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

[Andy Lutomirski <luto@kernel.org>]

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.

Now someone can add a sixth level without changing every code path in
the kernel that touches page tables.

--Andy