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Subject[PATCH] cxl/region: Fix null pointer dereference for resetting decoder
Date
Not all decoders have a reset callback.

The CXL specification allows a host bridge with a single root port to
have no explicit HDM decoders. Currently the region driver assumes there
are none. As such the CXL core creates a special pass through decoder
instance without a commit/reset callback.

Prior to this patch, the ->reset() callback was called unconditionally when
calling cxl_region_decode_reset. Thus a configuration with 1 Host Bridge,
1 Root Port, and one directly attached CXL type 3 device or multiple CXL
type 3 devices attached to downstream ports of a switch can cause a null
pointer dereference.

Before the fix, a kernel crash was observed when we destroy the region, and
a pass through decoder is reset.

The issue can be reproduced as below,
1) create a region with a CXL setup which includes a HB with a
single root port under which a memdev is attached directly.
2) destroy the region with cxl destroy-region regionX -f.

Fixes: 176baefb2eb5 ("cxl/hdm: Commit decoder state to hardware")
Signed-off-by: Fan Ni <fan.ni@samsung.com>
---
drivers/cxl/core/region.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/cxl/core/region.c b/drivers/cxl/core/region.c
index f9ae5ad284ff..3931793a13ac 100644
--- a/drivers/cxl/core/region.c
+++ b/drivers/cxl/core/region.c
@@ -131,7 +131,7 @@ static int cxl_region_decode_reset(struct cxl_region *cxlr, int count)
struct cxl_memdev *cxlmd = cxled_to_memdev(cxled);
struct cxl_port *iter = cxled_to_port(cxled);
struct cxl_ep *ep;
- int rc;
+ int rc = 0;

while (!is_cxl_root(to_cxl_port(iter->dev.parent)))
iter = to_cxl_port(iter->dev.parent);
@@ -143,7 +143,8 @@ static int cxl_region_decode_reset(struct cxl_region *cxlr, int count)

cxl_rr = cxl_rr_load(iter, cxlr);
cxld = cxl_rr->decoder;
- rc = cxld->reset(cxld);
+ if (cxld->reset)
+ rc = cxld->reset(cxld);
if (rc)
return rc;
}
@@ -186,7 +187,8 @@ static int cxl_region_decode_commit(struct cxl_region *cxlr)
iter = ep->next, ep = cxl_ep_load(iter, cxlmd)) {
cxl_rr = cxl_rr_load(iter, cxlr);
cxld = cxl_rr->decoder;
- cxld->reset(cxld);
+ if (cxld->reset)
+ cxld->reset(cxld);
}

cxled->cxld.reset(&cxled->cxld);
--
2.25.1
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