[PATCH 4/4] misc: fastrpc: fix use-after-free race in fastrpc_map_create

[srini@kernel.org]

From: Zhenghang Xiao <kipreyyy@gmail.com>

fastrpc_map_lookup returns a raw pointer after releasing fl->lock. The
caller fastrpc_map_create then calls fastrpc_map_get (kref_get_unless_zero)
on this unprotected pointer. A concurrent MEM_UNMAP can free the map
between the lock release and the kref operation, resulting in a
use-after-free on the freed slab object.

Restore the take_ref parameter to fastrpc_map_lookup so the reference
is acquired atomically under fl->lock before the pointer is exposed to
the caller.

Fixes: 10df039834f8 ("misc: fastrpc: Skip reference for DMA handles")
Cc: stable@vger.kernel.org
Signed-off-by: Zhenghang Xiao <kipreyyy@gmail.com>
Signed-off-by: Srinivas Kandagatla <srini@kernel.org>
---
 drivers/misc/fastrpc.c | 25 +++++++++++--------------
 1 file changed, 11 insertions(+), 14 deletions(-)

diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
index 47cf3d21b51d..f3a49384586d 100644
--- a/drivers/misc/fastrpc.c
+++ b/drivers/misc/fastrpc.c
@@ -388,7 +388,7 @@ static int fastrpc_map_get(struct fastrpc_map *map)
 
 
 static int fastrpc_map_lookup(struct fastrpc_user *fl, int fd,
-			    struct fastrpc_map **ppmap)
+			    struct fastrpc_map **ppmap, bool take_ref)
 {
 	struct fastrpc_map *map = NULL;
 	struct dma_buf *buf;
@@ -403,6 +403,12 @@ static int fastrpc_map_lookup(struct fastrpc_user *fl, int fd,
 		if (map->fd != fd || map->buf != buf)
 			continue;
 
+		if (take_ref) {
+			ret = fastrpc_map_get(map);
+			if (ret)
+				break;
+		}
+
 		*ppmap = map;
 		ret = 0;
 		break;
@@ -920,19 +926,10 @@ static int fastrpc_map_attach(struct fastrpc_user *fl, int fd,
 static int fastrpc_map_create(struct fastrpc_user *fl, int fd,
 			      u64 len, u32 attr, struct fastrpc_map **ppmap)
 {
-	struct fastrpc_session_ctx *sess = fl->sctx;
-	int err = 0;
-
-	if (!fastrpc_map_lookup(fl, fd, ppmap)) {
-		if (!fastrpc_map_get(*ppmap))
-			return 0;
-		dev_dbg(sess->dev, "%s: Failed to get map fd=%d\n",
-			__func__, fd);
-	}
-
-	err = fastrpc_map_attach(fl, fd, len, attr, ppmap);
+	if (!fastrpc_map_lookup(fl, fd, ppmap, true))
+		return 0;
 
-	return err;
+	return fastrpc_map_attach(fl, fd, len, attr, ppmap);
 }
 
 /*
@@ -1202,7 +1199,7 @@ static int fastrpc_put_args(struct fastrpc_invoke_ctx *ctx,
 	for (i = 0; i < FASTRPC_MAX_FDLIST; i++) {
 		if (!fdlist[i])
 			break;
-		if (!fastrpc_map_lookup(fl, (int)fdlist[i], &mmap))
+		if (!fastrpc_map_lookup(fl, (int)fdlist[i], &mmap, false))
 			fastrpc_map_put(mmap);
 	}
 
-- 
2.53.0