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From:   Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu, 29 Oct 2020 10:23:09 +0100
Message-ID: <CAKMK7uGVUf9RfTHKa8fpTUgGQ5iy+5toK+tQYp0TokKU=iM8pQ@mail.gmail.com>
Subject: Re: [PATCH v4 08/15] mm: Add unsafe_follow_pfn
To:     Christoph Hellwig <hch@infradead.org>
Cc:     DRI Development <dri-devel@lists.freedesktop.org>,
        LKML <linux-kernel@vger.kernel.org>,
        KVM list <kvm@vger.kernel.org>, Linux MM <linux-mm@kvack.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        linux-samsung-soc <linux-samsung-soc@vger.kernel.org>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@vger.kernel.org>,
        linux-s390 <linux-s390@vger.kernel.org>,
        Daniel Vetter <daniel.vetter@intel.com>,
        Jason Gunthorpe <jgg@ziepe.ca>,
        Kees Cook <keescook@chromium.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        John Hubbard <jhubbard@nvidia.com>,
        "J??r??me Glisse" <jglisse@redhat.com>, Jan Kara <jack@suse.cz>
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On Thu, Oct 29, 2020 at 9:56 AM Christoph Hellwig <hch@infradead.org> wrote:
>
> > +int unsafe_follow_pfn(struct vm_area_struct *vma, unsigned long address,
> > +     unsigned long *pfn)
>
> The one tab indent here looks weird, normally tis would be two tabs
> or aligned aftetthe opening brace.
>
> > +{
> > +#ifdef CONFIG_STRICT_FOLLOW_PFN
> > +     pr_info("unsafe follow_pfn usage rejected, see CONFIG_STRICT_FOLLOW_PFN\n");
> > +     return -EINVAL;
> > +#else
> > +     WARN_ONCE(1, "unsafe follow_pfn usage\n");
> > +     add_taint(TAINT_USER, LOCKDEP_STILL_OK);
> > +
> > +     return follow_pfn(vma, address, pfn);
> > +#endif
>
> Woudn't this be a pretty good use case of "if (IS_ENABLED(...)))"?
>
> Also I'd expect the inverse polarity of the config option, that is
> a USAFE_FOLLOW_PFN option to enable to unsafe behavior.

Was just about to send out v5, will apply your suggestions for that
using IS_ENABLED.

Wrt negative or positive Kconfig, I was following STRICT_DEVMEM symbol
as precedence. But easy to invert if there's strong feeling the other
way round, I'm not attached to either.

> > +/**
> > + * unsafe_follow_pfn - look up PFN at a user virtual address
> > + * @vma: memory mapping
> > + * @address: user virtual address
> > + * @pfn: location to store found PFN
> > + *
> > + * Only IO mappings and raw PFN mappings are allowed.
> > + *
> > + * Returns zero and the pfn at @pfn on success, -ve otherwise.
> > + */
> > +int unsafe_follow_pfn(struct vm_area_struct *vma, unsigned long address,
> > +     unsigned long *pfn)
> > +{
> > +     return follow_pfn(vma, address, pfn);
> > +}
> > +EXPORT_SYMBOL(unsafe_follow_pfn);
>
> Any reason this doesn't use the warn and disable logic?

I figured without an mmu there's not much guarantees anyway. But I
guess I can put it in here too for consistency.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch