misc: nsm: bound the device-reported response length

nsm_sendrecv_msg_locked() stores the virtqueue used-ring length reported
by the NSM device into msg->resp.len without bounding it to the response
buffer. A malicious or buggy backend can report a length larger than the
response buffer; parse_resp_raw() then copies that many bytes out of the
fixed buffer to user space, disclosing adjacent kernel heap (an
out-of-bounds read). The request path already floors its length in
fill_req_raw(); the response path lacks the symmetric check.

Clamp the stored length to the size of the response buffer. Well-behaved
devices report no more than the posted buffer size, so conforming traffic
is unaffected.

Fixes: b9873755a6 ("misc: Add Nitro Secure Module driver")
Cc: stable@vger.kernel.org
Signed-off-by: Bryam Vargas <hexlabsecurity@proton.me>
Reviewed-by: Alexander Graf <graf@amazon.com>
Link: https://patch.msgid.link/20260620-b4-disp-a54b7dd6-v1-1-79d1f236a854@proton.me
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/nsm.c

@@ -243,7 +243,7 @@ static int nsm_sendrecv_msg_locked(struct nsm *nsm)
 		goto cleanup;
 	}
 
-	msg->resp.len = len;
+	msg->resp.len = min_t(unsigned int, len, sizeof(msg->resp.data));
 
 	rc = 0;