[PATCH -next v4 3/4] arm64: mm: add support for page table check

[Pasha Tatashin]

On Tue, Apr 19, 2022 at 6:22 AM Anshuman Khandual
<anshuman.khandual at arm.com> wrote:
>
>
> On 4/18/22 09:14, Tong Tiangen wrote:
> > +#ifdef CONFIG_PAGE_TABLE_CHECK
> > +static inline bool pte_user_accessible_page(pte_t pte)
> > +{
> > +     return pte_present(pte) && (pte_user(pte) || pte_user_exec(pte));
> > +}
> > +
> > +static inline bool pmd_user_accessible_page(pmd_t pmd)
> > +{
> > +     return pmd_present(pmd) && (pmd_user(pmd) || pmd_user_exec(pmd));
> > +}
> > +
> > +static inline bool pud_user_accessible_page(pud_t pud)
> > +{
> > +     return pud_present(pud) && pud_user(pud);
> > +}
> > +#endif
> Wondering why check for these page table entry states when init_mm
> has already being excluded ? Should not user page tables be checked
> for in entirety for all updates ? what is the rationale for filtering
> out only pxx_user_access_page entries ?

The point is to prevent false sharing and memory corruption issues.
The idea of PTC to be simple and relatively independent  from the MM
state machine that catches invalid page sharing. I.e. if an R/W anon
page is accessible by user land, that page can never be mapped into
another process (internally shared anons are treated as named
mappings).

Therefore, we try not to rely on MM states, and ensure that when a
page-table entry is accessible by user it meets the required
assumptions: no false sharing, etc.

For example, one bug that was caught with PTC was where a driver on an
unload would put memory on a freelist but memory is still mapped in
user page table.

Pasha