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

[Andy Lutomirski]

On Fri, Jan 26, 2018 at 10:51 AM, Kirill A. Shutemov
<kirill@xxxxxxxxxxxxx> wrote:
> On Thu, Jan 25, 2018 at 01:12:14PM -0800, 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.
>
> Ouch. Sorry for this.
>
>>
>> Interestingly, the problem doesn't trigger with nopti.  I assume this
>> is because the kernel is mapped with global pages if we boot with
>> nopti.  The sequence of operations when we create a new task is that
>> we first load its mm while still running on the old stack (which
>> crashes if the old stack is unmapped in the new mm unless the TLB
>> saves us), then we call prepare_switch_to(), and then we switch to the
>> new stack.  prepare_switch_to() pokes the new stack directly, which
>> will populate the mapping through vmalloc_fault().  I assume that
>> we're getting lucky on non-PTI systems -- the old stack's TLB entry
>> stays alive long enough to make it all the way through
>> prepare_switch_to() and switch_to() so that we make it to a valid
>> stack.
>>
>> Fixes: b50858ce3e2a ("x86/mm/vmalloc: Add 5-level paging support")
>> Cc: stable@xxxxxxxxxxxxxxx
>> Reported-and-tested-by: Neil Berrington <neil.berrington@xxxxxxxxxxxx>
>> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
>> ---
>>  arch/x86/mm/tlb.c | 34 +++++++++++++++++++++++++++++-----
>>  1 file changed, 29 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c
>> index a1561957dccb..5bfe61a5e8e3 100644
>> --- a/arch/x86/mm/tlb.c
>> +++ b/arch/x86/mm/tlb.c
>> @@ -151,6 +151,34 @@ void switch_mm(struct mm_struct *prev, struct mm_struct *next,
>>       local_irq_restore(flags);
>>  }
>>
>> +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) {
>
> Can we have
>
>         if (PTRS_PER_P4D > 1)
>
> here instead? This way I wouldn't need to touch the code again for
> boot-time switching support.

Want to send a patch?

(Also, I haven't noticed a patch to fix up the SYSRET checking for
boot-time switching.  Have I just missed it?)

--Andy