mm/mempolicy.c: fix out of bounds write in mpol_parse_str()

stable inclusion
from stable-v4.19.102
commit 732ecd4aad51d336b49b9be431219d173ac826c8
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6L0EC
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=732ecd4aad51d336b49b9be431219d173ac826c8

--------------------------------

commit c7a91bc7 upstream.

What we are trying to do is change the '=' character to a NUL terminator
and then at the end of the function we restore it back to an '='.  The
problem is there are two error paths where we jump to the end of the
function before we have replaced the '=' with NUL.

We end up putting the '=' in the wrong place (possibly one element
before the start of the buffer).

Link: http://lkml.kernel.org/r/20200115055426.vdjwvry44nfug7yy@kili.mountain


Reported-by:  <syzbot+e64a13c5369a194d67df@syzkaller.appspotmail.com>
Fixes: 095f1fc4 ("mempolicy: rework shmem mpol parsing and display")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Dmitry Vyukov <dvyukov@google.com>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Lee Schermerhorn <lee.schermerhorn@hp.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
Reviewed-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Reviewed-by: tong tiangen <tongtiangen@huawei.com>
Signed-off-by: Yongqiang Liu <liuyongqiang13@huawei.com>