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


but HAVE_RCU_TABLE_FREE was enabled on x86 only in 4.14:

commit 9e52fc2b50de3a1c08b44f94c610fbe998c0031a
Author: Vitaly Kuznetsov <>
Date: Mon Aug 28 10:22:51 2017 +0200

x86/mm: Enable RCU based page table freeing (CONFIG_HAVE_RCU_TABLE_FREE=y)

and, if I understood correctly, Ajay is suggesting the patch for older
stable kernels (4.9 and 4.4 I would guess).


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