riscv: Fix kernel stack size when KASAN is enabled

We use Kconfig to select the kernel stack size, doubling the default
size if KASAN is enabled.

But that actually only works if KASAN is selected from the beginning,
meaning that if KASAN config is added later (for example using
menuconfig), CONFIG_THREAD_SIZE_ORDER won't be updated, keeping the
default size, which is not enough for KASAN as reported in [1].

So fix this by moving the logic to compute the right kernel stack into a
header.

Fixes: a7555f6b ("riscv: stack: Add config of thread stack size")
Reported-by: syzbot+ba9eac24453387a9d502@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/000000000000eb301906222aadc2@google.com/ [1]
Cc: stable@vger.kernel.org
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20240917150328.59831-1-alexghiti@rivosinc.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/Kconfig

@@ -777,8 +777,7 @@ config IRQ_STACKS
 config THREAD_SIZE_ORDER
 	int "Kernel stack size (in power-of-two numbers of page size)" if VMAP_STACK && EXPERT
 	range 0 4
-	default 1 if 32BIT && !KASAN
-	default 3 if 64BIT && KASAN
+	default 1 if 32BIT
 	default 2
 	help
 	  Specify the Pages of thread stack size (from 4KB to 64KB), which also

arch/riscv/include/asm/thread_info.h

@@ -13,7 +13,12 @@
 #include <linux/sizes.h>
 
 /* thread information allocation */
-#define THREAD_SIZE_ORDER	CONFIG_THREAD_SIZE_ORDER
+#ifdef CONFIG_KASAN
+#define KASAN_STACK_ORDER	1
+#else
+#define KASAN_STACK_ORDER	0
+#endif
+#define THREAD_SIZE_ORDER	(CONFIG_THREAD_SIZE_ORDER + KASAN_STACK_ORDER)
 #define THREAD_SIZE		(PAGE_SIZE << THREAD_SIZE_ORDER)
 
 /*