arm64: mm: Don't abuse memblock NOMAP to check for overlaps

Now that the linear region mapping routines respect existing table
mappings and contiguous block and page mappings, it is no longer needed
to fiddle with the memblock tables to set and clear the NOMAP attribute
in order to omit text and rodata when creating the linear map.

Instead, map the kernel text and rodata alias first with the desired
initial attributes and granularity, so that the loop iterating over the
memblocks will not remap it in a manner that prevents it from being
remapped with updated attributes later.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Kevin Brodsky <kevin.brodsky@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>

Author: ardbiesheuvel

arch/arm64/mm/mmu.c

@@ -1164,12 +1164,17 @@ static void __init map_mem(void)
 		flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
 
 	/*
-	 * Take care not to create a writable alias for the
-	 * read-only text and rodata sections of the kernel image.
-	 * So temporarily mark them as NOMAP to skip mappings in
-	 * the following for-loop
+	 * Map the linear alias of the [_text, __init_begin) interval first
+	 * so that its write permissions can be removed later without the need
+	 * to split any block mappings created by the loop below.
+	 *
+	 * Write permissions are needed for alternatives patching, and will be
+	 * removed later by mark_linear_text_alias_ro() above. This makes the
+	 * contents of the region accessible to subsystems such as hibernate,
+	 * but protects it from inadvertent modification or execution.
 	 */
-	memblock_mark_nomap(kernel_start, kernel_end - kernel_start);
+	__map_memblock(kernel_start, kernel_end, pgprot_tagged(PAGE_KERNEL),
+		       flags);
 
 	/* map all the memory banks */
 	for_each_mem_range(i, &start, &end) {
@@ -1181,17 +1186,6 @@ static void __init map_mem(void)
 		__map_memblock(start, end, pgprot_tagged(PAGE_KERNEL),
 			       flags);
 	}
-
-	/*
-	 * Map the linear alias of the [_text, __init_begin) interval
-	 * as non-executable now, and remove the write permission in
-	 * mark_linear_text_alias_ro() below (which will be called after
-	 * alternative patching has completed). This makes the contents
-	 * of the region accessible to subsystems such as hibernate,
-	 * but protects it from inadvertent modification or execution.
-	 */
-	__map_memblock(kernel_start, kernel_end, PAGE_KERNEL, 0);
-	memblock_clear_nomap(kernel_start, kernel_end - kernel_start);
 }
 
 void mark_rodata_ro(void)