linux-kernel - Re: [PATCH v1 1/5] powerpc/mm: sanity_check_fault() should work for all, not only BOOK3S

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Date:   Tue, 08 Sep 2020 18:43:48 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v1 1/5] powerpc/mm: sanity_check_fault() should work for
 all, not only BOOK3S

Excerpts from Christophe Leroy's message of August 7, 2020 3:15 am:
> The verification and message introduced by commit 374f3f5979f9
> ("powerpc/mm/hash: Handle user access of kernel address gracefully")
> applies to all platforms, it should not be limited to BOOK3S.
> 
> Make the BOOK3S version of sanity_check_fault() the one for all,
> and bail out earlier if not BOOK3S.
> 
> Fixes: 374f3f5979f9 ("powerpc/mm/hash: Handle user access of kernel address gracefully")
> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
> ---
>  arch/powerpc/mm/fault.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
> index 925a7231abb3..2efa34d7e644 100644
> --- a/arch/powerpc/mm/fault.c
> +++ b/arch/powerpc/mm/fault.c
> @@ -303,7 +303,6 @@ static inline void cmo_account_page_fault(void)
>  static inline void cmo_account_page_fault(void) { }
>  #endif /* CONFIG_PPC_SMLPAR */
>  
> -#ifdef CONFIG_PPC_BOOK3S
>  static void sanity_check_fault(bool is_write, bool is_user,
>  			       unsigned long error_code, unsigned long address)
>  {
> @@ -320,6 +319,9 @@ static void sanity_check_fault(bool is_write, bool is_user,
>  		return;
>  	}
>  
> +	if (!IS_ENABLED(CONFIG_PPC_BOOK3S))
> +		return;

Seems okay. Why is address == -1 special though? I guess it's because 
it may not be an exploit kernel reference but a buggy pointer underflow? 
In that case -1 doesn't seem like it would catch very much. Would it be 
better to test for high bit set for example ((long)address < 0) ?

Anyway for your patch

Reviewed-by: Nicholas Piggin <npiggin@...il.com>

> +
>  	/*
>  	 * For hash translation mode, we should never get a
>  	 * PROTFAULT. Any update to pte to reduce access will result in us
> @@ -354,10 +356,6 @@ static void sanity_check_fault(bool is_write, bool is_user,
>  
>  	WARN_ON_ONCE(error_code & DSISR_PROTFAULT);
>  }
> -#else
> -static void sanity_check_fault(bool is_write, bool is_user,
> -			       unsigned long error_code, unsigned long address) { }
> -#endif /* CONFIG_PPC_BOOK3S */
>  
>  /*
>   * Define the correct "is_write" bit in error_code based
> -- 
> 2.25.0
> 
>