Re: [PATCH v2 1/2] x86/mm/64: Fix vmapped stack syncing on very-large-memory 4-level systems

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Thu, Jan 25, 2018 at 02:00:22PM -0800, Andy Lutomirski wrote:
> On Thu, Jan 25, 2018 at 1:49 PM, Dave Hansen <dave.hansen@intel.com> wrote:
> > On 01/25/2018 01:12 PM, Andy Lutomirski wrote:
> >> Neil Berrington reported a double-fault on a VM with 768GB of RAM that
> >> uses large amounts of vmalloc space with PTI enabled.
> >>
> >> The cause is that load_new_mm_cr3() was never fixed to take the
> >> 5-level pgd folding code into account, so, on a 4-level kernel, the
> >> pgd synchronization logic compiles away to exactly nothing.
> >
> > You don't mention it, but we can normally handle vmalloc() faults in the
> > kernel that are due to unsynchronized page tables.  The thing that kills
> > us here is that we have an unmapped stack and we try to use that stack
> > when entering the page fault handler, which double faults.  The double
> > fault handler gets a new stack and saves us enough to get an oops out.
> >
> > Right?
> 
> Exactly.
> 
> There are two special code paths that can't use vmalloc_fault(): this
> one and switch_to().  The latter avoids explicit page table fiddling
> and just touches the new stack before loading it into rsp.
> 
> >
> >> +static void sync_current_stack_to_mm(struct mm_struct *mm)
> >> +{
> >> +     unsigned long sp = current_stack_pointer;
> >> +     pgd_t *pgd = pgd_offset(mm, sp);
> >> +
> >> +     if (CONFIG_PGTABLE_LEVELS > 4) {
> >> +             if (unlikely(pgd_none(*pgd))) {
> >> +                     pgd_t *pgd_ref = pgd_offset_k(sp);
> >> +
> >> +                     set_pgd(pgd, *pgd_ref);
> >> +             }
> >> +     } else {
> >> +             /*
> >> +              * "pgd" is faked.  The top level entries are "p4d"s, so sync
> >> +              * the p4d.  This compiles to approximately the same code as
> >> +              * the 5-level case.
> >> +              */
> >> +             p4d_t *p4d = p4d_offset(pgd, sp);
> >> +
> >> +             if (unlikely(p4d_none(*p4d))) {
> >> +                     pgd_t *pgd_ref = pgd_offset_k(sp);
> >> +                     p4d_t *p4d_ref = p4d_offset(pgd_ref, sp);
> >> +
> >> +                     set_p4d(p4d, *p4d_ref);
> >> +             }
> >> +     }
> >> +}
> >
> > We keep having to add these.  It seems like a real deficiency in the
> > mechanism that we're using for pgd folding.  Can't we get a warning or
> > something when we try to do a set_pgd() that's (silently) not doing
> > anything?  This exact same pattern bit me more than once with the
> > KPTI/KAISER patches.
> 
> Hmm, maybe.
> 
> What I'd really like to see is an entirely different API.  Maybe:
> 
> typedef struct {
>   opaque, but probably includes:
>   int depth;  /* 0 is root */
>   void *table;
> } ptbl_ptr;
> 
> ptbl_ptr root_table = mm_root_ptbl(mm);
> 
> set_ptbl_entry(root_table, pa, prot);
> 
> /* walk tables */
> ptbl_ptr pt = ...;
> ptentry_ptr entry;
> while (ptbl_has_children(pt)) {
>   pt = pt_next(pt, addr);
> }
> entry = pt_entry_at(pt, addr);
> /* do something with entry */
> 
> etc.

I thought about very similar design, but never got time to try it really.
It's not one-week-end type of project :/

-- 
 Kirill A. Shutemov