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

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Date:   Mon, 16 Dec 2019 14:04:43 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ajay Kaher <akaher@...are.com>
Cc:     gregkh@...uxfoundation.org, stable@...r.kernel.org,
        torvalds@...ux-foundation.org, punit.agrawal@....com,
        akpm@...ux-foundation.org, kirill.shutemov@...ux.intel.com,
        willy@...radead.org, will.deacon@....com, mszeredi@...hat.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        srivatsab@...are.com, srivatsa@...il.mit.edu, amakhalov@...are.com,
        srinidhir@...are.com, bvikas@...are.com, anishs@...are.com,
        vsirnapalli@...are.com, srostedt@...are.com,
        Vlastimil Babka <vbabka@...e.cz>,
        Oscar Salvador <osalvador@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH v3 8/8] x86, mm, gup: prevent get_page() race with munmap
 in paravirt guest

On Tue, Dec 17, 2019 at 02:15:48AM +0530, Ajay Kaher wrote:
> From: Vlastimil Babka <vbabka@...e.cz>
> 
> 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