ath10k: check idx validity in __ath10k_htt_rx_ring_fill_n()

ath10k: check idx validity in __ath10k_htt_rx_ring_fill_n()

The idx in __ath10k_htt_rx_ring_fill_n function lives in
consistent dma region writable by the device. Malfunctional
or malicious device could manipulate such idx to have a OOB
write. Either by
    htt->rx_ring.netbufs_ring[idx] = skb;
or by
    ath10k_htt_set_paddrs_ring(htt, paddr, idx);

The idx can also be negative as it's signed, giving a large
memory space to write to.

It's possibly exploitable by corruptting a legit pointer with
a skb pointer. And then fill skb with payload as rougue object.

Part of the log here. Sometimes it appears as UAF when writing
to a freed memory by chance.

 [   15.594376] BUG: unable to handle page fault for address: ffff887f5c1804f0
 [   15.595483] #PF: supervisor write access in kernel mode
 [   15.596250] #PF: error_code(0x0002) - not-present page
 [   15.597013] PGD 0 P4D 0
 [   15.597395] Oops: 0002 [#1] SMP KASAN PTI
 [   15.597967] CPU: 0 PID: 82 Comm: kworker/u2:2 Not tainted 5.6.0 #69
 [   15.598843] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
 BIOS rel-1.12.1-0-ga5cab58e9a3f-prebuilt.qemu.org 04/01/2014
 [   15.600438] Workqueue: ath10k_wq ath10k_core_register_work [ath10k_core]
 [   15.601389] RIP: 0010:__ath10k_htt_rx_ring_fill_n
 (linux/drivers/net/wireless/ath/ath10k/htt_rx.c:173) ath10k_core

Signed-off-by: Zekun Shen <bruceshenzk@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200623221105.3486-1-bruceshenzk@gmail.com