arm64: fixmap: check idx is definitely valid

Fixmap indices are in the interval (FIX_HOLE, __end_of_fixed_addresses),
but in __set_fixmap we only check idx <= __end_of_fixed_addresses, and
therefore indices <= FIX_HOLE are erroneously accepted. If called with
such an idx, __set_fixmap may corrupt page tables outside of the fixmap
region.

This patch ensures that we validate the idx against both endpoints of
the interval.

Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Kees Cook <keescook@chromium.org>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Laura Abbott <lauraa@codeaurora.org>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/mm/mmu.c

@@ -627,10 +627,7 @@ void __set_fixmap(enum fixed_addresses idx,
 	unsigned long addr = __fix_to_virt(idx);
 	pte_t *pte;
 
-	if (idx >= __end_of_fixed_addresses) {
-		BUG();
-		return;
-	}
+	BUG_ON(idx <= FIX_HOLE || idx >= __end_of_fixed_addresses);
 
 	pte = fixmap_pte(addr);