Re: [PATCH] arm: flush: don't abuse pfn_valid() to check if pfn is in RAM

[Robin Murphy]

On 2024-01-31 7:00 pm, Russell King (Oracle) wrote:
> On Wed, Jan 31, 2024 at 06:39:31PM +0000, Robin Murphy wrote:
> > On 31/01/2024 12:59 pm, Yongqiang Liu wrote:
> > > @@ -292,7 +293,7 @@ void __sync_icache_dcache(pte_t pteval)
> > >    		/* only flush non-aliasing VIPT caches for exec mappings */
> > >    		return;
> > >    	pfn = pte_pfn(pteval);
> > > -	if (!pfn_valid(pfn))
> > > +	if (!memblock_is_map_memory(PFN_PHYS(pfn)))
> > >    		return;
> > >    	folio = page_folio(pfn_to_page(pfn));
> >
> > Hmm, it's a bit odd in context, since pfn_valid() obviously pairs with this
> > pfn_to_page(), whereas it's not necessarily clear that
> > memblock_is_map_memory() implies pfn_valid().
> >
> > However, in this case we're starting from a PTE - rather than going off to
> > do a slow scan of memblock to determine whether a round-trip through
> > page_address() is going to give back a mapped VA, can we not trivially
> > identify that from whether the PTE itself is valid?
>
> Depends what you mean by "valid". If you're referring to pte_valid()
> and L_PTE_VALID then no.
>
> On 32-bit non-LPAE, the valid bit is the same as the present bit, and
> needs to be set for the PTE to not fault. Any PTE that is mapping
> something will be "valid" whether it is memory or not, whether it is
> backed by a page or not.
>
> pfn_valid() should be telling us whether the PFN is suitable to be
> passed to pfn_to_page(), and if we have a situation where pfn_valid()
> returns true, but pfn_to_page() returns an invalid page, then that in
> itself is a bug that needs to be fixed and probably has far reaching
> implications for the stability of the kernel.

Right, the problem here seems to be the opposite one, wherein we *do* 
often have a valid struct page for an address which is reserved and thus 
not mapped by the kernel, but seemingly we then take it down a path 
which assumes anything !PageHighmem() is lowmem and dereferences 
page_address() without looking.

However I realise I should have looked closer at the caller, and my idea 
is futile since the PTE here is for a userspace mapping, not a kernel 
VA, and is already pte_valid_user() && !pte_special(). Plus the fact 
that the stack trace indicates an mmap() path suggests it most likely is 
a legitimate mapping of some no-map carveout or MMIO region. Oh well. My 
first point still stands, though - I think at least a comment to clarify 
that assumption would be warranted.

Thanks,
Robin.