misc: fastrpc: fix memory corruption on open

commit d245f43 upstream.

The probe session-duplication overflow check incremented the session
count also when there were no more available sessions so that memory
beyond the fixed-size slab-allocated session array could be corrupted in
fastrpc_session_alloc() on open().

Fixes: f6f9279 ("misc: fastrpc: Add Qualcomm fastrpc basic driver model")
Cc: stable@vger.kernel.org      # 5.1
Signed-off-by: Johan Hovold <johan+linaro@kernel.org>
Link: https://lore.kernel.org/r/20220829080531.29681-3-johan+linaro@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/misc/fastrpc.c

@@ -1555,7 +1555,7 @@ static int fastrpc_cb_probe(struct platform_device *pdev)
 		spin_unlock_irqrestore(&cctx->lock, flags);
 		return -ENOSPC;
 	}
-	sess = &cctx->session[cctx->sesscount];
+	sess = &cctx->session[cctx->sesscount++];
 	sess->used = false;
 	sess->valid = true;
 	sess->dev = dev;
@@ -1568,13 +1568,12 @@ static int fastrpc_cb_probe(struct platform_device *pdev)
 		struct fastrpc_session_ctx *dup_sess;
 
 		for (i = 1; i < sessions; i++) {
-			if (cctx->sesscount++ >= FASTRPC_MAX_SESSIONS)
+			if (cctx->sesscount >= FASTRPC_MAX_SESSIONS)
 				break;
-			dup_sess = &cctx->session[cctx->sesscount];
+			dup_sess = &cctx->session[cctx->sesscount++];
 			memcpy(dup_sess, sess, sizeof(*dup_sess));
 		}
 	}
-	cctx->sesscount++;
 	spin_unlock_irqrestore(&cctx->lock, flags);
 	rc = dma_set_mask(dev, DMA_BIT_MASK(32));
 	if (rc) {