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

From: Zhenhua Huang <quic_zhenhuah@quicinc.com>
To: Marco Elver <elver@google.com>
Cc: <catalin.marinas@arm.com>, <will@kernel.org>, <glider@google.com>,
	<dvyukov@google.com>, <akpm@linux-foundation.org>,
	<robin.murphy@arm.com>, <mark.rutland@arm.com>,
	<jianyong.wu@arm.com>, <james.morse@arm.com>,
	<linux-arm-kernel@lists.infradead.org>,
	<kasan-dev@googlegroups.com>, <linux-mm@kvack.org>,
	<quic_pkondeti@quicinc.com>, <quic_guptap@quicinc.com>,
	<quic_tingweiz@quicinc.com>
Subject: Re: [PATCH] mm,kfence: decouple kfence from page granularity mapping judgement
Date: Thu, 9 Mar 2023 19:03:59 +0800	[thread overview]
Message-ID: <706340ef-1745-c1e4-be4d-358d5db4c05e@quicinc.com> (raw)
In-Reply-To: <CANpmjNOah6gSB_mRvhsK_9DpBtiYinRd5z34PU+tOFgSqXB8Sw@mail.gmail.com>

Thanks Marco.

On 2023/3/9 18:33, Marco Elver wrote:
> On Thu, 9 Mar 2023 at 09:05, Zhenhua Huang <quic_zhenhuah@quicinc.com> wrote:
>>
>> Kfence only needs its pool to be mapped as page granularity, previous
>> judgement was a bit over protected. Decouple it from judgement and do
>> page granularity mapping for kfence pool only [1].
>>
>> To implement this, also relocate the kfence pool allocation before the
>> linear mapping setting up, 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/1675750519-1064-1-git-send-email-quic_zhenhuah@quicinc.com/T/
>> Suggested-by: Mark Rutland <mark.rutland@arm.com>
>> Signed-off-by: Zhenhua Huang <quic_zhenhuah@quicinc.com>
>> ---
>>   arch/arm64/mm/mmu.c      | 24 ++++++++++++++++++++++++
>>   arch/arm64/mm/pageattr.c |  5 ++---
>>   include/linux/kfence.h   | 10 ++++++++--
>>   init/main.c              |  1 -
>>   mm/kfence/core.c         | 18 ++++++++++++++----
>>   5 files changed, 48 insertions(+), 10 deletions(-)
>>
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index 6f9d889..bd79691 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>
>> @@ -532,6 +533,9 @@ static void __init map_mem(pgd_t *pgdp)
>>          phys_addr_t kernel_end = __pa_symbol(__init_begin);
>>          phys_addr_t start, end;
>>          int flags = NO_EXEC_MAPPINGS;
>> +#ifdef CONFIG_KFENCE
>> +       phys_addr_t kfence_pool = 0;
>> +#endif
>>          u64 i;
>>
>>          /*
>> @@ -564,6 +568,12 @@ static void __init map_mem(pgd_t *pgdp)
>>          }
>>   #endif
>>
>> +#ifdef CONFIG_KFENCE
>> +       kfence_pool = kfence_alloc_pool();
>> +       if (kfence_pool)
>> +               memblock_mark_nomap(kfence_pool, KFENCE_POOL_SIZE);
>> +#endif
>> +
>>          /* map all the memory banks */
>>          for_each_mem_range(i, &start, &end) {
>>                  if (start >= end)
>> @@ -608,6 +618,20 @@ static void __init map_mem(pgd_t *pgdp)
>>                  }
>>          }
>>   #endif
>> +
>> +       /* Kfence pool needs page-level mapping */
>> +#ifdef CONFIG_KFENCE
>> +       if (kfence_pool) {
>> +               __map_memblock(pgdp, kfence_pool,
>> +                       kfence_pool + KFENCE_POOL_SIZE,
>> +                       pgprot_tagged(PAGE_KERNEL),
>> +                       NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS);
>> +               memblock_clear_nomap(kfence_pool, KFENCE_POOL_SIZE);
>> +               /* kfence_pool really mapped now */
>> +               kfence_set_pool(kfence_pool);
>> +       }
>> +#endif
>> +
>>   }
>>
>>   void mark_rodata_ro(void)
>> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
>> index 79dd201..61156d0 100644
>> --- a/arch/arm64/mm/pageattr.c
>> +++ b/arch/arm64/mm/pageattr.c
>> @@ -22,12 +22,11 @@ bool rodata_full __ro_after_init = IS_ENABLED(CONFIG_RODATA_FULL_DEFAULT_ENABLED
>>   bool can_set_direct_map(void)
>>   {
>>          /*
>> -        * rodata_full, DEBUG_PAGEALLOC and KFENCE require linear map to be
>> +        * rodata_full and DEBUG_PAGEALLOC require linear map to be
>>           * mapped at page granularity, so that it is possible to
>>           * protect/unprotect single pages.
>>           */
>> -       return (rodata_enabled && rodata_full) || debug_pagealloc_enabled() ||
>> -               IS_ENABLED(CONFIG_KFENCE);
>> +       return (rodata_enabled && rodata_full) || debug_pagealloc_enabled();
>>   }
>>
>>   static int change_page_range(pte_t *ptep, unsigned long addr, void *data)
>> diff --git a/include/linux/kfence.h b/include/linux/kfence.h
>> index 726857a..0252e74 100644
>> --- a/include/linux/kfence.h
>> +++ b/include/linux/kfence.h
>> @@ -61,7 +61,12 @@ static __always_inline bool is_kfence_address(const void *addr)
>>   /**
>>    * kfence_alloc_pool() - allocate the KFENCE pool via memblock
>>    */
>> -void __init kfence_alloc_pool(void);
>> +phys_addr_t __init kfence_alloc_pool(void);
>> +
>> +/**
>> + * kfence_set_pool() - KFENCE pool mapped and can be used
>> + */
>> +void __init kfence_set_pool(phys_addr_t addr);
>>
>>   /**
>>    * kfence_init() - perform KFENCE initialization at boot time
>> @@ -223,7 +228,8 @@ bool __kfence_obj_info(struct kmem_obj_info *kpp, void *object, struct slab *sla
>>   #else /* CONFIG_KFENCE */
>>
>>   static inline bool is_kfence_address(const void *addr) { return false; }
>> -static inline void kfence_alloc_pool(void) { }
>> +static inline phys_addr_t kfence_alloc_pool(void) { return (phys_addr_t)NULL; }
>> +static inline void kfence_set_pool(phys_addr_t addr) { }
>>   static inline void kfence_init(void) { }
>>   static inline void kfence_shutdown_cache(struct kmem_cache *s) { }
>>   static inline void *kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags) { return NULL; }
>> diff --git a/init/main.c b/init/main.c
>> index 4425d17..9aaf217 100644
>> --- a/init/main.c
>> +++ b/init/main.c
>> @@ -839,7 +839,6 @@ static void __init mm_init(void)
>>           */
>>          page_ext_init_flatmem();
>>          init_mem_debugging_and_hardening();
>> -       kfence_alloc_pool();
> 
> This breaks other architectures.

