linux-kernel - Re: [PATCH v3 1/3] mm/migrate_device.c: Flush TLB while holding PTL

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <6e31290a-5b72-ceb6-b11c-f8c1b066beaf@redhat.com>
Date:   Wed, 24 Aug 2022 14:35:51 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Alistair Popple <apopple@...dia.com>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        Peter Xu <peterx@...hat.com>,
        Nadav Amit <nadav.amit@...il.com>,
        huang ying <huang.ying.caritas@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Sierra Guiza, Alejandro (Alex)" <alex.sierra@....com>,
        Felix Kuehling <Felix.Kuehling@....com>,
        Jason Gunthorpe <jgg@...dia.com>,
        John Hubbard <jhubbard@...dia.com>,
        Ralph Campbell <rcampbell@...dia.com>,
        Matthew Wilcox <willy@...radead.org>,
        Karol Herbst <kherbst@...hat.com>,
        Lyude Paul <lyude@...hat.com>, Ben Skeggs <bskeggs@...hat.com>,
        Logan Gunthorpe <logang@...tatee.com>, paulus@...abs.org,
        linuxppc-dev@...ts.ozlabs.org, stable@...r.kernel.org
Subject: Re: [PATCH v3 1/3] mm/migrate_device.c: Flush TLB while holding PTL

On 24.08.22 14:26, Alistair Popple wrote:
> 
> David Hildenbrand <david@...hat.com> writes:
> 
>> On 24.08.22 05:03, Alistair Popple wrote:
>>> When clearing a PTE the TLB should be flushed whilst still holding the
>>> PTL to avoid a potential race with madvise/munmap/etc. For example
>>> consider the following sequence:
>>>
>>>   CPU0                          CPU1
>>>   ----                          ----
>>>
>>>   migrate_vma_collect_pmd()
>>>   pte_unmap_unlock()
>>>                                 madvise(MADV_DONTNEED)
>>>                                 -> zap_pte_range()
>>>                                 pte_offset_map_lock()
>>>                                 [ PTE not present, TLB not flushed ]
>>>                                 pte_unmap_unlock()
>>>                                 [ page is still accessible via stale TLB ]
>>>   flush_tlb_range()
>>>
>>> In this case the page may still be accessed via the stale TLB entry
>>> after madvise returns. Fix this by flushing the TLB while holding the
>>> PTL.
>>>
>>> Signed-off-by: Alistair Popple <apopple@...dia.com>
>>> Reported-by: Nadav Amit <nadav.amit@...il.com>
>>> Fixes: 8c3328f1f36a ("mm/migrate: migrate_vma() unmap page from vma while collecting pages")
>>> Cc: stable@...r.kernel.org
>>>
>>> ---
>>>
>>> Changes for v3:
>>>
>>>  - New for v3
>>> ---
>>>  mm/migrate_device.c | 5 +++--
>>>  1 file changed, 3 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/mm/migrate_device.c b/mm/migrate_device.c
>>> index 27fb37d..6a5ef9f 100644
>>> --- a/mm/migrate_device.c
>>> +++ b/mm/migrate_device.c
>>> @@ -254,13 +254,14 @@ static int migrate_vma_collect_pmd(pmd_t *pmdp,
>>>  		migrate->dst[migrate->npages] = 0;
>>>  		migrate->src[migrate->npages++] = mpfn;
>>>  	}
>>> -	arch_leave_lazy_mmu_mode();
>>> -	pte_unmap_unlock(ptep - 1, ptl);
>>>
>>>  	/* Only flush the TLB if we actually modified any entries */
>>>  	if (unmapped)
>>>  		flush_tlb_range(walk->vma, start, end);
>>>
>>> +	arch_leave_lazy_mmu_mode();
>>> +	pte_unmap_unlock(ptep - 1, ptl);
>>> +
>>>  	return 0;
>>>  }
>>>
>>>
>>> base-commit: ffcf9c5700e49c0aee42dcba9a12ba21338e8136
>>
>> I'm not a TLB-flushing expert, but this matches my understanding (and a
>> TLB flushing Linux documentation I stumbled over some while ago but
>> cannot quickly find).
>>
>> In the ordinary try_to_migrate_one() path, flushing would happen via
>> ptep_clear_flush() (just like we do for the anon_exclusive case here as
>> well), correct?
> 
> Correct.
> 

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Thanks,

David / dhildenb