can: kvaser_pciefd: refine error prone echo_skb_max handling logic

can: kvaser_pciefd: refine error prone echo_skb_max handling logic

commit 54ec8b08216f3be2cc98b33633d3c8ea79749895 upstream.

echo_skb_max should define the supported upper limit of echo_skb[]
allocated inside the netdevice's priv. The corresponding size value
provided by this driver to alloc_candev() is KVASER_PCIEFD_CAN_TX_MAX_COUNT
which is 17.

But later echo_skb_max is rounded up to the nearest power of two (for the
max case, that would be 32) and the tx/ack indices calculated further
during tx/rx may exceed the upper array boundary. Kasan reported this for
the ack case inside kvaser_pciefd_handle_ack_packet(), though the xmit
function has actually caught the same thing earlier.

 BUG: KASAN: slab-out-of-bounds in kvaser_pciefd_handle_ack_packet+0x2d7/0x92a drivers/net/can/kvaser_pciefd.c:1528
 Read of size 8 at addr ffff888105e4f078 by task swapper/4/0

 CPU: 4 UID: 0 PID: 0 Comm: swapper/4 Not tainted 6.15.0 #12 PREEMPT(voluntary)
 Call Trace:
  <IRQ>
 dump_stack_lvl lib/dump_stack.c:122
 print_report mm/kasan/report.c:521
 kasan_report mm/kasan/report.c:634
 kvaser_pciefd_handle_ack_packet drivers/net/can/kvaser_pciefd.c:1528
 kvaser_pciefd_read_packet drivers/net/can/kvaser_pciefd.c:1605
 kvaser_pciefd_read_buffer drivers/net/can/kvaser_pciefd.c:1656
 kvaser_pciefd_receive_irq drivers/net/can/kvaser_pciefd.c:1684
 kvaser_pciefd_irq_handler drivers/net/can/kvaser_pciefd.c:1733
 __handle_irq_event_percpu kernel/irq/handle.c:158
 handle_irq_event kernel/irq/handle.c:210
 handle_edge_irq kernel/irq/chip.c:833
 __common_interrupt arch/x86/kernel/irq.c:296
 common_interrupt arch/x86/kernel/irq.c:286
  </IRQ>

Tx max count definitely matters for kvaser_pciefd_tx_avail(), but for seq
numbers' generation that's not the case - we're free to calculate them as
would be more convenient, not taking tx max count into account. The only
downside is that the size of echo_skb[] should correspond to the max seq
number (not tx max count), so in some situations a bit more memory would
be consumed than could be.

Thus make the size of the underlying echo_skb[] sufficient for the rounded
max tx value.

Found by Linux Verification Center (linuxtesting.org) with Syzkaller.

Fixes: 8256e0ca6010 ("can: kvaser_pciefd: Fix echo_skb race")
Cc: stable@vger.kernel.org
Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
Link: https://patch.msgid.link/20250528192713.63894-1-pchelkin@ispras.ru
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>