linux-kernel - Re: [PATCH V1] nvme-pci: Fix NULL pointer dereference in nvme_pci_prp_iter

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Message-ID: <20260203052756.GA15839@lst.de>
Date: Tue, 3 Feb 2026 06:27:56 +0100
From: Christoph Hellwig <hch@....de>
To: Keith Busch <kbusch@...nel.org>
Cc: Christoph Hellwig <hch@....de>, Robin Murphy <robin.murphy@....com>,
	Pradeep P V K <pradeep.pragallapati@....qualcomm.com>,
	axboe@...nel.dk, sagi@...mberg.me, linux-nvme@...ts.infradead.org,
	linux-kernel@...r.kernel.org, nitin.rawat@....qualcomm.com,
	Leon Romanovsky <leon@...nel.org>,
	Marek Szyprowski <m.szyprowski@...sung.com>, iommu@...ts.linux.dev
Subject: Re: [PATCH V1] nvme-pci: Fix NULL pointer dereference in
 nvme_pci_prp_iter_next

On Mon, Feb 02, 2026 at 11:59:04AM -0700, Keith Busch wrote:
> In the case where this iteration caused dma_need_unmap() to toggle to
> true, this is the iteration that allocates the dma_vecs, and it
> initializes the first entry to this iter. But the next lines proceed to
> the save this iter in the next index, so it's doubly accounted for and
> will get unmapped twice in the completion.

Yeah.

> Also, if the allocation fails, we should set iter->status to
> BLK_STS_RESOURCE so the callers know why the iteration can't continue.
> Otherwise, the caller will think the request is badly formed if you
> return false from here without setting iter->status.
> 
> Here's my quick take. Boot tested with swiotlb enabled, but haven't
> tried to test the changing dma_need_unmap() scenario.

Looks much better.  Cosmetic nits below.

Pradeep, can you test this with your setup?

> +	if (!dma_use_iova(&iod->dma_state) && dma_need_unmap(dma_dev))
> +		return nvme_pci_prp_save_mapping(iter, req);

> +	if (!dma_use_iova(&iod->dma_state) && dma_need_unmap(nvmeq->dev->dev))
> +		if (!nvme_pci_prp_save_mapping(iter, req))
> +			return iter->status;

I'd move the dma_use_iova / dma_need_unmap checks into
nvme_pci_prp_save_mapping to simplify this a bit more.

>  
>  	/*
>  	 * PRP1 always points to the start of the DMA transfers.
> @@ -1218,6 +1231,8 @@ static blk_status_t nvme_prep_rq(struct request *req)
>  	iod->nr_descriptors = 0;
>  	iod->total_len = 0;
>  	iod->meta_total_len = 0;
> +	iod->nr_dma_vecs = 0;
> +	iod->dma_vecs = NULL;

I don't think we need the dma_vecs initialization here, as everything
is keyed off nr_dma_vecs.