fdc: Fix MSR.RQM flag

The RQM bit in MSR should be set whenever the guest is supposed to
access the FIFO, and it should be cleared in all other cases. This is
important so the guest can't continue writing/reading the FIFO beyond
the length that it's suppossed to access (see CVE-2015-3456).

Commit e9077462 fixed the CVE by adding code that avoids the buffer
overflow; however it doesn't correct the wrong behaviour of the floppy
controller which should already have cleared RQM.

Currently, RQM stays set all the time and during all phases while a
command is being processed. This is error-prone because the command has
to explicitly clear the flag if it doesn't need data (and indeed, the
two buggy commands that are the culprits for the CVE just forgot to do
that).

This patch clears RQM immediately as soon as all bytes that are expected
have been received. If the the FIFO is used in the next phase, the flag
has to be set explicitly there.

It also clear RQM after receiving all bytes even if the phase transition
immediately sets it again. While it's technically not necessary at the
moment because the state between clearing and setting RQM is not
observable by the guest, this is more explicit and matches how real
hardware works. It will actually become necessary in qemu once
asynchronous code paths are introduced.

This alone should have been enough to fix the CVE, but now we have two
lines of defense - even better.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 1432214378-31891-8-git-send-email-kwolf@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>
