ksmbd: validate response sizes in ipc_validate_msg()

ipc_validate_msg() computes the expected message size for each
response type by adding (or multiplying) attacker-controlled fields
from the daemon response to a fixed struct size in unsigned int
arithmetic.  Three cases can overflow:

  KSMBD_EVENT_RPC_REQUEST:
      msg_sz = sizeof(struct ksmbd_rpc_command) + resp->payload_sz;
  KSMBD_EVENT_SHARE_CONFIG_REQUEST:
      msg_sz = sizeof(struct ksmbd_share_config_response) +
               resp->payload_sz;
  KSMBD_EVENT_LOGIN_REQUEST_EXT:
      msg_sz = sizeof(struct ksmbd_login_response_ext) +
               resp->ngroups * sizeof(gid_t);

resp->payload_sz is __u32 and resp->ngroups is __s32.  Each addition
can wrap in unsigned int; the multiplication by sizeof(gid_t) mixes
signed and size_t, so a negative ngroups is converted to SIZE_MAX
before the multiply.  A wrapped value of msg_sz that happens to
equal entry->msg_sz bypasses the size check on the next line, and
downstream consumers (smb2pdu.c:6742 memcpy using rpc_resp->payload_sz,
kmemdup in ksmbd_alloc_user using resp_ext->ngroups) then trust the
unverified length.

Use check_add_overflow() on the RPC_REQUEST and SHARE_CONFIG_REQUEST
paths to detect integer overflow without constraining functional
payload size; userspace ksmbd-tools grows NDR responses in 4096-byte
chunks for calls like NetShareEnumAll, so a hard transport cap is
unworkable on the response side.  For LOGIN_REQUEST_EXT, reject
resp->ngroups outside the signed [0, NGROUPS_MAX] range up front and
report the error from ipc_validate_msg() so it fires at the IPC
boundary; with that bound the subsequent multiplication and addition
stay well below UINT_MAX.  The now-redundant ngroups check and
pr_err in ksmbd_alloc_user() are removed.

This is the response-side analogue of aab98e2 ("ksmbd: fix
integer overflows on 32 bit systems"), which hardened the request
side.

Fixes: 0626e66 ("cifsd: add server handler for central processing and tranport layers")
Fixes: a77e0e0 ("ksmbd: add support for supplementary groups")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-6
Assisted-by: Codex:gpt-5-4
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

Author: mjbommar

fs/smb/server/mgmt/user_config.c

@@ -56,12 +56,6 @@ struct ksmbd_user *ksmbd_alloc_user(struct ksmbd_login_response *resp,
 		goto err_free;
 
 	if (resp_ext) {
-		if (resp_ext->ngroups > NGROUPS_MAX) {
-			pr_err("ngroups(%u) from login response exceeds max groups(%d)\n",
-					resp_ext->ngroups, NGROUPS_MAX);
-			goto err_free;
-		}
-
 		user->sgid = kmemdup(resp_ext->____payload,
 				     resp_ext->ngroups * sizeof(gid_t),
 				     KSMBD_DEFAULT_GFP);

fs/smb/server/transport_ipc.c

@@ -13,6 +13,7 @@
 #include <net/genetlink.h>
 #include <linux/socket.h>
 #include <linux/workqueue.h>
+#include <linux/overflow.h>
 
 #include "vfs_cache.h"
 #include "transport_ipc.h"
@@ -496,7 +497,9 @@ static int ipc_validate_msg(struct ipc_msg_table_entry *entry)
 	{
 		struct ksmbd_rpc_command *resp = entry->response;
 
-		msg_sz = sizeof(struct ksmbd_rpc_command) + resp->payload_sz;
+		if (check_add_overflow(sizeof(struct ksmbd_rpc_command),
+				       resp->payload_sz, &msg_sz))
+			return -EINVAL;
 		break;
 	}
 	case KSMBD_EVENT_SPNEGO_AUTHEN_REQUEST:
@@ -515,8 +518,9 @@ static int ipc_validate_msg(struct ipc_msg_table_entry *entry)
 			if (resp->payload_sz < resp->veto_list_sz)
 				return -EINVAL;
 
-			msg_sz = sizeof(struct ksmbd_share_config_response) +
-					resp->payload_sz;
+			if (check_add_overflow(sizeof(struct ksmbd_share_config_response),
+					       resp->payload_sz, &msg_sz))
+				return -EINVAL;
 		}
 		break;
 	}
@@ -525,6 +529,12 @@ static int ipc_validate_msg(struct ipc_msg_table_entry *entry)
 		struct ksmbd_login_response_ext *resp = entry->response;
 
 		if (resp->ngroups) {
+			if (resp->ngroups < 0 ||
+			    resp->ngroups > NGROUPS_MAX) {
+				pr_err("ngroups(%d) from login response exceeds max groups(%d)\n",
+				       resp->ngroups, NGROUPS_MAX);
+				return -EINVAL;
+			}
 			msg_sz = sizeof(struct ksmbd_login_response_ext) +
 					resp->ngroups * sizeof(gid_t);
 		}