io_uring: fix not locked access to fixed buf table

io_uring: fix not locked access to fixed buf table

commit e9b4ed5c52df71f2dd523b4c07e20c560352a740 upstream.

We can look inside the fixed buffer table only while holding
->uring_lock, however in some cases we don't do the right async prep for
IORING_OP_{WRITE,READ}_FIXED ending up with NULL req->imu forcing making
an io-wq worker to try to resolve the fixed buffer without proper
locking.

Move req->imu setup into early req init paths, i.e. io_prep_rw(), which
is called unconditionally for rw requests and under uring_lock.

Fixes: 28c2b7b3c696b ("io_uring: add full-fledged dynamic buffers support")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>