mm: prevent get_user_pages() from overflowing page refcount

commit 8fde12ca79aff9b5ba951fce1a2641901b8d8e64 upstream.

If the page refcount wraps around past zero, it will be freed while
there are still four billion references to it.  One of the possible
avenues for an attacker to try to make this happen is by doing direct IO
on a page multiple times.  This patch makes get_user_pages() refuse to
take a new page reference if there are already more than two billion
references to the page.

Reported-by: Jann Horn <jannh@google.com>
Acked-by: Matthew Wilcox <willy@infradead.org>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[ 4.4.y backport notes:
  Ajay:     - Added local variable 'err' with-in follow_hugetlb_page()
              from 2be7cfed995e, to resolve compilation error
            - Added page_ref_count()
            - Added missing refcount overflow checks on x86 and s390
              (Vlastimil, thanks for this change)
  Srivatsa: - Replaced call to get_page_foll() with try_get_page_foll() ]
Signed-off-by: Srivatsa S. Bhat (VMware) <srivatsa@csail.mit.edu>
Signed-off-by: Ajay Kaher <akaher@vmware.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

0 files changed, 0 insertions, 0 deletions