[PATCH 5.15 042/107] misc: fastrpc: fix memory corruption on open

[Greg Kroah-Hartman]

From: Johan Hovold <johan+linaro@xxxxxxxxxx>

commit d245f43aab2b61195d8ebb64cef7b5a08c590ab4 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: f6f9279f2bf0 ("misc: fastrpc: Add Qualcomm fastrpc basic driver model")
Cc: stable@xxxxxxxxxxxxxxx      # 5.1
Signed-off-by: Johan Hovold <johan+linaro@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20220829080531.29681-3-johan+linaro@xxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 drivers/misc/fastrpc.c |    7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

--- a/drivers/misc/fastrpc.c
+++ b/drivers/misc/fastrpc.c
@@ -1555,7 +1555,7 @@ static int fastrpc_cb_probe(struct platf
 		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 platf
 		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) {