ecryptfs: forbid opening files without mmap handler

ecryptfs: forbid opening files without mmap handler

Orabug: 25256945

[ Upstream commit 2f36db71009304b3f0b95afacd8eba1f9f046b87 ]

This prevents users from triggering a stack overflow through a recursive
invocation of pagefault handling that involves mapping procfs files into
virtual memory.

Signed-off-by: Jann Horn <jannh@google.com>
Acked-by: Tyler Hicks <tyhicks@canonical.com>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
(cherry picked from commit ca1950cd168857c811f217a390ed8c91b57d2c09)
Signed-off-by: Dhaval Giani <dhaval.giani@oracle.com>