Re: [PATCH v3 4/5] powerpc/fault: Avoid heavy search_exception_tables() verification

["Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>]

On 12/8/20 2:07 PM, Christophe Leroy wrote:
> 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@csgroup.eu>
> Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
> ---
> v3: rebased
> v2: Squashed with the preceeding patch which was re-ordering tests that get removed in this patch.
> ---
>   arch/powerpc/mm/fault.c | 23 +++++++----------------
>   1 file changed, 7 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
> index 3fcd34c28e10..1770b41e4730 100644
> --- a/arch/powerpc/mm/fault.c
> +++ b/arch/powerpc/mm/fault.c
> @@ -210,28 +210,19 @@ 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 | DSISR_KEYFAULT)) &&
> -	    !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;
> -
> -	// 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))
> +	// 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 %s user page (%lx) - exploit attempt? (uid: %d)\n",
> +				    is_write ? "write" : "read", address,
> +				    from_kuid(&init_user_ns, current_uid()));
>   		return true;
> +	}


Should we update bad_kuap_fault to check for !is_kernel_addr() and 
error_code & (DSISIR_PROT_FAULT | DSISIR_KEYFAULT). I am wondering 
whether we can take another fault w.r.t kernel address/user address and 
end up reporting that as KUAP fault?

>   
> -	// 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;
>   }
>   
> 


-aneesh