tee: shm: fix use-after-free via temporarily dropped reference

tee: shm: fix use-after-free via temporarily dropped reference

BugLink: http://bugs.launchpad.net/bugs/1782846
commit bb765d1c331f62b59049d35607ed2e365802bef9 upstream.

Bump the file's refcount before moving the reference into the fd table,
not afterwards. The old code could drop the file's refcount to zero for a
short moment before calling get_file() via get_dma_buf().

This code can only be triggered on ARM systems that use Linaro's OP-TEE.

Fixes: 967c9cca2cc5 ("tee: generic TEE subsystem")
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>