linux-kernel - Re: [PATCH] powerpc/mm: Only read faulting instruction when necessary in do_page

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Message-ID: <a41ff27c-fedc-6621-01ed-d25f4d0813d1@c-s.fr>
Date:   Mon, 24 Apr 2017 12:26:11 +0200
From:   Christophe LEROY <christophe.leroy@....fr>
To:     Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Scott Wood <oss@...error.net>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] powerpc/mm: Only read faulting instruction when necessary
 in do_page_fault()



Le 24/04/2017 à 11:15, Michael Ellerman a écrit :
> Christophe Leroy <christophe.leroy@....fr> writes:
>
>> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
>> index 400f2d0d42f8..3d506589236c 100644
>> --- a/arch/powerpc/mm/fault.c
>> +++ b/arch/powerpc/mm/fault.c
>> @@ -356,8 +348,22 @@ int do_page_fault(struct pt_regs *regs, unsigned long address,
>>  		 * between the last mapped region and the stack will
>>  		 * expand the stack rather than segfaulting.
>>  		 */
>> -		if (address + 2048 < uregs->gpr[1] && !store_updates_sp(inst))
>> -			goto bad_area;
>> +		if (address + 2048 < uregs->gpr[1]) {
>> +			if (!inst) {
>
> That looks like it could lead to an infinite loop, if the instruction
> that caused the fault has been overwritten with NULL by another thread.

If the inst is NULL, store_updates_sp(inst) returns false so it jumps to 
bad_area_nosemaphore

Christophe

>
> Or did I miss something? (I only read the diff)
>
>> +				/*
>> +				 * We want to do this outside mmap_sem, because
>> +				 * reading code around nip can result in fault,
>> +				 * which will cause a deadlock when called with
>> +				 * mmap_sem held
>> +				 */
>> +				up_read(&mm->mmap_sem);
>> +				__get_user(inst,
>> +					   (unsigned int __user *)regs->nip);
>> +				if (!store_updates_sp(inst))
>> +					goto bad_area_nosemaphore;
>> +				goto retry;
>> +			}
>
>
> cheers
>