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

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Date:   Tue, 14 May 2019 11:20:25 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Anshuman Khandual <anshuman.khandual@....com>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        akpm@...ux-foundation.org, catalin.marinas@....com,
        will.deacon@....com
Cc:     mhocko@...e.com, mgorman@...hsingularity.net, james.morse@....com,
        mark.rutland@....com, robin.murphy@....com, cpandya@...eaurora.org,
        arunks@...eaurora.org, dan.j.williams@...el.com, osalvador@...e.de,
        cai@....pw, logang@...tatee.com, ira.weiny@...el.com
Subject: Re: [PATCH V3 3/4] arm64/mm: Inhibit huge-vmap with ptdump

On 14.05.19 11:00, Anshuman Khandual wrote:
> From: Mark Rutland <mark.rutland@....com>
> 
> 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.

I wonder if there is another way to protect from such a race happening.
(IOW, a lock). But as you say, it's a debug feature, so this is an easy fix.

Looks good to me (with limited arm64 code insight :) )

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


-- 

Thanks,

David / dhildenb