arm64: kaslr: ensure randomized quantities are clean to the PoC

kaslr_early_init() is called with the kernel mapped at its
link time offset, and if it returns with a non-zero offset,
the kernel is unmapped and remapped again at the randomized
offset.

During its execution, kaslr_early_init() also randomizes the
base of the module region and of the linear mapping of DRAM,
and sets two variables accordingly. However, since these
variables are assigned with the caches on, they may get lost
during the cache maintenance that occurs when unmapping and
remapping the kernel, so ensure that these values are cleaned
to the PoC.
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Fixes: f80fb3a3 ("arm64: add support for kernel ASLR")
Cc: <stable@vger.kernel.org> # v4.6+
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/kernel/kaslr.c

@@ -14,6 +14,7 @@
 #include <linux/sched.h>
 #include <linux/types.h>
 
+#include <asm/cacheflush.h>
 #include <asm/fixmap.h>
 #include <asm/kernel-pgtable.h>
 #include <asm/memory.h>
@@ -43,7 +44,7 @@ static __init u64 get_kaslr_seed(void *fdt)
 	return ret;
 }
 
-static __init const u8 *get_cmdline(void *fdt)
+static __init const u8 *kaslr_get_cmdline(void *fdt)
 {
 	static __initconst const u8 default_cmdline[] = CONFIG_CMDLINE;
 
@@ -109,7 +110,7 @@ u64 __init kaslr_early_init(u64 dt_phys)
 	 * Check if 'nokaslr' appears on the command line, and
 	 * return 0 if that is the case.
 	 */
-	cmdline = get_cmdline(fdt);
+	cmdline = kaslr_get_cmdline(fdt);
 	str = strstr(cmdline, "nokaslr");
 	if (str == cmdline || (str > cmdline && *(str - 1) == ' '))
 		return 0;
@@ -169,5 +170,8 @@ u64 __init kaslr_early_init(u64 dt_phys)
 	module_alloc_base += (module_range * (seed & ((1 << 21) - 1))) >> 21;
 	module_alloc_base &= PAGE_MASK;
 
+	__flush_dcache_area(&module_alloc_base, sizeof(module_alloc_base));
+	__flush_dcache_area(&memstart_offset_seed, sizeof(memstart_offset_seed));
+
 	return offset;
 }