Re: [PATCH V2] arm64/mm: Intercept pfn changes in set_pte_at()

[Mark Rutland]

Hi Annshuman,

On Mon, Jan 09, 2023 at 10:58:16AM +0530, Anshuman Khandual wrote:
> Changing pfn on a user page table mapped entry, without first going through
> break-before-make (BBM) procedure is unsafe. This just updates set_pte_at()
> to intercept such changes, via an updated pgattr_change_is_safe(). This new
> check happens via __check_racy_pte_update(), which has now been renamed as
> __check_safe_pte_update().
> 
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Signed-off-by: Anshuman Khandual <anshuman.khandual@xxxxxxx>
> ---
> This applies on v6.2-rc3. This patch had some test time on an internal CI
> system without any issues being reported.

Can you elaborate on this a little bit? It's not entirely clear what that
internal CI system has tested. It would be helpful if you could indicate:

* What sort of testing has been done by the CI system? e.g. is this just
  booting, running LTP, something else?

* Has this tried a bunch of configurations and/or machines?

* If any targetted stress tests have been used? e.g. stress-ng's memory system
  tests?

I'm assuming that's hitting LTP on a few machines/configs, which'd be
reasonable. It'd just be nice to confirm exactly what has been tested.

I've added this to my lcoal syzkaller instance's test branch, and I'll shout if
that hits anything over the weekend.

> Changes in V1:
> 
> https://lore.kernel.org/all/20221116031001.292236-1-anshuman.khandual@xxxxxxx/

Did you mean to list some cahnges here?

> 
>  arch/arm64/include/asm/pgtable.h | 8 ++++++--
>  arch/arm64/mm/mmu.c              | 8 +++++++-
>  2 files changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> index b4bbeed80fb6..832c9c8fb58f 100644
> --- a/arch/arm64/include/asm/pgtable.h
> +++ b/arch/arm64/include/asm/pgtable.h
> @@ -275,6 +275,7 @@ static inline void set_pte(pte_t *ptep, pte_t pte)
>  }
>  
>  extern void __sync_icache_dcache(pte_t pteval);
> +bool pgattr_change_is_safe(u64 old, u64 new);
>  
>  /*
>   * PTE bits configuration in the presence of hardware Dirty Bit Management
> @@ -292,7 +293,7 @@ extern void __sync_icache_dcache(pte_t pteval);
>   *   PTE_DIRTY || (PTE_WRITE && !PTE_RDONLY)
>   */
>  
> -static inline void __check_racy_pte_update(struct mm_struct *mm, pte_t *ptep,
> +static inline void __check_safe_pte_update(struct mm_struct *mm, pte_t *ptep,
>  					   pte_t pte)
>  {
>  	pte_t old_pte;
> @@ -318,6 +319,9 @@ static inline void __check_racy_pte_update(struct mm_struct *mm, pte_t *ptep,
>  	VM_WARN_ONCE(pte_write(old_pte) && !pte_dirty(pte),
>  		     "%s: racy dirty state clearing: 0x%016llx -> 0x%016llx",
>  		     __func__, pte_val(old_pte), pte_val(pte));
> +	VM_WARN_ONCE(!pgattr_change_is_safe(pte_val(old_pte), pte_val(pte)),
> +		     "%s: unsafe attribute change: 0x%016llx -> 0x%016llx",
> +		     __func__, pte_val(old_pte), pte_val(pte));
>  }
>  
>  static inline void __set_pte_at(struct mm_struct *mm, unsigned long addr,
> @@ -346,7 +350,7 @@ static inline void __set_pte_at(struct mm_struct *mm, unsigned long addr,
>  			mte_sync_tags(old_pte, pte);
>  	}
>  
> -	__check_racy_pte_update(mm, ptep, pte);
> +	__check_safe_pte_update(mm, ptep, pte);
>  
>  	set_pte(ptep, pte);
>  }
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 14c87e8d69d8..a1d16b35c4f6 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -133,7 +133,7 @@ static phys_addr_t __init early_pgtable_alloc(int shift)
>  	return phys;
>  }
>  
> -static bool pgattr_change_is_safe(u64 old, u64 new)
> +bool pgattr_change_is_safe(u64 old, u64 new)
>  {
>  	/*
>  	 * The following mapping attributes may be updated in live
> @@ -145,6 +145,12 @@ static bool pgattr_change_is_safe(u64 old, u64 new)
>  	if (old == 0 || new == 0)
>  		return true;

These checks above should really use pte_valid(); we were just being lazy when
this was originally written since for the init_*() cases the memory should be
zero initially.

So could you make that:

	if (!pte_valid(__pte(old)) || !pte_valid(__pte(new)))
		return true;

> +	/* If old and new ptes are valid, pfn should not change */
> +	if (pte_valid(__pte(old)) && pte_valid(__pte(new))) {
> +		if (pte_pfn(__pte(old)) != pte_pfn(__pte(new)))
> +			return false;
> +	}

With the above change, it's clear that both must be valid to get this far, and
this check can be reduced to:


	/* A live entry's pfn should not change */
	if (pte_pfn(__pte(old)) != pte_pfn(__pte(new)))
		return false;

With those changes:

Acked-by: Mark Rutland <mark.rutland@xxxxxxx>

Thanks,
Mark.