MIPS: relocatable: fix possible boot hangup with KASLR enabled

LLVM-built Linux triggered a boot hangup with KASLR enabled.

arch/mips/kernel/relocate.c:get_random_boot() uses linux_banner,
which is a string constant, as a random seed, but accesses it
as an array of unsigned long (in rotate_xor()).
When the address of linux_banner is not aligned to sizeof(long),
such access emits unaligned access exception and hangs the kernel.

Use PTR_ALIGN() to align input address to sizeof(long) and also
align down the input length to prevent possible access-beyond-end.

Fixes: 405bc8fd ("MIPS: Kernel: Implement KASLR using CONFIG_RELOCATABLE")
Cc: stable@vger.kernel.org # 4.7+
Signed-off-by: Alexander Lobakin <alobakin@pm.me>
Tested-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

arch/mips/kernel/relocate.c

@@ -187,8 +187,14 @@ static int __init relocate_exception_table(long offset)
 static inline __init unsigned long rotate_xor(unsigned long hash,
 					      const void *area, size_t size)
 {
-	size_t i;
-	unsigned long *ptr = (unsigned long *)area;
+	const typeof(hash) *ptr = PTR_ALIGN(area, sizeof(hash));
+	size_t diff, i;
+
+	diff = (void *)ptr - area;
+	if (unlikely(size < diff + sizeof(hash)))
+		return hash;
+
+	size = ALIGN_DOWN(size - diff, sizeof(hash));
 
 	for (i = 0; i < size / sizeof(hash); i++) {
 		/* Rotate by odd number of bits and XOR. */