wifi: brcmfmac: Fix oops due to NULL pointer dereference in brcmf_sdiod_sglist_rw()

wifi: brcmfmac: Fix oops due to NULL pointer dereference in brcmf_sdiod_sglist_rw()

This patch fixes a NULL pointer dereference bug in brcmfmac that occurs
when a high 'sd_sgentry_align' value applies (e.g. 512) and a lot of queued SKBs
are sent from the pkt queue.

The problem is the number of entries in the pre-allocated sgtable, it is
nents = max(rxglom_size, txglom_size) + max(rxglom_size, txglom_size) >> 4 + 1.
Given the default [rt]xglom_size=32 it's actually 35 which is too small.
Worst case, the pkt queue can end up with 64 SKBs. This occurs when a new SKB
is added for each original SKB if tailroom isn't enough to hold tail_pad.
At least one sg entry is needed for each SKB. So, eventually the "skb_queue_walk loop"
in brcmf_sdiod_sglist_rw may run out of sg entries. This makes sg_next return
NULL and this causes the oops.

The patch sets nents to max(rxglom_size, txglom_size) * 2 to be able handle
the worst-case.
Btw. this requires only 64-35=29 * 16 (or 20 if CONFIG_NEED_SG_DMA_LENGTH) = 464
additional bytes of memory.

Signed-off-by: Norbert van Bolhuis <nvbolhuis@gmail.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://patch.msgid.link/20241107132903.13513-1-nvbolhuis@gmail.com