linux-kernel - Re: [PATCH v1 4/5] powerpc/fault: Avoid heavy search_exception

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Date:   Tue, 08 Sep 2020 18:54:40 +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 4/5] powerpc/fault: Avoid heavy
 search_exception_tables() verification

Excerpts from Christophe Leroy's message of August 7, 2020 3:15 am:
> search_exception_tables() is an heavy operation, we have to avoid it.
> When KUAP is selected, we'll know the fault has been blocked by KUAP.
> Otherwise, it behaves just as if the address was already in the TLBs
> and no fault was generated.
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>

Sorry I missed reviewing this. Yes, we discussed this and decided
that it's not effective I think (and KUAP solves it properly).

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

> ---
>  arch/powerpc/mm/fault.c | 20 +++++---------------
>  1 file changed, 5 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
> index 525e0c2b5406..edde169ba3a6 100644
> --- a/arch/powerpc/mm/fault.c
> +++ b/arch/powerpc/mm/fault.c
> @@ -214,24 +214,14 @@ static bool bad_kernel_fault(struct pt_regs *regs, unsigned long error_code,
>  	if (address >= TASK_SIZE)
>  		return true;
>  
> -	if (!is_exec && (error_code & DSISR_PROTFAULT) &&
> -	    !search_exception_tables(regs->nip)) {
> +	// Read/write fault blocked by KUAP is bad, it can never succeed.
> +	if (bad_kuap_fault(regs, address, is_write)) {
>  		pr_crit_ratelimited("Kernel attempted to access user page (%lx) - exploit attempt? (uid: %d)\n",
> -				    address,
> -				    from_kuid(&init_user_ns, current_uid()));
> -	}
> -
> -	// Fault on user outside of certain regions (eg. copy_tofrom_user()) is bad
> -	if (!search_exception_tables(regs->nip))
> -		return true;
> -
> -	// Read/write fault in a valid region (the exception table search passed
> -	// above), but blocked by KUAP is bad, it can never succeed.
> -	if (bad_kuap_fault(regs, address, is_write))
> +				    address, from_kuid(&init_user_ns, current_uid()));
>  		return true;
> +	}
>  
> -	// What's left? Kernel fault on user in well defined regions (extable
> -	// matched), and allowed by KUAP in the faulting context.
> +	// What's left? Kernel fault on user and allowed by KUAP in the faulting context.
>  	return false;
>  }
>  
> -- 
> 2.25.0
> 
>