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From:   Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
To:     gregkh@linuxfoundation.org
Cc:     linux-kernel@vger.kernel.org, Ola Jeppsson <ola@snap.com>,
        Abel Vesa <abel.vesa@linaro.org>,
        Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Subject: [PATCH 3/3] misc: fastrpc: Fix use-after-free race condition for maps
Date:   Thu, 24 Nov 2022 17:49:41 +0000
Message-Id: <20221124174941.418450-4-srinivas.kandagatla@linaro.org>
In-Reply-To: <20221124174941.418450-1-srinivas.kandagatla@linaro.org>
References: <20221124174941.418450-1-srinivas.kandagatla@linaro.org>
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From: Ola Jeppsson <ola@snap.com>

It is possible that in between calling fastrpc_map_get() until
map->fl->lock is taken in fastrpc_free_map(), another thread can call
fastrpc_map_lookup() and get a reference to a map that is about to be
deleted.

Rewrite fastrpc_map_get() to only increase the reference count of a map
if it's non-zero. Propagate this to callers so they can know if a map is
about to be deleted.

Fixes this warning:
refcount_t: addition on 0; use-after-free.
WARNING: CPU: 5 PID: 10100 at lib/refcount.c:25 refcount_warn_saturate
...
Call trace:
 refcount_warn_saturate
 [fastrpc_map_get inlined]
 [fastrpc_map_lookup inlined]
 fastrpc_map_create
 fastrpc_internal_invoke
 fastrpc_device_ioctl
 __arm64_sys_ioctl
 invoke_syscall

Fixes: c68cfb718c8f ("misc: fastrpc: Add support for context Invoke method")
Signed-off-by: Ola Jeppsson <ola@snap.com>
Signed-off-by: Abel Vesa <abel.vesa@linaro.org>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
---
 drivers/misc/fastrpc.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
index 9ecbf673d321..80811e852d8f 100644
--- a/drivers/misc/fastrpc.c
+++ b/drivers/misc/fastrpc.c
@@ -332,10 +332,12 @@ static void fastrpc_map_put(struct fastrpc_map *map)
 		kref_put(&map->refcount, fastrpc_free_map);
 }
 
-static void fastrpc_map_get(struct fastrpc_map *map)
+static int fastrpc_map_get(struct fastrpc_map *map)
 {
-	if (map)
-		kref_get(&map->refcount);
+	if (!map)
+		return -ENOENT;
+
+	return kref_get_unless_zero(&map->refcount) ? 0 : -ENOENT;
 }
 
 
-- 
2.25.1