net: enetc: fix race condition in VF MAC address configuration

net: enetc: fix race condition in VF MAC address configuration

Sashiko reported a potential race condition between the VF message
handler and administrative VF MAC configuration from the host [1].

The VF message handler (enetc_msg_pf_set_vf_primary_mac_addr) runs
asynchronously in a workqueue context and accesses vf_state->flags
without any locking. Concurrently, the host can administratively
change the VF MAC address via enetc_pf_set_vf_mac(), which executes
under RTNL lock and modifies both vf_state->flags and hardware
registers.

This creates two race windows:

1) TOCTOU race on vf_state->flags: The check of ENETC_VF_FLAG_PF_SET_MAC
   and subsequent MAC programming are not atomic, allowing the flag state
   to change between check and use.

2) Torn MAC address writes: Hardware MAC programming requires multiple
   non-atomic register writes (__raw_writel for lower 32 bits and
   __raw_writew for upper 16 bits). Concurrent updates from VF mailbox
   and PF admin paths can interleave these operations, resulting in a
   corrupted MAC address being programmed into the hardware.

Fix by introducing a per-VF mutex to serialize access to vf_state and
hardware MAC register updates. Both enetc_pf_set_vf_mac() and
enetc_msg_pf_set_vf_primary_mac_addr() now acquire this lock before
accessing vf_state->flags or programming the MAC address, ensuring
atomic read-modify-write sequences and preventing register write
interleaving.

Link: https://sashiko.dev/#/patchset/20260511080805.2052495-1-wei.fang%40nxp.com
Fixes: beb74ac878c8 ("enetc: Add vf to pf messaging support")
Signed-off-by: Wei Fang <wei.fang@nxp.com>
Reviewed-by: Harshitha Ramamurthy <hramamurthy@google.com>
Link: https://patch.msgid.link/20260520064421.91569-6-wei.fang@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>