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SubjectRe: [PATCH v3 8/8] x86, mm, gup: prevent get_page() race with munmap in paravirt guest
Vlastimil Babka <> writes:

> On 12/16/19 2:47 PM, Peter Zijlstra wrote:
>> On Mon, Dec 16, 2019 at 02:30:44PM +0100, Vitaly Kuznetsov wrote:
>>> Peter Zijlstra <> writes:
>>>> On Tue, Dec 17, 2019 at 02:15:48AM +0530, Ajay Kaher wrote:
>>>>> From: Vlastimil Babka <>
>>>>> The x86 version of get_user_pages_fast() relies on disabled interrupts to
>>>>> synchronize gup_pte_range() between gup_get_pte(ptep); and get_page() against
>>>>> a parallel munmap. The munmap side nulls the pte, then flushes TLBs, then
>>>>> releases the page. As TLB flush is done synchronously via IPI disabling
>>>>> interrupts blocks the page release, and get_page(), which assumes existing
>>>>> reference on page, is thus safe.
>>>>> However when TLB flush is done by a hypercall, e.g. in a Xen PV guest, there is
>>>>> no blocking thanks to disabled interrupts, and get_page() can succeed on a page
>>>>> that was already freed or even reused.
>>>>> We have recently seen this happen with our 4.4 and 4.12 based kernels, with
>>>>> userspace (java) that exits a thread, where mm_release() performs a futex_wake()
>>>>> on tsk->clear_child_tid, and another thread in parallel unmaps the page where
>>>>> tsk->clear_child_tid points to. The spurious get_page() succeeds, but futex code
>>>>> immediately releases the page again, while it's already on a freelist. Symptoms
>>>>> include a bad page state warning, general protection faults acessing a poisoned
>>>>> list prev/next pointer in the freelist, or free page pcplists of two cpus joined
>>>>> together in a single list. Oscar has also reproduced this scenario, with a
>>>>> patch inserting delays before the get_page() to make the race window larger.
>>>>> Fix this by removing the dependency on TLB flush interrupts the same way as the
>>>> This is suppsed to be fixed by:
>>>> arch/x86/Kconfig: select HAVE_RCU_TABLE_FREE if PARAVIRT
>>> Yes,
> Well, that commit fixes the "page table can be freed under us" part. But
> this patch is about the "get_page() will succeed on a page that's being
> freed" part. Upstream fixed that unknowingly in 4.13 by a gup.c
> refactoring that would be too risky to backport fully.

(I also dislike receiving only this patch of the series, next time
please send the whole thing, it's only 8 patches, our mailfolders will
survive that)

When I was adding Hyper-V PV TLB flush to RHEL7 - which is 3.10 based -
in addition to adding page_cache_get_speculative() to
gup_get_pte()/gup_huge_pmd()/gup_huge_pud() I also had to synchronize
huge PMD split against gup_fast with the following hack:

+static void do_nothing(void *unused)
+static void serialize_against_pte_lookup(struct mm_struct *mm)
+ smp_mb();
+ smp_call_function_many(mm_cpumask(mm), do_nothing, NULL, 1);
void pmdp_splitting_flush(struct vm_area_struct *vma,
unsigned long address, pmd_t *pmdp)
@@ -434,9 +473,10 @@ void pmdp_splitting_flush(struct vm_area_struct *vma,
set = !test_and_set_bit(_PAGE_BIT_SPLITTING,
(unsigned long *)pmdp);
if (set) {
/* need tlb flush only to serialize against gup-fast */
flush_tlb_range(vma, address, address + HPAGE_PMD_SIZE);
+ if (pv_mmu_ops.flush_tlb_others != native_flush_tlb_others)
+ serialize_against_pte_lookup(vma->vm_mm);

I'm not sure which stable kernel you're targeting (and if you addressed this
with other patches in the series, if this is needed,...) so JFYI.

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