen/common/sched/core.c
+++ b/xen/common/sched/core.c
@@ -3017,7 +3017,7 @@ void __init scheduler_init(void)
     for ( i = 0; i < NUM_SCHEDULERS; i++)
     {
 #define sched_test_func(f)                               \
-        if ( !schedulers[i]->f )                         \
+        if ( schedulers[i] && !schedulers[i]->f )        \
         {                                                \
             printk("scheduler %s misses .%s, dropped\n", \
                    schedulers[i]->opt_name, #f);         \
@@ -3034,6 +3034,9 @@ void __init scheduler_init(void)
 
 #undef sched_test_func
 
+        if ( !schedulers[i] )
+            continue;
+
         if ( schedulers[i]->global_init && schedulers[i]->global_init() < 0 )
         {
             printk("scheduler %s failed initialization, dropped\n",
-- 
2.43.0