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

[Tyler Hicks]

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.

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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