linux-kernel - Re: [PATCH] crypto: ccp - Limit memory allocation in SEV_GET

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Date:   Fri, 30 Dec 2022 14:01:04 -0800 (PST)
From:   David Rientjes <rientjes@...gle.com>
To:     Herbert Xu <herbert@...dor.apana.org.au>
cc:     Peter Gonda <pgonda@...gle.com>, Andy Nguyen <theflow@...gle.com>,
        stable@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-crypto@...r.kernel.org, John Allen <john.allen@....com>,
        "Thomas . Lendacky" <thomas.lendacky@....com>
Subject: Re: [PATCH] crypto: ccp - Limit memory allocation in SEV_GET_ID2
 ioctl

On Wed, 28 Dec 2022, Herbert Xu wrote:

> On Tue, Dec 27, 2022 at 05:42:31PM -0800, David Rientjes wrote:
> >
> > The goal was to be more explicit about that, but setting __GFP_NOWARN 
> > would result in the same functional behavior.  If we're to go that route, 
> > it would likely be best to add a comment about the limitation.
> > 
> > That said, if AMD would prefer this to be an EINVAL instead of a ENOMEM by 
> > introducing a more formal limitation on the length that can be used, that 
> > would be preferred so that we don't need to rely on the page allocator's 
> > max length to enforce this arbitrarily.
> 
> Ideally the limit should be set according to the object that
> you're trying to allocate.  But if that is truly unlimited,
> and you don't want to see a warning, then GFP_NOWARN seems to
> fit the bill.
> 

AMD would be able to speak authoritatively on it, but I think the length 
of the ID isn't to be assumed by software because it will likely change 
later.

I don't think there's an active vulnerability with the currnet code so we 
can likely drop stable@...r.kernel.org for this.  The kzalloc() will fail 
if you try to allocate over 2MB.  If you try to allocate >32KB, the page 
allocator will attempt to reclaim but won't oom kill.  If you try to 
allocate <=32KB, there's the potential for oom kill if nothing is 
reclaimable, but if memory is that scarce I think we have bigger problems.

So __GFP_NOWARN will work, but I also think it's subtle enough that it 
warrants being coupled with a comment.