Re: [PATCH 5.4 v2] mm/hugetlb: fix races when looking up a CONT-PTE/PMD size hugetlb page

[Greg KH <greg@kroah.com>]

On Tue, Dec 13, 2022 at 06:06:09PM -0800, Samuel Mendoza-Jonas wrote:
> From: Baolin Wang <baolin.wang@linux.alibaba.com>
> 
> [ Upstream commit fac35ba763ed07ba93154c95ffc0c4a55023707f ]
> 
> On some architectures (like ARM64), it can support CONT-PTE/PMD size
> hugetlb, which means it can support not only PMD/PUD size hugetlb (2M and
> 1G), but also CONT-PTE/PMD size(64K and 32M) if a 4K page size specified.
> 
> So when looking up a CONT-PTE size hugetlb page by follow_page(), it will
> use pte_offset_map_lock() to get the pte entry lock for the CONT-PTE size
> hugetlb in follow_page_pte().  However this pte entry lock is incorrect
> for the CONT-PTE size hugetlb, since we should use huge_pte_lock() to get
> the correct lock, which is mm->page_table_lock.
> 
> That means the pte entry of the CONT-PTE size hugetlb under current pte
> lock is unstable in follow_page_pte(), we can continue to migrate or
> poison the pte entry of the CONT-PTE size hugetlb, which can cause some
> potential race issues, even though they are under the 'pte lock'.
> 
> For example, suppose thread A is trying to look up a CONT-PTE size hugetlb
> page by move_pages() syscall under the lock, however antoher thread B can
> migrate the CONT-PTE hugetlb page at the same time, which will cause
> thread A to get an incorrect page, if thread A also wants to do page
> migration, then data inconsistency error occurs.
> 
> Moreover we have the same issue for CONT-PMD size hugetlb in
> follow_huge_pmd().
> 
> To fix above issues, rename the follow_huge_pmd() as follow_huge_pmd_pte()
> to handle PMD and PTE level size hugetlb, which uses huge_pte_lock() to
> get the correct pte entry lock to make the pte entry stable.
> 
> Mike said:
> 
> Support for CONT_PMD/_PTE was added with bb9dd3df8ee9 ("arm64: hugetlb:
> refactor find_num_contig()").  Patch series "Support for contiguous pte
> hugepages", v4.  However, I do not believe these code paths were
> executed until migration support was added with 5480280d3f2d ("arm64/mm:
> enable HugeTLB migration for contiguous bit HugeTLB pages") I would go
> with 5480280d3f2d for the Fixes: targe.
> 
> Link: https://lkml.kernel.org/r/635f43bdd85ac2615a58405da82b4d33c6e5eb05.1662017562.git.baolin.wang@linux.alibaba.com
> Fixes: 5480280d3f2d ("arm64/mm: enable HugeTLB migration for contiguous bit HugeTLB pages")
> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> Suggested-by: Mike Kravetz <mike.kravetz@oracle.com>
> Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Muchun Song <songmuchun@bytedance.com>
> Cc: <stable@vger.kernel.org>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: Sasha Levin <sashal@kernel.org>
> [5.4: Fixup contextual diffs before pin_user_pages()]
> Signed-off-by: Samuel Mendoza-Jonas <samjonas@amazon.com>
> ---
> v2: Fix up stray out label that is now unused.

Now queued up, thanks.

greg k-h