arm64: kaslr: Reserve size of ARM64_MEMSTART_ALIGN in linear region

[ Upstream commit c8a43c18 ]

When KASLR is enabled (CONFIG_RANDOMIZE_BASE=y), the top 4K of kernel
virtual address space may be mapped to physical addresses despite being
reserved for ERR_PTR values.

Fix the randomization of the linear region so that we avoid mapping the
last page of the virtual address space.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: liyueyi <liyueyi@live.com>
[will: rewrote commit message; merged in suggestion from Ard]
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Sasha Levin (Microsoft) <sashal@kernel.org>

arch/arm64/mm/init.c

@@ -450,7 +450,7 @@ void __init arm64_memblock_init(void)
 		 * memory spans, randomize the linear region as well.
 		 */
 		if (memstart_offset_seed > 0 && range >= ARM64_MEMSTART_ALIGN) {
-			range = range / ARM64_MEMSTART_ALIGN + 1;
+			range /= ARM64_MEMSTART_ALIGN;
 			memstart_addr -= ARM64_MEMSTART_ALIGN *
 					 ((range * memstart_offset_seed) >> 16);
 		}