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

[Marco Elver <elver@google.com>]

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.

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.

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