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

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;
 	}
 
 	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