Re: [PATCH V3 3/4] arm64/mm: Inhibit huge-vmap with ptdump

[Ard Biesheuvel]

On Tue, 14 May 2019 at 11:00, Anshuman Khandual
<anshuman.khandual@xxxxxxx> wrote:
>
> From: Mark Rutland <mark.rutland@xxxxxxx>
>
> The arm64 ptdump code can race with concurrent modification of the
> kernel page tables. At the time this was added, this was sound as:
>
> * Modifications to leaf entries could result in stale information being
>   logged, but would not result in a functional problem.
>
> * Boot time modifications to non-leaf entries (e.g. freeing of initmem)
>   were performed when the ptdump code cannot be invoked.
>
> * At runtime, modifications to non-leaf entries only occurred in the
>   vmalloc region, and these were strictly additive, as intermediate
>   entries were never freed.
>
> However, since commit:
>
>   commit 324420bf91f6 ("arm64: add support for ioremap() block mappings")
>
> ... it has been possible to create huge mappings in the vmalloc area at
> runtime, and as part of this existing intermediate levels of table my be
> removed and freed.
>
> It's possible for the ptdump code to race with this, and continue to
> walk tables which have been freed (and potentially poisoned or
> reallocated). As a result of this, the ptdump code may dereference bogus
> addresses, which could be fatal.
>
> Since huge-vmap is a TLB and memory optimization, we can disable it when
> the runtime ptdump code is in use to avoid this problem.
>

The reason I added this originally is so that we don't trigger a
warning when unmapping vmappings that use 2 MB block mappings, which
happens when we free the kernel's .init segment. The ability to create
such mappings is indeed an optimization that the kernel mapping code
does not rely on.


> Fixes: 324420bf91f60582 ("arm64: add support for ioremap() block mappings")
> Signed-off-by: Mark Rutland <mark.rutland@xxxxxxx>
> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>

Acked-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxx>

> ---
>  arch/arm64/mm/mmu.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index ef82312..37a902c 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -955,13 +955,18 @@ void *__init fixmap_remap_fdt(phys_addr_t dt_phys)
>
>  int __init arch_ioremap_pud_supported(void)
>  {
> -       /* only 4k granule supports level 1 block mappings */
> -       return IS_ENABLED(CONFIG_ARM64_4K_PAGES);
> +       /*
> +        * Only 4k granule supports level 1 block mappings.
> +        * SW table walks can't handle removal of intermediate entries.
> +        */
> +       return IS_ENABLED(CONFIG_ARM64_4K_PAGES) &&
> +              !IS_ENABLED(CONFIG_ARM64_PTDUMP_DEBUGFS);
>  }
>
>  int __init arch_ioremap_pmd_supported(void)
>  {
> -       return 1;
> +       /* See arch_ioremap_pud_supported() */
> +       return !IS_ENABLED(CONFIG_ARM64_PTDUMP_DEBUGFS);
>  }
>
>  int pud_set_huge(pud_t *pudp, phys_addr_t phys, pgprot_t prot)
> --
> 2.7.4
>
>
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