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

On Mon, 2026-09-21 at 17:57 +0100, Simon Horman wrote:

On Sun, Sep 20, 2026 at 02:34:49PM +0800, Dongliang Qin wrote:

quoted

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 <redacted>
---
 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.

That sounds reasonable, the rds_ib_teardown_mr() at the top already leaves ibmr->sg
and ibmr->sg_len cleared.  So if we move the ib_dma_map_sg() below it and do the check
and goto there, then we dont need the unmap in the out path.  Just clear ibmr->sg and
ibmr->sg_len.  I think that would be a lot simpler.

Thanks for working on this!
Allison

quoted

 
 	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