Re: [PATCH] wifi: brcmfmac: Fix oops due to NULL pointer dereference in 'brcmf_sdiod_sglist_rw'

[Kalle Valo <kvalo@kernel.org>]

Norbert van Bolhuis <nvbolhuis@gmail.com> writes:

> 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.

BTW it would be good to fix (in a separate patch) the sg handling so
that the kernel won't oops when sg entries rung. That's not really
robust behaviour.

> 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.

s-o-b missing, please read our documentation from the link below.

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