Re: Potential race in TLB flush batching?

[Nadav Amit <nadav.amit@gmail.com>]

Mel Gorman <mgorman@suse.de> wrote:

> On Tue, Jul 11, 2017 at 03:40:02AM -0700, Nadav Amit wrote:
>> Mel Gorman <mgorman@suse.de> wrote:
>> 
>>>>> That is the same to a race whereby there is no batching mechanism and the
>>>>> racing operation happens between a pte clear and a flush as ptep_clear_flush
>>>>> is not atomic. All that differs is that the race window is a different size.
>>>>> The application on CPU1 is buggy in that it may or may not succeed the write
>>>>> but it is buggy regardless of whether a batching mechanism is used or not.
>>>> 
>>>> Thanks for your quick and detailed response, but I fail to see how it can
>>>> happen without batching. Indeed, the PTE clear and flush are not ???atomic???,
>>>> but without batching they are both performed under the page table lock
>>>> (which is acquired in page_vma_mapped_walk and released in
>>>> page_vma_mapped_walk_done). Since the lock is taken, other cores should not
>>>> be able to inspect/modify the PTE. Relevant functions, e.g., zap_pte_range
>>>> and change_pte_range, acquire the lock before accessing the PTEs.
>>> 
>>> I was primarily thinking in terms of memory corruption or data loss.
>>> However, we are still protected although it's not particularly obvious why.
>>> 
>>> On the reclaim side, we are either reclaiming clean pages (which ignore
>>> the accessed bit) or normal reclaim. If it's clean pages then any parallel
>>> write must update the dirty bit at minimum. If it's normal reclaim then
>>> the accessed bit is checked and if cleared in try_to_unmap_one, it uses a
>>> ptep_clear_flush_young_notify so the TLB gets flushed. We don't reclaim
>>> the page in either as part of page_referenced or try_to_unmap_one but
>>> clearing the accessed bit flushes the TLB.
>> 
>> Wait. Are you looking at the x86 arch function? The TLB is not flushed when
>> the access bit is cleared:
>> 
>> int ptep_clear_flush_young(struct vm_area_struct *vma,
>>                           unsigned long address, pte_t *ptep)
>> {
>>        /*
>>         * On x86 CPUs, clearing the accessed bit without a TLB flush
>>         * doesn't cause data corruption. [ It could cause incorrect
>>         * page aging and the (mistaken) reclaim of hot pages, but the
>>         * chance of that should be relatively low. ]
>>         *                 
>>         * So as a performance optimization don't flush the TLB when
>>         * clearing the accessed bit, it will eventually be flushed by
>>         * a context switch or a VM operation anyway. [ In the rare
>>         * event of it not getting flushed for a long time the delay
>>         * shouldn't really matter because there's no real memory
>>         * pressure for swapout to react to. ]
>>         */
>>        return ptep_test_and_clear_young(vma, address, ptep);
>> }
> 
> I forgot this detail, thanks for correcting me.
> 
>>> On the mprotect side then, as the page was first accessed, clearing the
>>> accessed bit incurs a TLB flush on the reclaim side before the second write.
>>> That means any TLB entry that exists cannot have the accessed bit set so
>>> a second write needs to update it.
>>> 
>>> While it's not clearly documented, I checked with hardware engineers
>>> at the time that an update of the accessed or dirty bit even with a TLB
>>> entry will check the underlying page tables and trap if it's not present
>>> and the subsequent fault will then fail on sigsegv if the VMA protections
>>> no longer allow the write.
>>> 
>>> So, on one side if ignoring the accessed bit during reclaim, the pages
>>> are clean so any access will set the dirty bit and trap if unmapped in
>>> parallel. On the other side, the accessed bit if set cleared the TLB and
>>> if not set, then the hardware needs to update and again will trap if
>>> unmapped in parallel.
>> 
>> 
>> Yet, even regardless to the TLB flush it seems there is still a possible
>> race:
>> 
>> CPU0				CPU1
>> ----				----
>> ptep_clear_flush_young_notify
>> ==> PTE.A==0
>> 				access PTE
>> 				==> PTE.A=1
>> prep_get_and_clear
>> 				change mapping (and PTE)
>> 				Use stale TLB entry
> 
> So I think you're right and this is a potential race. The first access can
> be a read or a write as it's a problem if the mprotect call restricts
> access.
> 
>>> If this guarantee from hardware was every shown to be wrong or another
>>> architecture wanted to add batching without the same guarantee then mprotect
>>> would need to do a local_flush_tlb if no pages were updated by the mprotect
>>> but right now, this should not be necessary.
>>> 
>>>> Can you please explain why you consider the application to be buggy?
>>> 
>>> I considered it a bit dumb to mprotect for READ/NONE and then try writing
>>> the same mapping. However, it will behave as expected.
>> 
>> I don???t think that this is the only scenario. For example, the application
>> may create a new memory mapping of a different file using mmap at the same
>> memory address that was used before, just as that memory is reclaimed.
> 
> That requires the existing mapping to be unmapped which will flush the
> TLB and parallel mmap/munmap serialises on mmap_sem. The race appears to
> be specific to mprotect which avoids the TLB flush if no pages were updated.

