linux-kernel - Re: [PATCH v1 3/5] powerpc/fault: Reorder tests in bad_kernel

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Date:   Tue, 08 Sep 2020 18:46:15 +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 3/5] powerpc/fault: Reorder tests in bad_kernel_fault()

Excerpts from Christophe Leroy's message of August 7, 2020 3:15 am:
> Check address earlier to simplify the following test.

Good logic reduction.

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

> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
> ---
>  arch/powerpc/mm/fault.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
> index 9ef9ee244f72..525e0c2b5406 100644
> --- a/arch/powerpc/mm/fault.c
> +++ b/arch/powerpc/mm/fault.c
> @@ -210,17 +210,17 @@ static bool bad_kernel_fault(struct pt_regs *regs, unsigned long error_code,
>  		return true;
>  	}
>  
> -	if (!is_exec && address < TASK_SIZE && (error_code & DSISR_PROTFAULT) &&
> +	// Kernel fault on kernel address is bad
> +	if (address >= TASK_SIZE)
> +		return true;
> +
> +	if (!is_exec && (error_code & DSISR_PROTFAULT) &&
>  	    !search_exception_tables(regs->nip)) {
>  		pr_crit_ratelimited("Kernel attempted to access user page (%lx) - exploit attempt? (uid: %d)\n",
>  				    address,
>  				    from_kuid(&init_user_ns, current_uid()));
>  	}
>  
> -	// Kernel fault on kernel address is bad
> -	if (address >= TASK_SIZE)
> -		return true;
> -
>  	// Fault on user outside of certain regions (eg. copy_tofrom_user()) is bad
>  	if (!search_exception_tables(regs->nip))
>  		return true;
> -- 
> 2.25.0
> 
>