Re: [PATCH v2] mm,kfence: decouple kfence from page granularity mapping judgement

[Marco Elver <elver@google.com>]

On Thu, 9 Mar 2023 at 16:38, 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, 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/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      | 44 ++++++++++++++++++++++++++++++++++++++++++++
>  arch/arm64/mm/pageattr.c |  5 ++---
>  include/linux/kfence.h   |  7 +++++++
>  mm/kfence/core.c         |  9 +++++++++
>  4 files changed, 62 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 6f9d889..46afe3f 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>
> @@ -525,6 +526,33 @@ 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 = 0;
> +
> +       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 0;
> +       }
> +
> +       return kfence_pool;
> +}
> +
> +#else
> +
> +static phys_addr_t arm64_kfence_alloc_pool(void)
> +{
> +       return (phys_addr_t)NULL;

Just return "0" - which the above function does as well on error. Or
the above function should also do (phys_addr_t)NULL for consistency.

> +}
> +
> +#endif
> +
>  static void __init map_mem(pgd_t *pgdp)
>  {
>         static const u64 direct_map_end = _PAGE_END(VA_BITS_MIN);
> @@ -532,6 +560,7 @@ 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;
> +       phys_addr_t kfence_pool = 0;
>         u64 i;
>
>         /*
> @@ -564,6 +593,10 @@ static void __init map_mem(pgd_t *pgdp)
>         }
>  #endif
>
> +       kfence_pool = arm64_kfence_alloc_pool();
> +       if (kfence_pool)
> +               memblock_mark_nomap(kfence_pool, KFENCE_POOL_SIZE);
> +
>         /* map all the memory banks */
>         for_each_mem_range(i, &start, &end) {
>                 if (start >= end)
> @@ -608,6 +641,17 @@ static void __init map_mem(pgd_t *pgdp)
>                 }
>         }
>  #endif
> +
> +       /* Kfence pool needs page-level mapping */
> +       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);
> +       }
>  }
>
>  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..d982ac2 100644
> --- a/include/linux/kfence.h
> +++ b/include/linux/kfence.h
> @@ -64,6 +64,11 @@ static __always_inline bool is_kfence_address(const void *addr)
>  void __init kfence_alloc_pool(void);
>
>  /**
> + * kfence_set_pool() - KFENCE pool mapped and can be used

I don't understand the comment. Maybe just "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 +227,10 @@ 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..a17c20c2 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 __kfence_pool already initialized in some arch, abort */

Abort sounds like it's a failure condition, but it's actually ok.

Maybe just write:

 /* Check if the pool has already been initialized by arch; if so,
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);
> +}
> +

The rest looks good.