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From: David Hildenbrand <david@redhat.com>
To: Anshuman Khandual <anshuman.khandual@arm.com>,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, akpm@linux-foundation.org,
	catalin.marinas@arm.com, will.deacon@arm.com
Cc: mhocko@suse.com, mgorman@techsingularity.net,
	james.morse@arm.com, mark.rutland@arm.com, robin.murphy@arm.com,
	cpandya@codeaurora.org, arunks@codeaurora.org,
	dan.j.williams@intel.com, osalvador@suse.de, cai@lca.pw,
	logang@deltatee.com, ira.weiny@intel.com
Subject: Re: [PATCH V3 3/4] arm64/mm: Inhibit huge-vmap with ptdump
Date: Tue, 14 May 2019 11:20:25 +0200	[thread overview]
Message-ID: <ad42eae0-29e1-8f09-a9ce-c36d2fd2e8e7@redhat.com> (raw)
In-Reply-To: <1557824407-19092-4-git-send-email-anshuman.khandual@arm.com>

On 14.05.19 11:00, Anshuman Khandual wrote:
> From: Mark Rutland <mark.rutland@arm.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@arm.com>
> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will.deacon@arm.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

  reply	other threads:[~2019-05-14  9:20 UTC|newest]

Thread overview: 28+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-14  9:00 [PATCH V3 0/4] arm64/mm: Enable memory hot remove Anshuman Khandual
2019-05-14  9:00 ` [PATCH V3 1/4] mm/hotplug: Reorder arch_remove_memory() call in __remove_memory() Anshuman Khandual
2019-05-14  9:05   ` David Hildenbrand
2019-05-14  9:00 ` [PATCH V3 2/4] arm64/mm: Hold memory hotplug lock while walking for kernel page table dump Anshuman Khandual
2019-05-14  9:16   ` David Hildenbrand
2019-05-14 15:40   ` Mark Rutland
2019-05-15  1:56     ` Anshuman Khandual
2019-05-15 16:58   ` Michal Hocko
2019-05-16 10:23     ` Mark Rutland
2019-05-16 11:05       ` Michal Hocko
2019-05-22 16:42         ` Mark Rutland
2019-05-24  6:06           ` Anshuman Khandual
2019-05-27  7:20           ` Michal Hocko
2019-05-28 14:09             ` Mark Rutland
2019-05-16 11:06       ` Anshuman Khandual
2019-05-16 11:16         ` Michal Hocko
2019-05-23  8:40           ` David Hildenbrand
2019-05-24  5:43             ` Anshuman Khandual
2019-05-14  9:00 ` [PATCH V3 3/4] arm64/mm: Inhibit huge-vmap with ptdump Anshuman Khandual
2019-05-14  9:20   ` David Hildenbrand [this message]
2019-05-16  8:38   ` Ard Biesheuvel
2019-05-14  9:00 ` [PATCH V3 4/4] arm64/mm: Enable memory hot remove Anshuman Khandual
2019-05-14  9:08   ` David Hildenbrand
2019-05-15 11:49   ` Mark Rutland
2019-05-16  5:34     ` Anshuman Khandual
2019-05-16 10:57       ` Mark Rutland
2019-05-17  3:15         ` Anshuman Khandual
2019-05-14  9:10 ` [PATCH V3 0/4] " David Hildenbrand

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