Nice catch. Thanks!

> 
>>          report_meminit();
>>          kmsan_init_shadow();
>>          stack_depot_early_init();
>> diff --git a/mm/kfence/core.c b/mm/kfence/core.c
>> index 5349c37..dd5cdd5 100644
>> --- a/mm/kfence/core.c
>> +++ b/mm/kfence/core.c
>> @@ -809,15 +809,25 @@ static void toggle_allocation_gate(struct work_struct *work)
>>
>>   /* === Public interface ===================================================== */
>>
>> -void __init kfence_alloc_pool(void)
>> +phys_addr_t __init kfence_alloc_pool(void)
>>   {
> 
> You could just return here:
> 
>    if (__kfence_pool)
>      return; /* Initialized earlier by arch init code. */

Yeah.

> 
> ... and see my comments below.
> 
>> +       phys_addr_t kfence_pool;
>>          if (!kfence_sample_interval)
>> -               return;
>> +               return 0;
>>
>> -       __kfence_pool = memblock_alloc(KFENCE_POOL_SIZE, PAGE_SIZE);
>> +       kfence_pool = memblock_phys_alloc(KFENCE_POOL_SIZE, PAGE_SIZE);
>>
>> -       if (!__kfence_pool)
>> +       if (!kfence_pool) {
>>                  pr_err("failed to allocate pool\n");
>> +               return 0;
>> +       }
>> +
>> +       return kfence_pool;
>> +}
>> +
>> +void __init kfence_set_pool(phys_addr_t addr)
>> +{
>> +       __kfence_pool = phys_to_virt(addr);
>>   }
> 
> I would suggest leaving kfence_alloc_pool() to return nothing (with
> the addition above), and just set __kfence_pool as before.
> __kfence_pool itself is exported by include/linux/kfence.h, so if you
> call kfence_alloc_pool() in arm64 earlier, you can access
> __kfence_pool to get the allocated pool.

Shall we add one new function like arm64_kfence_alloc_pool() ? The 
reason is linear mapping at that time not set up and we must alloc phys 
addr based on memblock. We can't use common kfence_alloc_pool()..

> 
> Because at that point, KFENCE isn't yet running, that only happens
> after kfence_init() much later.
> 
> With these changes, you should be able to make arm64 work the way you
> want, and not break other architectures where we don't need arch init
> code to allocate the pool.
> 
> Thanks,
> -- Marco