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

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Tue, 08 Dec 2020 20:22:35 +0530

Christophe Leroy <[email-masked]> 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 <[email-masked]>
> Reviewed-by: Nicholas Piggin <[email-masked]>
> ---
> 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;
> +	}

With this I am wondering whether the WARN() in bad_kuap_fault() is
needed. A direct access of userspace address will trigger this, whereas
previously we used bad_kuap_fault() only to identify incorrect restore
of AMR register (ie, to identify kernel bugs). Hence a WARN() there was
useful. We loose that differentiation now?

>  
> -	// 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