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

[Zhenhua Huang <quic_zhenhuah@quicinc.com>]

On 2023/3/14 16:41, Marco Elver wrote:
> On Tue, 14 Mar 2023 at 08:05, Zhenhua Huang <quic_zhenhuah@quicinc.com> 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;
>> +
>> +       kfence_pool = memblock_phys_alloc(KFENCE_POOL_SIZE, PAGE_SIZE);
>> +       if (!kfence_pool)
>> +               pr_err("failed to allocate kfence pool\n");
>> +
>> +       return kfence_pool;
>> +}
>> +
>> +#else
>> +
>> +static phys_addr_t arm64_kfence_alloc_pool(void)
>> +{
>> +       return 0;
>> +}
>> +
>> +#endif
>> +
>> +phys_addr_t early_kfence_pool;
> 
> The compiler will not optimize out this global. This now exists in all
> arm64 kernel builds.
> 
> Furthermore, there's no need for this to be phys_addr_t. Nothing
> outside map_mem() needs the address, so this can just be a bool.

Seems we need this early_kfence_bool to be explicit phys_addr_t as we 
need to mark/clear NOMAP for the region, so that it will not do linear 
mapping in the for loop.

> 
> I'd recommend moving the variable under CONFIG_KFENCE, and in the asm
> header, just having a static inline helper function e.g.
> arm64_kfence_early_pool(). That helper just returns false in the
> !CONFIG_KFENCE case.
> 
>>   static void __init map_mem(pgd_t *pgdp)
>>   {
>>          static const u64 direct_map_end = _PAGE_END(VA_BITS_MIN);
>> @@ -543,6 +572,10 @@ static void __init map_mem(pgd_t *pgdp)
>>           */
>>          BUILD_BUG_ON(pgd_index(direct_map_end - 1) == pgd_index(direct_map_end));
>>
>> +       early_kfence_pool = arm64_kfence_alloc_pool();
>> +       if (early_kfence_pool)
>> +               memblock_mark_nomap(early_kfence_pool, KFENCE_POOL_SIZE);
>> +
>>          if (can_set_direct_map())
>>                  flags |= NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS;
>>
>> @@ -608,6 +641,17 @@ static void __init map_mem(pgd_t *pgdp)
>>                  }
>>          }
>>   #endif
>> +
>> +       /* Kfence pool needs page-level mapping */
>> +       if (early_kfence_pool) {
>> +               __map_memblock(pgdp, early_kfence_pool,
>> +                       early_kfence_pool + KFENCE_POOL_SIZE,
>> +                       pgprot_tagged(PAGE_KERNEL),
>> +                       NO_BLOCK_MAPPINGS | NO_CONT_MAPPINGS);
>> +               memblock_clear_nomap(early_kfence_pool, KFENCE_POOL_SIZE);
>> +               /* kfence_pool really mapped now */
>> +               kfence_set_pool(early_kfence_pool);
>> +       }
>>   }
>>
>>   void mark_rodata_ro(void)
>> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
>> index 79dd201..7ce5295 100644
>> --- a/arch/arm64/mm/pageattr.c
>> +++ b/arch/arm64/mm/pageattr.c
>> @@ -7,10 +7,12 @@
>>   #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>
>>   #include <asm/tlbflush.h>
>> +#include <asm/kfence.h>
>>
>>   struct page_change_data {
>>          pgprot_t set_mask;
>> @@ -22,12 +24,15 @@ 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.
>> +        *
>> +        * Kfence pool requires page granularity mapping also if we init it
>> +        * late.
>>           */
>>          return (rodata_enabled && rodata_full) || debug_pagealloc_enabled() ||
>> -               IS_ENABLED(CONFIG_KFENCE);
>> +           (IS_ENABLED(CONFIG_KFENCE) && !early_kfence_pool);
>>   }
>>
>>   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..570d4e3 100644
>> --- a/include/linux/kfence.h
>> +++ b/include/linux/kfence.h
>> @@ -64,6 +64,12 @@ static __always_inline bool is_kfence_address(const void *addr)
>>   void __init kfence_alloc_pool(void);
>>
>>   /**
>> + * kfence_set_pool() - allows an arch to set the
>> + * KFENCE pool during early init
>> + */
>> +void __init kfence_set_pool(phys_addr_t addr);
>> +
>> +/**
>>    * kfence_init() - perform KFENCE initialization at boot time
>>    *
>>    * Requires that kfence_alloc_pool() was called before. This sets up the
>> @@ -222,8 +228,10 @@ bool __kfence_obj_info(struct kmem_obj_info *kpp, void *object, struct slab *sla
>>
>>   #else /* CONFIG_KFENCE */
>>
>> +#define KFENCE_POOL_SIZE 0
>>   static inline bool is_kfence_address(const void *addr) { return false; }
>>   static inline void kfence_alloc_pool(void) { }
>> +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/mm/kfence/core.c b/mm/kfence/core.c
>> index 5349c37..0765395 100644
>> --- a/mm/kfence/core.c
>> +++ b/mm/kfence/core.c
>> @@ -814,12 +814,21 @@ void __init kfence_alloc_pool(void)
>>          if (!kfence_sample_interval)
>>                  return;
>>
>> +       /* if the pool has already been initialized by arch, skip the below */
>> +       if (__kfence_pool)
>> +               return;
>> +
>>          __kfence_pool = memblock_alloc(KFENCE_POOL_SIZE, PAGE_SIZE);
>>
>>          if (!__kfence_pool)
>>                  pr_err("failed to allocate pool\n");
>>   }
>>
>> +void __init kfence_set_pool(phys_addr_t addr)
>> +{
>> +       __kfence_pool = phys_to_virt(addr);
>> +}
> 
> Please move this function to the header as a static inline function,
> because nothing else other than arm64 needs this, having this function
> be here introduces a .text size increase for everyone.
> 
> The function is so short that having it as a static inline function is
> fine, and will save a few bytes of .text.

Reasonable! Thanks!

> 
>>   static void kfence_init_enable(void)
>>   {
>>          if (!IS_ENABLED(CONFIG_KFENCE_STATIC_KEYS))
>> --
>> 2.7.4
>>

Thanks,
Zhenhua