Why? As far as I see the chain of calls during munmap is somewhat like:

do_munmap
=>unmap_region
==>tlb_gather_mmu
===>unmap_vmas
====>unmap_page_range
...
=====>zap_pte_range 	- this one batches only present PTEs
===>free_pgtables	- this one is only if page-tables are removed
===>pte_free_tlb
==>tlb_finish_mmu
===>tlb_flush_mmu
====>tlb_flush_mmu_tlbonly

zap_pte_range will check if pte_none and can find it is - if a concurrent
try_to_unmap_one already cleared the PTE. In this case it will not update
the range of the mmu_gather and would not indicate that a flush of the PTE
is needed. Then, tlb_flush_mmu_tlbonly will find that no PTE was cleared
(tlb->end == 0) and avoid flush, or may just flush fewer PTEs than actually
needed.

Due to this behavior, it raises a concern that in other cases as well, when
mmu_gather is used, a PTE flush may be missed.

>> The
>> application can (inadvertently) cause such a scenario by using MAP_FIXED.
>> But even without MAP_FIXED, running mmap->munmap->mmap can reuse the same
>> virtual address.
> 
> With flushes in between.
> 
>>> Such applications are safe due to how the accessed bit is handled by the
>>> software (flushes TLB if clearing young) and hardware (traps if updating
>>> the accessed or dirty bit and the underlying PTE was unmapped even if
>>> there is a TLB entry).
>> 
>> I don???t think it is so. And I also think there are many additional
>> potentially problematic scenarios.
> 
> I believe it's specific to mprotect but can be handled by flushing the
> local TLB when mprotect updates no pages. Something like this;
> 
> ---8<---
> mm, mprotect: Flush the local TLB if mprotect potentially raced with a parallel reclaim
> 
> Nadav Amit identified a theoritical race between page reclaim and mprotect
> due to TLB flushes being batched outside of the PTL being held. He described
> the race as follows
> 
>        CPU0                            CPU1
>        ----                            ----
>                                        user accesses memory using RW PTE
>                                        [PTE now cached in TLB]
>        try_to_unmap_one()
>        ==> ptep_get_and_clear()
>        ==> set_tlb_ubc_flush_pending()
>                                        mprotect(addr, PROT_READ)
>                                        ==> change_pte_range()
>                                        ==> [ PTE non-present - no flush ]
> 
>                                        user writes using cached RW PTE
>        ...
> 
>        try_to_unmap_flush()
> 
> The same type of race exists for reads when protecting for PROT_NONE.
> This is not a data integrity issue as the TLB is always flushed before any
> IO is queued or a page is freed but it is a correctness issue as a process
> restricting access with mprotect() may still be able to access the data
> after the syscall returns due to a stale TLB entry. Handle this issue by
> flushing the local TLB if reclaim is potentially batching TLB flushes and
> mprotect altered no pages.
> 
> Signed-off-by: Mel Gorman <mgorman@suse.de>
> Cc: stable@vger.kernel.org # v4.4+
> ---
> mm/internal.h |  5 ++++-
