ACK/CMNT: [Pull][SRU][CVE-2019-11487][Xenial] Avoid overflowing page reference count

[Connor Kuehl]

On 8/9/19 3:03 PM, Tyler Hicks wrote:
> On 2019-08-06 14:04:52, Connor Kuehl wrote:
>> https://people.canonical.com/~ubuntu-security/cve/2019/CVE-2019-11487.html
>>
>>  From the link above:
>>
>> "The Linux kernel before 5.1-rc5 allows page->_refcount reference count
>> overflow, with resultant use-after-free issues, if about 140 GiB of RAM
>> exists. This is related to fs/fuse/dev.c, fs/pipe.c, fs/splice.c,
>> include/linux/mm.h, include/linux/pipe_fs_i.h, kernel/trace/trace.c,
>> mm/gup.c, and mm/hugetlb.c. It can occur with FUSE requests."
>>
>> Bionic has already received these patches during an upstream sync:
>> https://bugs.launchpad.net/bugs/1838459
>>
>> Xenial felt like a bit of an involved backport that touched the mm subsystem,
>> and because of that, I think it is possible there's a breakage risk here that
>> I don't see as I'm not very familiar with mm.
> 
> This all looks good to me. The only thing that I noticed was a spaces vs
> tab issue on the line that calls atomic_inc() in "mm: add
> 'try_get_page()' helper function". That's not a blocker, IMO, but maybe
> you could fix it up before it is pulled.

I'll fix this in a V2.

> 
> Acked-by: Tyler Hicks <tyhicks at canonical.com>
> 
> Tyler
> 
>>
>> It builds on all arches and I boot-tested amd64.
>>
>> * mm: prevent get_user_pages() from overflowing page refcount
>>
>>    This is the patch I'm most concerned about. It appears to be based on a
>>    refactoring patch which split a function into a number of others. Rather
>>    than try to backport the refactoring patch(es). I've backported the code
>>    to what I think is equivalent to what the refactored one does.
>>
>>    In my backport, I didn't see an equivalent error path for this hunk from
>>    the original patch, so it was omitted:
>>
>> diff --git a/mm/gup.c b/mm/gup.c
>> index 75029649baca..81e0bdefa2cc 100644
>> --- a/mm/gup.c
>> +++ b/mm/gup.c
>> @@ -295,7 +299,10 @@ static struct page *follow_pmd_mask(struct vm_area_struct *vma,
>>   			if (pmd_trans_unstable(pmd))
>>   				ret = -EBUSY;
>>   		} else {
>> -			get_page(page);
>> +			if (unlikely(!try_get_page(page))) {
>> +				spin_unlock(ptl);
>> +				return ERR_PTR(-ENOMEM);
>> +			}
>>   			spin_unlock(ptl);
>>   			lock_page(page);
>>   			ret = split_huge_page(page);
>>
>> * mm, gup: ensure real head page is ref-counted when using hugepages
>>
>>    This patch wasn't named as part of the CVE fix itself, but bringing this one in
>>    made it easier to apply the last fix "mm: prevent get_user_pages() from overflowing page refcount"
>>
>> * mm: make page ref count overflow check tighter and more explicit
>>
>>    Minor offset adjustments, and used atomic_read instead of a wrapper function
>>    that is introduced in a later commit (that commit makes a large number of changes
>>    so I felt it was better to just use the mechanism that it ultimately uses).
>>
>> * mm: add 'try_get_page()' helper function
>>
>>    Offset adjustment, directly use atomic_read and atomic_inc rather than the
>>    page_ref wrappers that aren't introduced until a later and larger commit.
>>
>> * fs: prevent page refcount overflow in pipe_buf_get
>>
>>    Offset adjustment and manually change the function signature for buffer_pipe_buf_get
>>
>> * pipe: add pipe_buf_get() helper
>>
>>    Clean cherry pick. Needed for "fs: prevent page refcount overflow in pipe_buf_get"
>>
>> ----------------------------------------------------------------
>> The following changes since commit 1ef87ecb69472da81f394f8229ec3e100b306252:
>>
>>    Linux 4.4.186 (2019-08-05 18:19:52 +0200)
>>
>> are available in the Git repository at:
>>
>>    git://git.launchpad.net/~connork/+git/xenial CVE-2019-11487
>>
>> for you to fetch changes up to 0af916186a039ec8029b12f64023f2786ab8fa6c:
>>
>>    mm: prevent get_user_pages() from overflowing page refcount (2019-08-06 13:29:12 -0700)
>>
>> ----------------------------------------------------------------
>> Linus Torvalds (3):
>>        mm: add 'try_get_page()' helper function
>>        mm: make page ref count overflow check tighter and more explicit
>>        mm: prevent get_user_pages() from overflowing page refcount
>>
>> Matthew Wilcox (1):
>>        fs: prevent page refcount overflow in pipe_buf_get
>>
>> Miklos Szeredi (1):
>>        pipe: add pipe_buf_get() helper
>>
>> Punit Agrawal (1):
>>        mm, gup: ensure real head page is ref-counted when using hugepages
>>
>>   fs/fuse/dev.c             | 12 ++++++------
>>   fs/pipe.c                 |  4 ++--
>>   fs/splice.c               | 12 ++++++++++--
>>   include/linux/mm.h        | 15 +++++++++++++-
>>   include/linux/pipe_fs_i.h | 17 ++++++++++++++--
>>   kernel/trace/trace.c      |  6 +++++-
>>   mm/gup.c                  | 50 ++++++++++++++++++++++++++++++++++-------------
>>   mm/hugetlb.c              | 16 ++++++++++++++-
>>   8 files changed, 103 insertions(+), 29 deletions(-)
>>
>> -- 
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