Re: [PATCH v1 1/4] kasan: introduce set_pmd_early_shadow()

[JÃrgen GroÃ]

On 15.01.20 11:54, Sergey Dyasli wrote:
> Hi Juergen,
>
> On 08/01/2020 15:20, Sergey Dyasli wrote:
> > It is incorrect to call pmd_populate_kernel() multiple times for the
> > same page table. Xen notices it during kasan_populate_early_shadow():
> >
> >      (XEN) mm.c:3222:d155v0 mfn 3704b already pinned
> >
> > This happens for kasan_early_shadow_pte when USE_SPLIT_PTE_PTLOCKS is
> > enabled. Fix this by introducing set_pmd_early_shadow() which calls
> > pmd_populate_kernel() only once and uses set_pmd() afterwards.
> >
> > Signed-off-by: Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>
>
> Looks like the plan to use set_pmd() directly has failed: it's an
> arch-specific function and can't be used in arch-independent code
> (as kbuild test robot has proven).
>
> Do you see any way out of this other than disabling SPLIT_PTE_PTLOCKS
> for PV KASAN?

Change set_pmd_early_shadow() like the following:

#ifdef CONFIG_XEN_PV
static inline void set_pmd_early_shadow(pmd_t *pmd, pte_t *early_shadow)
{
	static bool pmd_populated = false;

	if (likely(pmd_populated)) {
		set_pmd(pmd, __pmd(__pa(early_shadow) | _PAGE_TABLE));
	} else {
		pmd_populate_kernel(&init_mm, pmd, early_shadow);
		pmd_populated = true;
	}
}
#else
static inline void set_pmd_early_shadow(pmd_t *pmd, pte_t *early_shadow)
{
	pmd_populate_kernel(&init_mm, pmd, early_shadow);
}
#endif

... and move it to include/xen/xen-ops.h and call it with
lm_alias(kasan_early_shadow_pte) as the second parameter.


Juergen