linux-kernel - Re: [PATCH] iommu: Up front sanity check in the arm_lpae

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Message-ID: <20201207120527.GA4474@willie-the-truck>
Date:   Mon, 7 Dec 2020 12:05:27 +0000
From:   Will Deacon <will@...nel.org>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Keqian Zhu <zhukeqian1@...wei.com>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        iommu@...ts.linux-foundation.org, Marc Zyngier <maz@...nel.org>,
        Joerg Roedel <joro@...tes.org>,
        Catalin Marinas <catalin.marinas@....com>,
        James Morse <james.morse@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Mark Brown <broonie@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexios Zavras <alexios.zavras@...el.com>,
        wanghaibin.wang@...wei.com, jiangkunkun@...wei.com
Subject: Re: [PATCH] iommu: Up front sanity check in the arm_lpae_map

On Mon, Dec 07, 2020 at 12:01:09PM +0000, Robin Murphy wrote:
> On 2020-12-05 08:29, Keqian Zhu wrote:
> > ... then we have more chance to detect wrong code logic.
> 
> I don't follow that justification - it's still the same check with the same
> outcome, so how does moving it have any effect on the chance to detect
> errors?
> 
> AFAICS the only difference it would make is to make some errors *less*
> obvious - if a sufficiently broken caller passes an empty prot value
> alongside an invalid size or already-mapped address, this will now quietly
> hide the warnings from the more serious condition(s).
> 
> Yes, it will bail out a bit faster in the specific case where the prot value
> is the only thing wrong, but since when do we optimise for fundamentally
> incorrect API usage?

I thought it was the other way round -- doesn't this patch move the "empty
prot" check later, so we have a chance to check the size and addresses
first?

Will

> > Signed-off-by: Keqian Zhu <zhukeqian1@...wei.com>
> > ---
> >   drivers/iommu/io-pgtable-arm.c | 8 ++++----
> >   1 file changed, 4 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/iommu/io-pgtable-arm.c b/drivers/iommu/io-pgtable-arm.c
> > index a7a9bc08dcd1..8ade72adab31 100644
> > --- a/drivers/iommu/io-pgtable-arm.c
> > +++ b/drivers/iommu/io-pgtable-arm.c
> > @@ -444,10 +444,6 @@ static int arm_lpae_map(struct io_pgtable_ops *ops, unsigned long iova,
> >   	arm_lpae_iopte prot;
> >   	long iaext = (s64)iova >> cfg->ias;
> > -	/* If no access, then nothing to do */
> > -	if (!(iommu_prot & (IOMMU_READ | IOMMU_WRITE)))
> > -		return 0;
> > -
> >   	if (WARN_ON(!size || (size & cfg->pgsize_bitmap) != size))
> >   		return -EINVAL;
> > @@ -456,6 +452,10 @@ static int arm_lpae_map(struct io_pgtable_ops *ops, unsigned long iova,
> >   	if (WARN_ON(iaext || paddr >> cfg->oas))
> >   		return -ERANGE;
> > +	/* If no access, then nothing to do */
> > +	if (!(iommu_prot & (IOMMU_READ | IOMMU_WRITE)))
> > +		return 0;
> > +
> >   	prot = arm_lpae_prot_to_pte(data, iommu_prot);
> >   	ret = __arm_lpae_map(data, iova, paddr, size, prot, lvl, ptep, gfp);
> >   	/*
> >