[PATCH] arm64: Fix Kconfig dependencies for RANDOMIZE_BASE

[Ard Biesheuvel]

On 26 July 2016 at 19:45, Catalin Marinas <catalin.marinas at arm.com> wrote:
> On Tue, Jul 26, 2016 at 10:16:55AM -0700, Jeff Vander Stoep wrote:
>> Selecting CONFIG_RANDOMIZE_BASE=y and CONFIG_MODULES=n causes a
>> build error due to dependencies on modules. This patch makes KASLR
>> module config options dependent on CONFIG_MODULES=y.
>>
>> Signed-off-by: Jeff Vander Stoep <jeffv at google.com>
>> ---
>>  arch/arm64/Kconfig | 3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>> index 7e23f95..c9eff79 100644
>> --- a/arch/arm64/Kconfig
>> +++ b/arch/arm64/Kconfig
>> @@ -891,7 +891,6 @@ config RELOCATABLE
>>
>>  config RANDOMIZE_BASE
>>       bool "Randomize the address of the kernel image"
>> -     select ARM64_MODULE_PLTS
>>       select RELOCATABLE
>>       help
>>         Randomizes the virtual address at which the kernel image is
>
> I thought we need the module PLTs once we randomize the kernel base,
> independent of whether we randomize the modules base or not.

Indeed. The module region is always randomized with KASLR, but either
fully (i.e., anywhere in the vmalloc region) or partially, in which
case a random 128 MB range is selected that also covers vmlinux's
.text segment, allowing jumps without veneers. However, since this
range is part of the ordinary vmalloc space, there is a >0 probability
that other allocations may exhaust it before you have loaded all your
modules, in which case the module loader will fall back to ordinary
vmalloc region allocations, which are most likely too far away to be
resolved without PLTs

> Could we do
> something like:
>
>         select ARM64_MODULE_PLTS if MODULES
>
> Whichever way we fix this, we probably need:
>
> Fixes: f80fb3a3d508 ("arm64: add support for kernel ASLR")
> Cc: <stable at vger.kernel.org> # 4.6+
>

With these changes

Acked-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>