linux-kernel - [PATCH 2/3] fastrpc: move fl->maps list removal into fastrpc_free

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Date:   Wed, 17 Feb 2021 22:20:54 -0500
From:   Jonathan Marek <jonathan@...ek.ca>
To:     linux-arm-msm@...r.kernel.org
Cc:     srinivas.kandagatla@...aro.org, Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org (open list)
Subject: [PATCH 2/3] fastrpc: move fl->maps list removal into fastrpc_free_map

This fixes the incredibly broken behavior of fastrpc_context_free(),
which calls fastrpc_map_put() but does not remove the map from the list
when it is free'd, causing use-after-free errors.

fl->lock needs to be held not just for list_del(), but also kref_put, to
avoid a race condition with fastrpc_map_find() logic.

Signed-off-by: Jonathan Marek <jonathan@...ek.ca>
---
 drivers/misc/fastrpc.c | 27 +++++++++++++--------------
 1 file changed, 13 insertions(+), 14 deletions(-)

diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
index 4fabea0c1551..170352b43ab6 100644
--- a/drivers/misc/fastrpc.c
+++ b/drivers/misc/fastrpc.c
@@ -245,19 +245,21 @@ static void fastrpc_free_map(struct kref *ref)
 		dma_buf_put(map->buf);
 	}
 
+	list_del(&map->node);
+
 	kfree(map);
 }
 
 static void fastrpc_map_put(struct fastrpc_map *map)
 {
-	if (map)
-		kref_put(&map->refcount, fastrpc_free_map);
+	spin_lock(&map->fl->lock);
+	kref_put(&map->refcount, fastrpc_free_map);
+	spin_unlock(&map->fl->lock);
 }
 
 static void fastrpc_map_get(struct fastrpc_map *map)
 {
-	if (map)
-		kref_get(&map->refcount);
+	kref_get(&map->refcount);
 }
 
 static int fastrpc_map_find(struct fastrpc_user *fl, int fd,
@@ -351,8 +353,10 @@ static void fastrpc_context_free(struct kref *ref)
 	ctx = container_of(ref, struct fastrpc_invoke_ctx, refcount);
 	cctx = ctx->cctx;
 
-	for (i = 0; i < ctx->nscalars; i++)
-		fastrpc_map_put(ctx->maps[i]);
+	for (i = 0; i < ctx->nscalars; i++) {
+		if (ctx->maps[i])
+			fastrpc_map_put(ctx->maps[i]);
+	}
 
 	if (ctx->buf)
 		fastrpc_buf_free(ctx->buf);
@@ -1103,12 +1107,8 @@ static int fastrpc_init_create_process(struct fastrpc_user *fl,
 	fl->init_mem = NULL;
 	fastrpc_buf_free(imem);
 err_alloc:
-	if (map) {
-		spin_lock(&fl->lock);
-		list_del(&map->node);
-		spin_unlock(&fl->lock);
+	if (map)
 		fastrpc_map_put(map);
-	}
 err:
 	kfree(args);
 
@@ -1185,10 +1185,9 @@ static int fastrpc_device_release(struct inode *inode, struct file *file)
 		fastrpc_context_put(ctx);
 	}
 
-	list_for_each_entry_safe(map, m, &fl->maps, node) {
-		list_del(&map->node);
+	list_for_each_entry_safe(map, m, &fl->maps, node)
 		fastrpc_map_put(map);
-	}
+	WARN_ON(!list_empty(&fl->maps));
 
 	list_for_each_entry_safe(buf, b, &fl->mmaps, node) {
 		list_del(&buf->node);
-- 
2.26.1