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Date:   Wed, 19 Jan 2022 19:35:33 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     "Russell King (Oracle)" <linux@armlinux.org.uk>
Cc:     Robin Murphy <robin.murphy@arm.com>,
        Yury Norov <yury.norov@gmail.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Nicholas Piggin <npiggin@gmail.com>,
        Ding Tianhong <dingtianhong@huawei.com>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Alexey Klimov <aklimov@redhat.com>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org
Subject: Re: [PATCH] vmap(): don't allow invalid pages
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 <YehfHsuMzvy//Szi@shell.armlinux.org.uk>
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On Wed, Jan 19, 2022 at 06:57:34PM +0000, Russell King (Oracle) wrote:
> On Wed, Jan 19, 2022 at 06:01:24PM +0000, Matthew Wilcox wrote:
> > On Wed, Jan 19, 2022 at 05:54:15PM +0000, Russell King (Oracle) wrote:
> > > On Wed, Jan 19, 2022 at 04:27:32PM +0000, Matthew Wilcox wrote:
> > > > On Wed, Jan 19, 2022 at 01:28:14PM +0000, Robin Murphy wrote:
> > > > > > +		if (WARN_ON(!pfn_valid(page_to_pfn(page))))
> > > > > 
> > > > > Is it page_to_pfn() guaranteed to work without blowing up if page is invalid
> > > > > in the first place? Looking at the CONFIG_SPARSEMEM case I'm not sure that's
> > > > > true...
> > > > 
> > > > Even if it does blow up, at least it's blowing up here where someone
> > > > can start to debug it, rather than blowing up on first access, where
> > > > we no longer have the invlid struct page pointer.
> > > > 
> > > > I don't think we have a 'page_valid' function which will tell us whether
> > > > a random pointer is actually a struct page or not.
> > > 
> > > Isn't it supposed to be:
> > > 
> > > 	if (!pfn_valid(pfn)) {
> > > 		handle invalid pfn;
> > > 	}
> > > 
> > > 	page = pfn_to_page(pfn);
> > > 
> > > Anything else - even trying to convert an invalid page back to a pfn,
> > > could well be unreliable (sparsemem or discontigmem). 
> > 
> > This function is passed an array of pages.  We have no way of doing
> > what you propose.
> 
> You can't go from a struct page to "this is valid", it's too late by the
> time you call vmap() - that's my fundamental point.

Yes, and we have debugging code in __virt_to_phys() that would have
caught this, had Yury enabled CONFIG_DEBUG_VIRTUAL.  My point is that
in this instance, page_to_pfn() doesn't crash, which lets vmap() set
up a mapping to a completely bogus physical address.  We're better
off checking pfn_valid() here than not.

> If the translation from a PFN to a struct page can return pointers to
> something that isn't a valid struct page, then it can also (with
> sparsemem) return a pointer to _another_ struct page that could well
> be valid depending on how the struct page arrays are laid out in
> memory.

Sure, it's not going to catch everything.  But I don't think that
letting perfect be the enemy of the good here is the right approach.