Re: [RFC PATCH 2/6] x86/mm: temporary mm struct

[Nadav Amit]

at 8:41 AM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:

> On Wed, Aug 29, 2018 at 2:49 AM, Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
>> On Wed, 29 Aug 2018 01:11:43 -0700
>> Nadav Amit <namit@xxxxxxxxxx> wrote:
>> 
>>> From: Andy Lutomirski <luto@xxxxxxxxxx>
>>> 
>>> Sometimes we want to set a temporary page-table entries (PTEs) in one of
>>> the cores, without allowing other cores to use - even speculatively -
>>> these mappings. There are two benefits for doing so:
>>> 
>>> (1) Security: if sensitive PTEs are set, temporary mm prevents their use
>>> in other cores. This hardens the security as it prevents exploding a
>>> dangling pointer to overwrite sensitive data using the sensitive PTE.
>>> 
>>> (2) Avoiding TLB shootdowns: the PTEs do not need to be flushed in
>>> remote page-tables.
>>> 
>>> To do so a temporary mm_struct can be used. Mappings which are private
>>> for this mm can be set in the userspace part of the address-space.
>>> During the whole time in which the temporary mm is loaded, interrupts
>>> must be disabled.
>>> 
>>> The first use-case for temporary PTEs, which will follow, is for poking
>>> the kernel text.
>>> 
>>> [ Commit message was written by Nadav ]
>>> 
>>> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
>>> Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
>>> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
>>> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>>> Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
>>> ---
>>> arch/x86/include/asm/mmu_context.h | 20 ++++++++++++++++++++
>>> 1 file changed, 20 insertions(+)
>>> 
>>> diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
>>> index eeeb9289c764..96afc8c0cf15 100644
>>> --- a/arch/x86/include/asm/mmu_context.h
>>> +++ b/arch/x86/include/asm/mmu_context.h
>>> @@ -338,4 +338,24 @@ static inline unsigned long __get_current_cr3_fast(void)
>>>      return cr3;
>>> }
>>> 
>>> +typedef struct {
>>> +     struct mm_struct *prev;
>>> +} temporary_mm_state_t;
>>> +
>>> +static inline temporary_mm_state_t use_temporary_mm(struct mm_struct *mm)
>>> +{
>>> +     temporary_mm_state_t state;
>>> +
>>> +     lockdep_assert_irqs_disabled();
>>> +     state.prev = this_cpu_read(cpu_tlbstate.loaded_mm);
>>> +     switch_mm_irqs_off(NULL, mm, current);
>>> +     return state;
>>> +}
>> 
>> Hmm, why don't we return mm_struct *prev directly?
> 
> I did it this way to make it easier to add future debugging stuff
> later.  Also, when I first wrote this, I stashed the old CR3 instead
> of the old mm_struct, and it seemed like callers should be insulated
> from details like this.

Andy, please let me know if you want me to change it somehow, and please
provide your signed-off-by.