[PATCH net] s390/ism: Zerorize dmb at allocation

[Alexandra Winter <wintera@linux.ibm.com>]

Sashiko reported [1] that 'Missing __GFP_ZERO in folio_alloc() causes
uninitialized kernel memory to be exposed in the receive message buffer'.
An ism dmb is receive-only, so the data is not leaked to a remote peer. In
general the smc kernel module (dibs client) will only push newly received
data to userspace. We still should not have uninitialized data in a receive
buffer.

Since
commit 750afb08ca71 ("cross-tree: phase out dma_zalloc_coherent()")
dma_alloc_coherent no longer required the __GFP_ZERO flag, but when
commit 83781384a96b ("s390/ism: Properly fix receive message buffer allocation")
switched to folio_alloc(), it should have added back the __GFP_ZERO flag.

Add __GFP_ZERO flag and state in dibs.h that register_dbm() provides
a zerorized buffer (dibs_lo already does).

Link: https://lore.kernel.org/linux-s390/20260903143746.A5CC41F00A3A@smtp.kernel.org/ [1]
Fixes: 83781384a96b ("s390/ism: Properly fix receive message buffer allocation")
Signed-off-by: Alexandra Winter <wintera@linux.ibm.com>
Reviewed-by: Julian Ruess <julianr@linux.ibm.com>
---
 drivers/s390/net/ism_drv.c | 3 ++-
 include/linux/dibs.h       | 6 +++---
 2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/s390/net/ism_drv.c b/drivers/s390/net/ism_drv.c
index 035b233abb4e..405abd99c820 100644
--- a/drivers/s390/net/ism_drv.c
+++ b/drivers/s390/net/ism_drv.c
@@ -256,7 +256,8 @@ static int ism_alloc_dmb(struct ism_dev *ism, struct dibs_dmb *dmb)
 		return -EINVAL;
 
 	folio = folio_alloc(GFP_KERNEL | __GFP_NOWARN | __GFP_NOMEMALLOC |
-			    __GFP_NORETRY, get_order(dmb->dmb_len));
+			    __GFP_NORETRY | __GFP_ZERO,
+			    get_order(dmb->dmb_len));
 
 	if (!folio) {
 		rc = -ENOMEM;
diff --git a/include/linux/dibs.h b/include/linux/dibs.h
index d3e0777f25ae..0c10c224bcca 100644
--- a/include/linux/dibs.h
+++ b/include/linux/dibs.h
@@ -261,12 +261,12 @@ struct dibs_dev_ops {
 	 *	@vlan_id: deprecated, ignored if device does not support vlan
 	 * Upon return in addition the following fields will be valid:
 	 *	@dmb_tok: for usage by remote and local devices and clients
-	 *	@cpu_addr: allocated buffer
+	 *	@cpu_addr: allocated, zerorized buffer
 	 *	@idx: dmb index, unique per dibs device
 	 *	@dma_addr: to be used by device driver,if applicable
 	 *
-	 * Allocate a dmb buffer and register it with this device and for this
-	 * client.
+	 * Allocate and zerorize a dmb buffer and register it with this device
+	 * and for this client.
 	 * Return: zero on success
 	 */
 	int (*register_dmb)(struct dibs_dev *dev, struct dibs_dmb *dmb,
-- 
2.53.0