Re: [PATCH 4/5 v10] ARM: Initialize the mapping of KASan shadow memory

[Russell King - ARM Linux admin <linux@armlinux.org.uk>]

On Mon, Jun 29, 2020 at 04:07:06PM +0200, Linus Walleij wrote:
> Asking for help here!
> 
> I have a problem with populating PTEs for the LPAE usecase using
> Versatile Express Cortex A15 (TC1) in QEMU.
> 
> In this loop of the patch:
> 
> On Mon, Jun 15, 2020 at 11:05 AM Linus Walleij <linus.walleij@linaro.org> wrote:
> 
> > +static void __init kasan_pte_populate(pmd_t *pmdp, unsigned long addr,
> > +                                     unsigned long end, int node, bool early)
> > +{
> > +       unsigned long next;
> > +       pte_t *ptep = pte_offset_kernel(pmdp, addr);
> 
> (...)
> 
> > +       do {
> > +               next = pmd_addr_end(addr, end);
> > +               kasan_pte_populate(pmdp, addr, next, node, early);
> > +       } while (pmdp++, addr = next, addr != end && pmd_none(READ_ONCE(*pmdp)));
> 
> I first populate the PMD for 0x6ee00000 .. 0x6f000000
> and this works fine, and the PTEs are all initialized.
> pte_offset_kernel() returns something reasonable.
> (0x815F5000).
> 
> Next the kernel processes the PMD for
> 0x6f000000 .. 0x6f200000 and now I run into trouble,
> because pte_offset_kernel() suddenly returns a NULL
> pointer 0x00000000.

That means there is no PTE table allocated which covers 0x6f000000.

"pmdp" points at the previous level's table entry that points at the
pte, and all pte_offset*() does is load that entry, convert it to a
pte_t pointer type, and point it to the appropriate entry for the
address.  So, pte_offset*() is an accessor that takes a pointer to
the preceding level's entry for "addr", and returns a pointer to
the pte_t entry in the last level of page table for "addr".

It is the responsibility of the caller to pte_offset*() to ensure
either by explicit tests, or prior knowledge, that pmd_val(*pmdp)
is a valid PTE table entry.

Since generic kernel code can't use "prior knowledge", it has to do
the full checks (see, mm/vmalloc.c vunmap_pte_range() and higher
levels etc using pmd_none_or_clear_bad() for example - whether you
can use _clear_bad() depends whether you intend to clear "bad" entries.
Beware that the 1MB sections on non-LPAE will appear as "bad" entries
since we can't "walk" them to PTE level, and they're certainly not
"none" entries.)

-- 
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