[PATCH] misc: ibmvmc: fix integer underflow in max_hmc_index calculation

[Junrui Luo]

The ibmvmc_process_capabilities() function contains an integer underflow
vulnerability that may trigger out-of-bounds memory access in the kernel.

When processing a VMC_MSG_CAP_RESP message, if the hypervisor sets
max_hmc to 0, the following calculation causes an integer underflow:

  ibmvmc.max_hmc_index = min_t(u8, ibmvmc_max_hmcs, crq->max_hmc) - 1;

Fix by adding validation to ensure max_hmc is at least 1 before the
subtraction.

Reported-by: Yuhao Jiang <danisjiang@xxxxxxxxx>
Reported-by: Junrui Luo <moonafterrain@xxxxxxxxxxx>
Fixes: 0eca353e7ae7 ("misc: IBM Virtual Management Channel Driver (VMC)")
Signed-off-by: Junrui Luo <moonafterrain@xxxxxxxxxxx>
---
 drivers/misc/ibmvmc.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/drivers/misc/ibmvmc.c b/drivers/misc/ibmvmc.c
index e5f935b5249d..5848d93ace4f 100644
--- a/drivers/misc/ibmvmc.c
+++ b/drivers/misc/ibmvmc.c
@@ -1711,6 +1711,12 @@ static void ibmvmc_process_capabilities(struct crq_server_adapter *adapter,
 		return;
 	}
 
+	if (crq->max_hmc == 0) {
+		dev_err(adapter->dev, "init failed, invalid max_hmc value of 0\n");
+		ibmvmc.state = ibmvmc_state_failed;
+		return;
+	}
+
 	ibmvmc.max_mtu = min_t(u32, ibmvmc_max_mtu, be32_to_cpu(crq->max_mtu));
 	ibmvmc.max_buffer_pool_size = min_t(u16, ibmvmc_max_buf_pool_size,
 					    be16_to_cpu(crq->pool_size));

---
base-commit: ea1013c1539270e372fc99854bc6e4d94eaeff66
change-id: 20251219-fixes-591c346c9599

Best regards,
-- 
Junrui Luo <moonafterrain@xxxxxxxxxxx>