powerpc/kasan: Force thread size increase with KASAN

KASAN causes increased stack usage, which can lead to stack overflows.

The logic in Kconfig to suggest a larger default doesn't work if a user
has CONFIG_EXPERT enabled and has an existing .config with a smaller
value.

Follow the lead of x86 and arm64, and force the thread size to be
increased when KASAN is enabled.

That also has the effect of enlarging the stack for 64-bit KASAN builds,
which is also desirable.

Fixes: edbadaf ("powerpc/kasan: Fix stack overflow by increasing THREAD_SHIFT")
Reported-by: Erhard Furtner <erhard_f@mailbox.org>
Reported-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[mpe: Use MIN_THREAD_SHIFT as suggested by Christophe]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220601143114.133524-1-mpe@ellerman.id.au

arch/powerpc/Kconfig

@@ -790,7 +790,6 @@ config THREAD_SHIFT
 	range 13 15
 	default "15" if PPC_256K_PAGES
 	default "14" if PPC64
-	default "14" if KASAN
 	default "13"
 	help
 	  Used to define the stack size. The default is almost always what you

arch/powerpc/include/asm/thread_info.h

@@ -14,10 +14,16 @@
 
 #ifdef __KERNEL__
 
-#if defined(CONFIG_VMAP_STACK) && CONFIG_THREAD_SHIFT < PAGE_SHIFT
+#ifdef CONFIG_KASAN
+#define MIN_THREAD_SHIFT	(CONFIG_THREAD_SHIFT + 1)
+#else
+#define MIN_THREAD_SHIFT	CONFIG_THREAD_SHIFT
+#endif
+
+#if defined(CONFIG_VMAP_STACK) && MIN_THREAD_SHIFT < PAGE_SHIFT
 #define THREAD_SHIFT		PAGE_SHIFT
 #else
-#define THREAD_SHIFT		CONFIG_THREAD_SHIFT
+#define THREAD_SHIFT		MIN_THREAD_SHIFT
 #endif
 
 #define THREAD_SIZE		(1 << THREAD_SHIFT)