On Thu, Jan 25, 2018 at 1:49 PM, Dave Hansen <dave.hansen@xxxxxxxxx> 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