Re: [PATCH v8] mm,kfence: decouple kfence from page granularity mapping judgement

[Zhenhua Huang <quic_zhenhuah@quicinc.com>]

On 2023/3/14 18:08, Zhenhua Huang wrote:
> 
> 
> On 2023/3/14 16:36, Pavan Kondeti wrote:
>> On Tue, Mar 14, 2023 at 03:05:02PM +0800, Zhenhua Huang wrote:
>>> Kfence only needs its pool to be mapped as page granularity, if it is
>>> inited early. Previous judgement was a bit over protected. From [1], 
>>> Mark
>>> suggested to "just map the KFENCE region a page granularity". So I
>>> decouple it from judgement and do page granularity mapping for kfence
>>> pool only. Need to be noticed that late init of kfence pool still 
>>> requires
>>> page granularity mapping.
>>>
>>> Page granularity mapping in theory cost more(2M per 1GB) memory on arm64
>>> platform. Like what I've tested on QEMU(emulated 1GB RAM) with
>>> gki_defconfig, also turning off rodata protection:
>>> Before:
>>> [root@liebao ]# cat /proc/meminfo
>>> MemTotal:         999484 kB
>>> After:
>>> [root@liebao ]# cat /proc/meminfo
>>> MemTotal:        1001480 kB
>>>
>>> To implement this, also relocate the kfence pool allocation before the
>>> linear mapping setting up, arm64_kfence_alloc_pool is to allocate phys
>>> addr, __kfence_pool is to be set after linear mapping set up.
>>>
>>> LINK: [1] 
>>> https://lore.kernel.org/linux-arm-kernel/Y+IsdrvDNILA59UN@FVFF77S0Q05N/
>>> Suggested-by: Mark Rutland <mark.rutland@arm.com>
>>> Signed-off-by: Zhenhua Huang <quic_zhenhuah@quicinc.com>
>>> ---
>>>   arch/arm64/include/asm/kfence.h |  2 ++
>>>   arch/arm64/mm/mmu.c             | 44 
>>> +++++++++++++++++++++++++++++++++++++++++
>>>   arch/arm64/mm/pageattr.c        |  9 +++++++--
>>>   include/linux/kfence.h          |  8 ++++++++
>>>   mm/kfence/core.c                |  9 +++++++++
>>>   5 files changed, 70 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/arch/arm64/include/asm/kfence.h 
>>> b/arch/arm64/include/asm/kfence.h
>>> index aa855c6..f1f9ca2d 100644
>>> --- a/arch/arm64/include/asm/kfence.h
>>> +++ b/arch/arm64/include/asm/kfence.h
>>> @@ -10,6 +10,8 @@
>>>   #include <asm/set_memory.h>
>>> +extern phys_addr_t early_kfence_pool;
>>> +
>>>   static inline bool arch_kfence_init_pool(void) { return true; }
>>>   static inline bool kfence_protect_page(unsigned long addr, bool 
>>> protect)
>>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>>> index 6f9d889..7fbf2ed 100644
>>> --- a/arch/arm64/mm/mmu.c
>>> +++ b/arch/arm64/mm/mmu.c
>>> @@ -24,6 +24,7 @@
>>>   #include <linux/mm.h>
>>>   #include <linux/vmalloc.h>
>>>   #include <linux/set_memory.h>
>>> +#include <linux/kfence.h>
>>>   #include <asm/barrier.h>
>>>   #include <asm/cputype.h>
>>> @@ -38,6 +39,7 @@
>>>   #include <asm/ptdump.h>
>>>   #include <asm/tlbflush.h>
>>>   #include <asm/pgalloc.h>
>>> +#include <asm/kfence.h>
>>>   #define NO_BLOCK_MAPPINGS    BIT(0)
>>>   #define NO_CONT_MAPPINGS    BIT(1)
>>> @@ -525,6 +527,33 @@ static int __init enable_crash_mem_map(char *arg)
>>>   }
>>>   early_param("crashkernel", enable_crash_mem_map);
>>> +#ifdef CONFIG_KFENCE
>>> +
>>> +static phys_addr_t arm64_kfence_alloc_pool(void)
>>> +{
>>> +    phys_addr_t kfence_pool;
>>> +
>>> +    if (!kfence_sample_interval)
>>> +        return 0;
>>> +
>>
>> Are you sure that kernel commandline param are processed this early?
>> AFAICS, start_kernel()->parse_args() process the kernel arguments. We
>> are here before that. without your patch, mm_init() which takes care of
>> allocating kfence memory is called after parse_args().
>>
>> Can you check your patch with kfence.sample_interval=0 appended to
>> kernel commandline?
>>
> 
> Thanks Pavan. I have tried and you're correct. Previously I thought it's 
> parsed by the way:
> setup_arch()->parse_early_param(earlier)->parse_early_options-> 
> do_early_param
> Unfortunately seems not take effect.
> 
> Then the only way left is we always allocate the kfence pool early? as 
> we can't get sample_invertal at this early stage.

 From logs, it seems early param can take effect before doing linear 
mapping set up. Let me think about it :) Thanks for pointing this out!

> 
>>> +    kfence_pool = memblock_phys_alloc(KFENCE_POOL_SIZE, PAGE_SIZE);
>>> +    if (!kfence_pool)
>>> +        pr_err("failed to allocate kfence pool\n");
>>> +
>> For whatever reason, if this allocation fails, what should be done? We
>> end up not calling kfence_set_pool(). kfence_alloc_pool() is going to
>> attempt allocation again but we did not setup page granularity. That
>> means, we are enabling KFENCE without meeting pre-conditions. Can you
>> check this?
> 
> In this scenario, early_kfence_pool should be false(0) and we will end 
> up using page granularity mapping? should be fine IMO.
> 
>>
>>> +    return kfence_pool;
>>> +}
>>> +
>>
>> Thanks,
>> Pavan
> 
> Thanks,
> Zhenhua