FROMLIST: misc: fastrpc: Add reference counting for fastrpc_user structure

Add reference counting using kref to the fastrpc_user structure to
prevent use-after-free issues when contexts are freed from workqueue
after device release.

The issue occurs when fastrpc_device_release() frees the user structure
while invoke contexts are still pending in the workqueue. When the
workqueue later calls fastrpc_context_free(), it attempts to access
buf->fl->cctx in fastrpc_buf_free(), leading to a use-after-free:

  pc : fastrpc_buf_free+0x38/0x80 [fastrpc]
  lr : fastrpc_context_free+0xa8/0x1b0 [fastrpc]
  ...
  fastrpc_context_free+0xa8/0x1b0 [fastrpc]
  fastrpc_context_put_wq+0x78/0xa0 [fastrpc]
  process_one_work+0x180/0x450
  worker_thread+0x26c/0x388

Implement proper reference counting to fix this:
- Initialize kref in fastrpc_device_open()
- Take a reference in fastrpc_context_alloc() for each context
- Release the reference in fastrpc_context_free() when context is freed
- Release the initial reference in fastrpc_device_release()

This ensures the user structure remains valid as long as there are
contexts holding references to it, preventing the race condition.

Link: https://lore.kernel.org/all/20260226151121.818852-1-anandu.e@oss.qualcomm.com/
Signed-off-by: Anandu Krishnan E <anandu.e@oss.qualcomm.com>

Author: Anandu Krishnan E

drivers/misc/fastrpc.c

@@ -327,6 +327,8 @@ struct fastrpc_user {
 	spinlock_t lock;
 	/* lock for allocations */
 	struct mutex mutex;
+	/* Reference count */
+	struct kref refcount;
 };
 
 static void fastrpc_free_map(struct kref *ref)
@@ -495,15 +497,36 @@ static void fastrpc_channel_ctx_put(struct fastrpc_channel_ctx *cctx)
 	kref_put(&cctx->refcount, fastrpc_channel_ctx_free);
 }
 
+static void fastrpc_user_free(struct kref *ref)
+{
+	struct fastrpc_user *fl = container_of(ref, struct fastrpc_user, refcount);
+
+	fastrpc_channel_ctx_put(fl->cctx);
+	mutex_destroy(&fl->mutex);
+	kfree(fl);
+}
+
+static void fastrpc_user_get(struct fastrpc_user *fl)
+{
+	kref_get(&fl->refcount);
+}
+
+static void fastrpc_user_put(struct fastrpc_user *fl)
+{
+	kref_put(&fl->refcount, fastrpc_user_free);
+}
+
 static void fastrpc_context_free(struct kref *ref)
 {
 	struct fastrpc_invoke_ctx *ctx;
 	struct fastrpc_channel_ctx *cctx;
+	struct fastrpc_user *fl;
 	unsigned long flags;
 	int i;
 
 	ctx = container_of(ref, struct fastrpc_invoke_ctx, refcount);
 	cctx = ctx->cctx;
+	fl = ctx->fl;
 
 	for (i = 0; i < ctx->nbufs; i++)
 		fastrpc_map_put(ctx->maps[i]);
@@ -519,6 +542,8 @@ static void fastrpc_context_free(struct kref *ref)
 	kfree(ctx->olaps);
 	kfree(ctx);
 
+	/* Release the reference taken in fastrpc_context_alloc() */
+	fastrpc_user_put(fl);
 	fastrpc_channel_ctx_put(cctx);
 }
 
@@ -628,6 +653,8 @@ static struct fastrpc_invoke_ctx *fastrpc_context_alloc(
 
 	/* Released in fastrpc_context_put() */
 	fastrpc_channel_ctx_get(cctx);
+	/* Take a reference to user, released in fastrpc_context_free() */
+	fastrpc_user_get(user);
 
 	ctx->sc = sc;
 	ctx->retval = -1;
@@ -658,6 +685,7 @@ static struct fastrpc_invoke_ctx *fastrpc_context_alloc(
 	spin_lock(&user->lock);
 	list_del(&ctx->node);
 	spin_unlock(&user->lock);
+	fastrpc_user_put(user);
 	fastrpc_channel_ctx_put(cctx);
 	kfree(ctx->maps);
 	kfree(ctx->olaps);
@@ -1675,11 +1703,9 @@ static int fastrpc_device_release(struct inode *inode, struct file *file)
 	}
 
 	fastrpc_session_free(cctx, fl->sctx);
-	fastrpc_channel_ctx_put(cctx);
-
-	mutex_destroy(&fl->mutex);
-	kfree(fl);
 	file->private_data = NULL;
+	/* Release the reference taken in fastrpc_device_open */
+	fastrpc_user_put(fl);
 
 	return 0;
 }
@@ -1723,6 +1749,7 @@ static int fastrpc_device_open(struct inode *inode, struct file *filp)
 	spin_lock_irqsave(&cctx->lock, flags);
 	list_add_tail(&fl->user, &cctx->users);
 	spin_unlock_irqrestore(&cctx->lock, flags);
+	kref_init(&fl->refcount);
 
 	return 0;
 }