riscv: mm: Update mmap_rnd_bits_max

ARCH_MMAP_RND_BITS_MAX is based on Sv39, which leaves a few
potential bits of mmap randomness on the table if we end up enabling
4/5-level paging. Update mmap_rnd_bits_max to take the final address
space size into account. This increases mmap_rnd_bits_max from 24 to
33 with Sv48/57.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>
Acked-by: Palmer Dabbelt <palmer@rivosinc.com>
Link: https://lore.kernel.org/r/20230929211155.3910949-6-samitolvanen@google.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Author: samitolvanen

arch/riscv/mm/init.c

@@ -767,6 +767,11 @@ static int __init print_no5lvl(char *p)
 }
 early_param("no5lvl", print_no5lvl);
 
+static void __init set_mmap_rnd_bits_max(void)
+{
+	mmap_rnd_bits_max = MMAP_VA_BITS - PAGE_SHIFT - 3;
+}
+
 /*
  * There is a simple way to determine if 4-level is supported by the
  * underlying hardware: establish 1:1 mapping in 4-level page table mode
@@ -1081,6 +1086,7 @@ asmlinkage void __init setup_vm(uintptr_t dtb_pa)
 
 #if defined(CONFIG_64BIT) && !defined(CONFIG_XIP_KERNEL)
 	set_satp_mode(dtb_pa);
+	set_mmap_rnd_bits_max();
 #endif
 
 	/*