arm: fixmap: implement __set_fixmap()

This is used from set_fixmap() and clear_fixmap() via asm-generic/fixmap.h.
Also makes sure that the fixmap allocation fits into the expected range.

Based on patch by Rabin Vincent.
Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Rabin Vincent <rabin@rab.in>
Acked-by: Nicolas Pitre <nico@linaro.org>

arch/arm/include/asm/fixmap.h

@@ -14,6 +14,8 @@ enum fixed_addresses {
 	__end_of_fixed_addresses
 };
 
+void __set_fixmap(enum fixed_addresses idx, phys_addr_t phys, pgprot_t prot);
+
 #include <asm-generic/fixmap.h>
 
 #endif

arch/arm/mm/mmu.c

@@ -22,6 +22,7 @@
 #include <asm/cputype.h>
 #include <asm/sections.h>
 #include <asm/cachetype.h>
+#include <asm/fixmap.h>
 #include <asm/sections.h>
 #include <asm/setup.h>
 #include <asm/smp_plat.h>
@@ -392,6 +393,29 @@ SET_MEMORY_FN(rw, pte_set_rw)
 SET_MEMORY_FN(x, pte_set_x)
 SET_MEMORY_FN(nx, pte_set_nx)
 
+/*
+ * To avoid TLB flush broadcasts, this uses local_flush_tlb_kernel_range().
+ * As a result, this can only be called with preemption disabled, as under
+ * stop_machine().
+ */
+void __set_fixmap(enum fixed_addresses idx, phys_addr_t phys, pgprot_t prot)
+{
+	unsigned long vaddr = __fix_to_virt(idx);
+	pte_t *pte = pte_offset_kernel(pmd_off_k(vaddr), vaddr);
+
+	/* Make sure fixmap region does not exceed available allocation. */
+	BUILD_BUG_ON(FIXADDR_START + (__end_of_fixed_addresses * PAGE_SIZE) >
+		     FIXADDR_END);
+	BUG_ON(idx >= __end_of_fixed_addresses);
+
+	if (pgprot_val(prot))
+		set_pte_at(NULL, vaddr, pte,
+			pfn_pte(phys >> PAGE_SHIFT, prot));
+	else
+		pte_clear(NULL, vaddr, pte);
+	local_flush_tlb_kernel_range(vaddr, vaddr + PAGE_SIZE);
+}
+
 /*
  * Adjust the PMD section entries according to the CPU in use.
  */