[PATCH] mm: kfence: export kfence_enabled as global variables

[Zhenhua Huang]

Thanks Mark!

On 2023/2/7 18:48, Mark Rutland wrote:
> On Tue, Feb 07, 2023 at 03:46:53PM +0800, Zhenhua Huang wrote:
>> Thanks Marco!
>>
>> On 2023/2/7 15:19, Marco Elver wrote:
>>> On Tue, 7 Feb 2023 at 07:15, Zhenhua Huang <quic_zhenhuah at quicinc.com> wrote:
>>>>
>>>> Export the variable to ease the judgement of whether kfence enabled
>>>> at runtime. It should be more precise than through kernel config
>>>> "CONFIG_KFENCE".
>>>>
>>>> For example We can disable kfence at runtime using bootargs
>>>> "kfence.sample_interval=0" but CONFIG_KFENCE enabled.
>>>> It was false positive.
>>>>
>>>> Signed-off-by: Zhenhua Huang <quic_zhenhuah at quicinc.com>
>>>> ---
>>>>    arch/arm64/mm/pageattr.c | 3 ++-
>>>>    include/linux/kfence.h   | 2 ++
>>>>    mm/kfence/core.c         | 2 +-
>>>>    3 files changed, 5 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
>>>> index 79dd201..208d780 100644
>>>> --- a/arch/arm64/mm/pageattr.c
>>>> +++ b/arch/arm64/mm/pageattr.c
>>>> @@ -7,6 +7,7 @@
>>>>    #include <linux/module.h>
>>>>    #include <linux/sched.h>
>>>>    #include <linux/vmalloc.h>
>>>> +#include <linux/kfence.h>
>>>>
>>>>    #include <asm/cacheflush.h>
>>>>    #include <asm/set_memory.h>
>>>> @@ -27,7 +28,7 @@ bool can_set_direct_map(void)
>>>>            * protect/unprotect single pages.
>>>>            */
>>>>           return (rodata_enabled && rodata_full) || debug_pagealloc_enabled() ||
>>>> -               IS_ENABLED(CONFIG_KFENCE);
>>>> +               kfence_enabled;
>>>
>>> Unfortunately this won't work, because it's possible to enable KFENCE
>>> after the kernel has booted with e.g.: echo 100 >
>>> /sys/module/kfence/parameters/sample_interval
>> Yeah, got it. Thanks for catching it.
>>>
>>> What is the problem you have encountered? Is the page-granular direct
>>> map causing issues?
>> We're working on a low memory target, page-granular mapping costed more (2M
>> per 1GB) memory. Due to GKI constraints, it is not easy to disable
>> CONFIG_KFENCE. So my intention was to move the judgement to runtime
>> configurable w/ CONFIG_KFENCE on...
>>
>> Do you have any further suggestion/proposal on this? Many Thanks!
> 
> Just to check, the cost is because we're mapping *all* of memory at page
> granulatrity, right? If we were to just map the KFENCE region a page
> granularity, would that be a sufficient saving?
Yes,that's expected saving which I would like to address here.
> 
> We didn't do that so far because it was simpler to just map everything at page
> granularity (and that's also required by rodata_full, which I though android
> used?).
Yes, by default we're setting rodata_full. While for low-ram target, we 
also can lower the security requirement. This is why I am pursuing..
BTW, rodata_full can be easily runtime configured through bootargs.
> 
> If it's really important (and rodata_full isn't being used), we could try to do
> that.
> 
> To do that we'd need to choose the KFENCE region *before* arm64 creates the
> fine-grain translation tables, which probable needs an arch_ hook.
It sounds a good solution. Let me also dig into this. Many Thanks!
> 
> Thanks,
> Mark.
>