Re: [PATCH v1 4/5] powerpc/fault: Avoid heavy search_exception_tables() verification

[Aneesh Kumar K.V]

Christophe Leroy <christophe.leroy@xxxxxxxxxx> writes:

> 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@xxxxxxxxxx>
> ---
>  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;

We still need to keep this ? Without that we detect the lack of
exception tables pretty late.



> -
> -	// 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