linux-kernel - Re: [PATCH v3 4/5] powerpc/fault: Avoid heavy search_exception

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Message-ID: <b532a9c6-97de-031d-f880-901a117cc95c@csgroup.eu>
Date:   Tue, 8 Dec 2020 16:07:32 +0100
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>, npiggin@...il.com
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v3 4/5] powerpc/fault: Avoid heavy
 search_exception_tables() verification



Le 08/12/2020 à 15:52, Aneesh Kumar K.V a écrit :
> Christophe Leroy <christophe.leroy@...roup.eu> 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@...roup.eu>
>> Reviewed-by: Nicholas Piggin <npiggin@...il.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;
>> +	}
> 
> 
> 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?

Yes, I wanted to remove the WARN(), see 
https://patchwork.ozlabs.org/project/linuxppc-dev/patch/cc9129bdda1dbc2f0a09cf45fece7d0b0e690784.1605541983.git.christophe.leroy@csgroup.eu/
but I understood from Michael that maybe it was not a good idea, so I left it aside for now when 
rebasing to v3.

Yes previously we were able to differentiate between a direct access of userspace and a valid access 
triggering a KUAP fault, but at the cost of the heavy search_exception_tables().
The issue was reported by Nick through https://github.com/linuxppc/issues/issues/317

Should be perform the search_exception_tables() once we have hit the KUAP fault and WARN() only in 
that case ?

I was wondering also if we should keep the WARN() only when CONFIG_PPC_KUAP_DEBUG is set ?

> 
> 
>>   
>> -	// 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