Subject: Re: [PATCH net v1] net: stmmac: TSO: Fix unbalanced DMA map/unmap for non-paged SKB data

From: Simon Horman <[email-masked]>
Date: Mon, 21 Oct 2024 13:26:01 +0100

On Mon, Oct 21, 2024 at 02:10:23PM +0800, Furong Xu wrote:
> In case the non-paged data of a SKB carries protocol header and protocol
> payload to be transmitted on a certain platform that the DMA AXI address
> width is configured to 40-bit/48-bit, or the size of the non-paged data
> is bigger than TSO_MAX_BUFF_SIZE on a certain platform that the DMA AXI
> address width is configured to 32-bit, then this SKB requires at least
> two DMA transmit descriptors to serve it.
> 
> For example, three descriptors are allocated to split one DMA buffer
> mapped from one piece of non-paged data:
>     dma_desc[N + 0],
>     dma_desc[N + 1],
>     dma_desc[N + 2].
> Then three elements of tx_q->tx_skbuff_dma[] will be allocated to hold
> extra information to be reused in stmmac_tx_clean():
>     tx_q->tx_skbuff_dma[N + 0],
>     tx_q->tx_skbuff_dma[N + 1],
>     tx_q->tx_skbuff_dma[N + 2].
> Now we focus on tx_q->tx_skbuff_dma[entry].buf, which is the DMA buffer
> address returned by DMA mapping call. stmmac_tx_clean() will try to
> unmap the DMA buffer _ONLY_IF_ tx_q->tx_skbuff_dma[entry].buf
> is a valid buffer address.
> 
> The expected behavior that saves DMA buffer address of this non-paged
> data to tx_q->tx_skbuff_dma[entry].buf is:
>     tx_q->tx_skbuff_dma[N + 0].buf = NULL;
>     tx_q->tx_skbuff_dma[N + 1].buf = NULL;
>     tx_q->tx_skbuff_dma[N + 2].buf = dma_map_single();
> Unfortunately, the current code misbehaves like this:
>     tx_q->tx_skbuff_dma[N + 0].buf = dma_map_single();
>     tx_q->tx_skbuff_dma[N + 1].buf = NULL;
>     tx_q->tx_skbuff_dma[N + 2].buf = NULL;
> 
> On the stmmac_tx_clean() side, when dma_desc[N + 0] is closed by the
> DMA engine, tx_q->tx_skbuff_dma[N + 0].buf is a valid buffer address
> obviously, then the DMA buffer will be unmapped immediately.
> There may be a rare case that the DMA engine does not finish the
> pending dma_desc[N + 1], dma_desc[N + 2] yet. Now things will go
> horribly wrong, DMA is going to access a unmapped/unreferenced memory
> region, corrupted data will be transmited or iommu fault will be
> triggered :(
> 
> In contrast, the for-loop that maps SKB fragments behaves perfectly
> as expected, and that is how the driver should do for both non-paged
> data and paged frags actually.
> 
> This patch corrects DMA map/unmap sequences by fixing the array index
> for tx_q->tx_skbuff_dma[entry].buf when assigning DMA buffer address.
> 
> Tested and verified on DWXGMAC CORE 3.20a
> 
> Reported-by: Suraj Jaiswal <[email-masked]>
> Fixes: f748be531d70 ("stmmac: support new GMAC4")
> Signed-off-by: Furong Xu <[email-masked]>

Thanks for the very thorough explanation, much appreciated.

Reviewed-by: Simon Horman <[email-masked]>