Re: [PATCH net] rds: ib: Clear the sg list when mapping an MR fails

[Simon Horman <horms@kernel.org>]

On Sun, Sep 20, 2026 at 02:34:49PM +0800, Dongliang Qin wrote:
> rds_ib_map_frmr() stores the caller's scatterlist in the MR before DMA
> mapping and registration can fail. On failure, __rds_rdma_map() unpins
> the pages and frees the scatterlist, but rds_ib_free_frmr() can still
> return the MR to the pool with the stale pointer set.
> 
> This leaves the pool with a dangling scatterlist and can lead to local
> privilege escalation. KASAN detects the resulting use-after-free when the
> MR is later torn down:
> 
> BUG: KASAN: slab-use-after-free in __rds_ib_teardown_mr
> Read of size 8
> 
> Call Trace:
>  __rds_ib_teardown_mr
>  rds_ib_unreg_frmr
>  rds_ib_flush_mr_pool
>  rds_ib_flush_mrs
>  rds_free_mr
>  rds_setsockopt
> 
> Clear the scatterlist fields before returning an error so the caller
> retains sole ownership of the scatterlist and its pinned pages. Route a
> zero-length DMA mapping through the same cleanup path, and skip unmap
> when no mapping was created.
> 
> Fixes: 1659185fb4d0 ("RDS: IB: Support Fastreg MR (FRMR) memory registration mode")
> Cc: stable@vger.kernel.org
> Signed-off-by: Dongliang Qin <cccccccccccc777777@gmail.com>
> ---
>  net/rds/ib_frmr.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/net/rds/ib_frmr.c b/net/rds/ib_frmr.c
> index bd8611911..fea387952 100644
> --- a/net/rds/ib_frmr.c
> +++ b/net/rds/ib_frmr.c
> @@ -213,7 +213,8 @@ static int rds_ib_map_frmr(struct rds_ib_device *rds_ibdev,
>  					 DMA_BIDIRECTIONAL);
>  	if (unlikely(!ibmr->sg_dma_len)) {
>  		pr_warn("RDS/IB: %s failed!\n", __func__);
> -		return -EBUSY;
> +		ret = -EBUSY;
> +		goto out_unmap;
>  	}

Hi Dongliang,

This approach leads to the complexity of a condition in the out_unmap
label.  I wonder if a cleaner approach would be to, instead, only store the
scatterlist in the MR once mapping has succeeded.

With such an approach I think that ibmr->sg and ibmr->sg_len would still
need to be cleared in the error path. But neither the condition in the
out_unmap label nor the hunk above would not be necessary. And I wonder
if that would be a cleaner approach.

>  
>  	frmr->sg_byte_len = 0;
> @@ -261,9 +262,13 @@ static int rds_ib_map_frmr(struct rds_ib_device *rds_ibdev,
>  	return ret;
>  
>  out_unmap:
> -	ib_dma_unmap_sg(rds_ibdev->dev, ibmr->sg, ibmr->sg_len,
> -			DMA_BIDIRECTIONAL);
> -	ibmr->sg_dma_len = 0;
> +	if (ibmr->sg_dma_len) {
> +		ib_dma_unmap_sg(rds_ibdev->dev, ibmr->sg, ibmr->sg_len,
> +				DMA_BIDIRECTIONAL);
> +		ibmr->sg_dma_len = 0;
> +	}
> +	ibmr->sg = NULL;
> +	ibmr->sg_len = 0;
>  	return ret;
>  }
>  
> -- 
> 2.43.0
>