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

[Zhenhua Huang <quic_zhenhuah@xxxxxxxxxxx>]

Thanks Marco!

On 2023/3/9 19:09, Marco Elver wrote:
> On Thu, 9 Mar 2023 at 12:04, Zhenhua Huang <quic_zhenhuah@xxxxxxxxxxx> wrote:
> > Thanks Marco.
> >
> > On 2023/3/9 18:33, Marco Elver wrote:
> > > On Thu, 9 Mar 2023 at 09:05, Zhenhua Huang <quic_zhenhuah@xxxxxxxxxxx> 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@xxxxxxxxxxx/T/
> > > > Suggested-by: Mark Rutland <mark.rutland@xxxxxxx>
> > > > Signed-off-by: Zhenhua Huang <quic_zhenhuah@xxxxxxxxxxx>
> > > > ---
> > > >    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()..
>
> Ah right - well, you can initialize __kfence_pool however you like
> within arm64 init code. Just teaching kfence_alloc_pool() to do
> nothing if it's already initialized should be enough. Within
> arch/arm64/mm/mmu.c it might be nice to factor out some bits into a
> helper like arm64_kfence_alloc_pool(), but would just stick to
> whatever is simplest.

Many thanks Marco. Let me conclude as following:
1. put arm64_kfence_alloc_pool() within arch/arm64/mm/mmu.c as it's 
arch_ specific codes.
2. leave kfence_set_pool() to set _kfence_pool within kfence driver, as 
it may become common part.

The reason we still need #2 is because _kfence_pool only can be used 
after mapping set up, it must be late than pool allocation. Do you have 
any further suggestion?

> Thanks,
> -- Marco