[PATCH 2/2] profiling: Check prof_buffer in profile_tick()

[Chen Zhongjin]

KASAN reported a UAF problem in profile_tick():

  BUG: KASAN: use-after-free in profile_tick+0x5c/0x80
  Read of size 8 at addr ffff888100928aa0 by task bash/1108

  CPU: 2 PID: 1108 Comm: bash Not tainted 5.10.0+ #72
  Call Trace:
   <IRQ>
   dump_stack+0x93/0xc5
   print_address_description.constprop.0+0x1c/0x3c0
   kasan_report.cold+0x37/0x74
   check_memory_region+0x161/0x1c0
   profile_tick+0x5c/0x80
   tick_sched_timer+0xcd/0x100
   __hrtimer_run_queues+0x23e/0x480
   hrtimer_interrupt+0x1c2/0x440
   asm_call_irq_on_stack+0xf/0x20
   </IRQ>
  ...

It is beacause in profiling_store(), profile_init() is possible to fail
and free prof_cpu_mask. However prof_cpu_mask is not set to NULL and
cpumask_available(prof_cpu_mask) will return true in profile_tick().
Then cpumask_test_cpu() will dereference prof_cpu_mask and trigger the
KASAN warning.

There is no interface to disable profile_tick() even though profile_init
has been already failed. And when CONFIG_CPUMASK_OFFSTACK=n, cpumask_var_t
is an array pointer which can not be set to NULL. So prof_cpu_mask can be
a freed pointner or uncleaned array here, and cpumask_available() can't
promise it is safe to use it.

To fix this, add check for prof_buffer in profile_tick() before check
cpumask_available(prof_cpu_mask). Because prof_cpu_mask is available only
after prof_buffer is allocated successfully.

Signed-off-by: Chen Zhongjin <chenzhongjin@xxxxxxxxxx>
---
 kernel/profile.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/profile.c b/kernel/profile.c
index 8a77769bc4b4..cca47dc2b1db 100644
--- a/kernel/profile.c
+++ b/kernel/profile.c
@@ -334,7 +334,7 @@ void profile_tick(int type)
 {
 	struct pt_regs *regs = get_irq_regs();
 
-	if (!user_mode(regs) && cpumask_available(prof_cpu_mask) &&
+	if (!user_mode(regs) && prof_buffer && cpumask_available(prof_cpu_mask) &&
 	    cpumask_test_cpu(smp_processor_id(), prof_cpu_mask))
 		profile_hit(type, (void *)profile_pc(regs));
 }
-- 
2.17.1