[T/X][CVE-2017-15127] CVE-2017-15127 (false-positive?)

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[T/X][CVE-2017-15127] CVE-2017-15127 (false-positive?)

Khaled Elmously
CVE-2017-15127

I think this may be a false-positive.

The CVE is marked as needed in trusty and xenial in the CVE matrix, but it seems to me that this doesn't actually affect trusty or xenial. This vulnerability appears to be related to the implementation of a feature called "UFFDIO_COPY operation for huge pages" - something which doesn't appear to exist in trusty or xenial. The fix for the CVE is to modify a function called hugetlb_mcopy_atomic_pte() (which gets called during the UFFDIO_COPY operation on a hugetlb vma) by changing the location of the goto label called 'out_release_nounlock' so as to avoid a call to unlock_page() if add_to_page_cache() returns with error - yet that 'out_release_nounlock' label doesn't exist in trusty or xenial's HEAD as it doesn't have the "UFFDIO_COPY for huge pages" feature at all (as implemented in 60d4d2d2b40e44cd36bfb6049e8d9e2055a24f8a). A function exists in current trusty/xenial called hugetlb_no_page() which shares some code structure with the vulnerable hugetlb_mcopy_atomic_pte() function, but it doesn't seem to suffer from the double unlock_page() problem. In the current trusty/xenial code, in hugetlb_no_page(), if huge_add_to_page_cache() returns an error, a call to put_page() is made and the error returned to the calling function, which is exactly what the new code does as well in both places where huge_add_to_page_cache() is called. I.e., no extraneous unlock_page() happens.

Based on the above, I believe trusty and xenial are not affected by this CVE.

(As I'm effectively proposing an empty patch as the 'fix' for this CVE, I thought I'd post my analysis to the mailing list and request 2 ACKs)

Thanks
Khalid

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