net/smc: check smcd_v2_ext_offset when receiving proposal msg

When receiving proposal msg in server, the field smcd_v2_ext_offset in
proposal msg is from the remote client and can not be fully trusted.
Once the value of smcd_v2_ext_offset exceed the max value, there has
the chance to access wrong address, and crash may happen.

This patch checks the value of smcd_v2_ext_offset before using it.

Fixes: 5c21c4ccafe8 ("net/smc: determine accepted ISM devices")
Signed-off-by: Guangguan Wang <[e-mail]>
Reviewed-by: Wen Gu <[e-mail]>
Reviewed-by: D. Wythe <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/smc/af_smc.c

···
 	pclc_smcd = smc_get_clc_msg_smcd(pclc);
 	smc_v2_ext = smc_get_clc_v2_ext(pclc);
 	smcd_v2_ext = smc_get_clc_smcd_v2_ext(smc_v2_ext);
+	if (!pclc_smcd || !smc_v2_ext || !smcd_v2_ext)
+		goto not_found;
 
 	mutex_lock(&smcd_dev_list.mutex);
 	if (pclc_smcd->ism.chid) {

net/smc/smc_clc.h

···
 static inline struct smc_clc_smcd_v2_extension *
 smc_get_clc_smcd_v2_ext(struct smc_clc_v2_extension *prop_v2ext)
 {
+	u16 max_offset = offsetof(struct smc_clc_msg_proposal_area, pclc_smcd_v2_ext) -
+		offsetof(struct smc_clc_msg_proposal_area, pclc_v2_ext) -
+		offsetof(struct smc_clc_v2_extension, hdr) -
+		offsetofend(struct smc_clnt_opts_area_hdr, smcd_v2_ext_offset);
+
 	if (!prop_v2ext)
 		return NULL;
-	if (!ntohs(prop_v2ext->hdr.smcd_v2_ext_offset))
+	if (!ntohs(prop_v2ext->hdr.smcd_v2_ext_offset) ||
+	    ntohs(prop_v2ext->hdr.smcd_v2_ext_offset) > max_offset)
 		return NULL;
 
 	return (struct smc_clc_smcd_v2_extension *)