riscv: read-only pages should not be writable

If EFI pages are marked as read-only,
we should remove the _PAGE_WRITE flag.

The current code overwrites an unused value.

Fixes: b91540d52a ("RISC-V: Add EFI runtime services")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Link: https://lore.kernel.org/r/20220528014132.91052-1-heinrich.schuchardt@canonical.com
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

arch/riscv/kernel/efi.c

@@ -65,7 +65,7 @@ static int __init set_permissions(pte_t *ptep, unsigned long addr, void *data)
 
 	if (md->attribute & EFI_MEMORY_RO) {
 		val = pte_val(pte) & ~_PAGE_WRITE;
-		val = pte_val(pte) | _PAGE_READ;
+		val |= _PAGE_READ;
 		pte = __pte(val);
 	}
 	if (md->attribute & EFI_MEMORY_XP) {