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

[Vlastimil Babka <vbabka@suse.cz>]

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 <peterz@infradead.org> writes:
>>
>>> On Tue, Dec 17, 2019 at 02:15:48AM +0530, Ajay Kaher wrote:
>>>> From: Vlastimil Babka <vbabka@suse.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
>>>
>>
>> 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.

>> but HAVE_RCU_TABLE_FREE was enabled on x86 only in 4.14:
>>
>> commit 9e52fc2b50de3a1c08b44f94c610fbe998c0031a
>> Author: Vitaly Kuznetsov <vkuznets@redhat.com>
>> 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).
> 
> It wasn't at all clear this was targeted at old kernels (I only got this
> one patch).
> 
> And why can't those necro kernels do backports of the upstream solution?
>