Subject: Re: [PATCH] vmap(): don't allow invalid pages

From: "Russell King (Oracle)" <[email-masked]>
Date: Wed, 19 Jan 2022 18:57:34 +0000

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.

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.

To repeat: once you have a struct page, it's too late to determine if
the struct page is valid.

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