> mm/mprotect.c | 12 ++++++++++--
> mm/rmap.c     | 20 ++++++++++++++++++++
> 3 files changed, 34 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/internal.h b/mm/internal.h
> index 0e4f558412fb..9b7d1a597816 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -498,6 +498,7 @@ extern struct workqueue_struct *mm_percpu_wq;
> #ifdef CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
> void try_to_unmap_flush(void);
> void try_to_unmap_flush_dirty(void);
> +void batched_unmap_protection_update(void);
> #else
> static inline void try_to_unmap_flush(void)
> {
> @@ -505,7 +506,9 @@ static inline void try_to_unmap_flush(void)
> static inline void try_to_unmap_flush_dirty(void)
> {
> }
> -
> +static inline void batched_unmap_protection_update()
> +{
> +}
> #endif /* CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH */
> 
> extern const struct trace_print_flags pageflag_names[];
> diff --git a/mm/mprotect.c b/mm/mprotect.c
> index 8edd0d576254..3de353d4b5fb 100644
> --- a/mm/mprotect.c
> +++ b/mm/mprotect.c
> @@ -254,9 +254,17 @@ static unsigned long change_protection_range(struct vm_area_struct *vma,
> 				 dirty_accountable, prot_numa);
> 	} while (pgd++, addr = next, addr != end);
> 
> -	/* Only flush the TLB if we actually modified any entries: */
> -	if (pages)
> +	/*
> +	 * Only flush all TLBs if we actually modified any entries. If no
> +	 * pages are modified, then call batched_unmap_protection_update
> +	 * if the context is a mprotect() syscall.
> +	 */
> +	if (pages) {
> 		flush_tlb_range(vma, start, end);
> +	} else {
> +		if (!prot_numa)
> +			batched_unmap_protection_update();
> +	}
> 	clear_tlb_flush_pending(mm);
> 
> 	return pages;
> diff --git a/mm/rmap.c b/mm/rmap.c
> index d405f0e0ee96..02cb035e4ce6 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -643,6 +643,26 @@ static bool should_defer_flush(struct mm_struct *mm, enum ttu_flags flags)
> 
> 	return should_defer;
> }
> +
> +/*
> + * This is called after an mprotect update that altered no pages. Batched
> + * unmap releases the PTL before a flush occurs leaving a window where
> + * an mprotect that reduces access rights can still access the page after
> + * mprotect returns via a stale TLB entry. Avoid this possibility by flushing
> + * the local TLB if mprotect updates no pages so that the the caller of
> + * mprotect always gets expected behaviour. It's overkill and unnecessary to
> + * flush all TLBs as a separate thread accessing the data that raced with
> + * both reclaim and mprotect as there is no risk of data corruption and
> + * the exact timing of a parallel thread seeing a protection update without
> + * any serialisation on the application side is always uncertain.
> + */
> +void batched_unmap_protection_update(void)
> +{
> +	count_vm_tlb_event(NR_TLB_LOCAL_FLUSH_ALL);
> +	local_flush_tlb();
> +	trace_tlb_flush(TLB_LOCAL_SHOOTDOWN, TLB_FLUSH_ALL);
> +}
> +
> #else
> static void set_tlb_ubc_flush_pending(struct mm_struct *mm, bool writable)
> {

I don’t think this solution is enough. I am sorry for not providing a
solution, but I don’t see an easy one.

Thanks,
Nadav

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>