Re: TLB and PTE coherency during munmap

[Max Filippov <jcmvbkbc@gmail.com>]

On Fri, May 31, 2013 at 5:26 AM, Max Filippov <jcmvbkbc@gmail.com> wrote:
> On Wed, May 29, 2013 at 2:15 PM, Catalin Marinas
> <catalin.marinas@arm.com> wrote:
>> On Wed, May 29, 2013 at 05:15:28AM +0100, Max Filippov wrote:
>>> On Tue, May 28, 2013 at 6:35 PM, Catalin Marinas <catalin.marinas@arm.com> wrote:
>>> > On 26 May 2013 03:42, Max Filippov <jcmvbkbc@gmail.com> wrote:
>>> >> Is it intentional that threads of a process that invoked munmap syscall
>>> >> can see TLB entries pointing to already freed pages, or it is a bug?
>>> >
>>> > If it happens, this would be a bug. It means that a process can access
>>> > a physical page that has been allocated to something else, possibly
>>> > kernel data.
>>> >
>>> >> I'm talking about zap_pmd_range and zap_pte_range:
>>> >>
>>> >>       zap_pmd_range
>>> >>         zap_pte_range
>>> >>           arch_enter_lazy_mmu_mode
>>> >>             ptep_get_and_clear_full
>>> >>             tlb_remove_tlb_entry
>>> >>             __tlb_remove_page
>>> >>           arch_leave_lazy_mmu_mode
>>> >>         cond_resched
>>> >>
>>> >> With the default arch_{enter,leave}_lazy_mmu_mode, tlb_remove_tlb_entry
>>> >> and __tlb_remove_page there is a loop in the zap_pte_range that clears
>>> >> PTEs and frees corresponding pages, but doesn't flush TLB, and
>>> >> surrounding loop in the zap_pmd_range that calls cond_resched. If a thread
>>> >> of the same process gets scheduled then it is able to see TLB entries
>>> >> pointing to already freed physical pages.
>>> >
>>> > It looks to me like cond_resched() here introduces a possible bug but
>>> > it depends on the actual arch code, especially the
>>> > __tlb_remove_tlb_entry() function. On ARM we record the range in
>>> > tlb_remove_tlb_entry() and queue the pages to be removed in
>>> > __tlb_remove_page(). It pretty much acts like tlb_fast_mode() == 0
>>> > even for the UP case (which is also needed for hardware speculative
>>> > TLB loads). The tlb_finish_mmu() takes care of whatever pages are left
>>> > to be freed.
>>> >
>>> > With a dummy __tlb_remove_tlb_entry() and tlb_fast_mode() == 1,
>>> > cond_resched() in zap_pmd_range() would cause problems.
>>>
>>> So, looks like most architectures in the UP configuration should have
>>> this issue (unless they flush TLB in the switch_mm, even when switching
>>> to the same mm):
>>
>> switch_mm() wouldn't be called if switching to the same mm. You could do
>
> Hmm... Strange, but as far as I can tell from the context_switch it would.
>
>> it in switch_to() but it's not efficient (or before returning to user
>> space on the same processor).
>>
>> Do you happen to have a user-space test for this? Something like one
>
> I only had mtest05 from LTP that triggered TLB/PTE inconsistency, but
> not anything that would really try to peek at the freed page. I can make
> such test though.
>
>> thread does an mmap(), writes some poison value, munmap(). The other
>> thread keeps checking the poison value while trapping and ignoring any
>> SIGSEGV. If it's working correctly, the second thread should either get
>> a SIGSEGV or read the poison value.

I've made a number of such tests and had them running for a couple of
days. Checking thread never read anything other than poison value so far.

-- 
Thanks.
-- Max

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