kdb: Make memory allocations more robust

Currently kdb uses in_interrupt() to determine whether its library
code has been called from the kgdb trap handler or from a saner calling
context such as driver init. This approach is broken because
in_interrupt() alone isn't able to determine kgdb trap handler entry from
normal task context. This can happen during normal use of basic features
such as breakpoints and can also be trivially reproduced using:
echo g > /proc/sysrq-trigger

We can improve this by adding check for in_dbg_master() instead which
explicitly determines if we are running in debugger context.

Cc: stable@vger.kernel.org
Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Link: https://lore.kernel.org/r/1611313556-4004-1-git-send-email-sumit.garg@linaro.org
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>

kernel/debug/kdb/kdb_private.h

@@ -230,7 +230,7 @@ extern struct task_struct *kdb_curr_task(int);
 
 #define kdb_task_has_cpu(p) (task_curr(p))
 
-#define GFP_KDB (in_interrupt() ? GFP_ATOMIC : GFP_KERNEL)
+#define GFP_KDB (in_dbg_master() ? GFP_ATOMIC : GFP_KERNEL)
 
 extern void *debug_kmalloc(size_t size, gfp_t flags);
 extern void debug_kfree(void *);