[char-misc for 4.19] mei: fix use-after-free in mei_cl_write

[Tomas Winkler]

From: John Hubbard <jhubbard@xxxxxxxxxx>

KASAN reports a use-after-free during startup, in mei_cl_write:

    BUG: KASAN: use-after-free in mei_cl_write+0x601/0x870 [mei]
       (drivers/misc/mei/client.c:1770)

This is caused by commit 98e70866aacb ("mei: add support for variable
length mei headers."), which changed the return value from len, to
buf->size. That ends up using a stale buf pointer, because blocking
call, the cb (callback) is deleted in me_cl_complete() function.

However, fortunately, len remains unchanged throughout the function
(and I don't see anything else that would require re-reading buf->size
either), so the fix is to simply revert the change, and return len, as
before.

Fixes: 98e70866aacb ("mei: add support for variable length mei headers.")
CC: Arnd Bergmann <arnd@xxxxxxxx>
CC: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: John Hubbard <jhubbard@xxxxxxxxxx>
Signed-off-by: Tomas Winkler <tomas.winkler@xxxxxxxxx>
---
V2: 1. Add better explaination in the commit message.
    2. Add Fixes: marker

BTW: The usage len is changed in furhter patches that were not
yet merged.

 drivers/misc/mei/client.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/misc/mei/client.c b/drivers/misc/mei/client.c
index 4ab6251d418e..ebdcf0b450e2 100644
--- a/drivers/misc/mei/client.c
+++ b/drivers/misc/mei/client.c
@@ -1767,7 +1767,7 @@ ssize_t mei_cl_write(struct mei_cl *cl, struct mei_cl_cb *cb)
 		}
 	}
 
-	rets = buf->size;
+	rets = len;
 err:
 	cl_dbg(dev, cl, "rpm: autosuspend\n");
 	pm_runtime_mark_last_busy(dev->dev);
-- 
2.14.4