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

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Message-ID: <79034c4c-ba06-4961-b41b-a43e5f5946af@oss.qualcomm.com>
Date: Tue, 3 Feb 2026 19:35:41 +0530
From: Pradeep Pragallapati <pradeep.pragallapati@....qualcomm.com>
To: Christoph Hellwig <hch@....de>, Keith Busch <kbusch@...nel.org>
Cc: Robin Murphy <robin.murphy@....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 2/3/2026 10:57 AM, Christoph Hellwig wrote:
> 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?
Sure, testing has started, and I will share the findings soon.
Also, I did not pick up the initialization of dma_vecs during testing.

> 
>> +	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.