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

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Date:   Mon, 24 Apr 2017 19:15:20 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Christophe Leroy <christophe.leroy@....fr>,
        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()

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.

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