[PATCH v2 09/12] cachefiles: defer exposing anon_fd until after copy_to_user() succeeds

[libaokun at huaweicloud.com]

From: Baokun Li <libaokun1 at huawei.com>

After installing the anonymous fd, we can now see it in userland and close
it. However, at this point we may not have gotten the reference count of
the cache, but we will put it during colse fd, so this may cause a cache
UAF.

So grab the cache reference count before fd_install(). In addition, by
kernel convention, fd is taken over by the user land after fd_install(),
and the kernel should not call close_fd() after that, i.e., it should call
fd_install() after everything is ready, thus fd_install() is called after
copy_to_user() succeeds.

Fixes: c8383054506c ("cachefiles: notify the user daemon when looking up cookie")
Suggested-by: Hou Tao <houtao1 at huawei.com>
Signed-off-by: Baokun Li <libaokun1 at huawei.com>
---
 fs/cachefiles/ondemand.c | 53 +++++++++++++++++++++++++---------------
 1 file changed, 33 insertions(+), 20 deletions(-)

diff --git a/fs/cachefiles/ondemand.c b/fs/cachefiles/ondemand.c
index d2d4e27fca6f..3a36613e00a7 100644
--- a/fs/cachefiles/ondemand.c
+++ b/fs/cachefiles/ondemand.c
@@ -4,6 +4,11 @@
 #include <linux/uio.h>
 #include "internal.h"
 
+struct anon_file {
+	struct file *file;
+	int fd;
+};
+
 static inline void cachefiles_req_put(struct cachefiles_req *req)
 {
 	if (refcount_dec_and_test(&req->ref))
@@ -263,14 +268,14 @@ int cachefiles_ondemand_restore(struct cachefiles_cache *cache, char *args)
 	return 0;
 }
 
-static int cachefiles_ondemand_get_fd(struct cachefiles_req *req)
+static int cachefiles_ondemand_get_fd(struct cachefiles_req *req,
+				      struct anon_file *anon_file)
 {
 	struct cachefiles_object *object;
 	struct cachefiles_cache *cache;
 	struct cachefiles_open *load;
-	struct file *file;
 	u32 object_id;
-	int ret, fd;
+	int ret;
 
 	object = cachefiles_grab_object(req->object,
 			cachefiles_obj_get_ondemand_fd);
@@ -282,16 +287,16 @@ static int cachefiles_ondemand_get_fd(struct cachefiles_req *req)
 	if (ret < 0)
 		goto err;
 
-	fd = get_unused_fd_flags(O_WRONLY);
-	if (fd < 0) {
-		ret = fd;
+	anon_file->fd = get_unused_fd_flags(O_WRONLY);
+	if (anon_file->fd < 0) {
+		ret = anon_file->fd;
 		goto err_free_id;
 	}
 
-	file = anon_inode_getfile("[cachefiles]", &cachefiles_ondemand_fd_fops,
-				  object, O_WRONLY);
-	if (IS_ERR(file)) {
-		ret = PTR_ERR(file);
+	anon_file->file = anon_inode_getfile("[cachefiles]",
+				&cachefiles_ondemand_fd_fops, object, O_WRONLY);
+	if (IS_ERR(anon_file->file)) {
+		ret = PTR_ERR(anon_file->file);
 		goto err_put_fd;
 	}
 
@@ -299,16 +304,15 @@ static int cachefiles_ondemand_get_fd(struct cachefiles_req *req)
 	if (object->ondemand->ondemand_id > 0) {
 		spin_unlock(&object->ondemand->lock);
 		/* Pair with check in cachefiles_ondemand_fd_release(). */
-		file->private_data = NULL;
+		anon_file->file->private_data = NULL;
 		ret = -EEXIST;
 		goto err_put_file;
 	}
 
-	file->f_mode |= FMODE_PWRITE | FMODE_LSEEK;
-	fd_install(fd, file);
+	anon_file->file->f_mode |= FMODE_PWRITE | FMODE_LSEEK;
 
 	load = (void *)req->msg.data;
-	load->fd = fd;
+	load->fd = anon_file->fd;
 	object->ondemand->ondemand_id = object_id;
 	spin_unlock(&object->ondemand->lock);
 
@@ -317,9 +321,11 @@ static int cachefiles_ondemand_get_fd(struct cachefiles_req *req)
 	return 0;
 
 err_put_file:
-	fput(file);
+	fput(anon_file->file);
+	anon_file->file = NULL;
 err_put_fd:
-	put_unused_fd(fd);
+	put_unused_fd(anon_file->fd);
+	anon_file->fd = ret;
 err_free_id:
 	xa_erase(&cache->ondemand_ids, object_id);
 err:
@@ -376,6 +382,7 @@ ssize_t cachefiles_ondemand_daemon_read(struct cachefiles_cache *cache,
 	struct cachefiles_msg *msg;
 	size_t n;
 	int ret = 0;
+	struct anon_file anon_file;
 	XA_STATE(xas, &cache->reqs, cache->req_id_next);
 
 	xa_lock(&cache->reqs);
@@ -409,7 +416,7 @@ ssize_t cachefiles_ondemand_daemon_read(struct cachefiles_cache *cache,
 	xa_unlock(&cache->reqs);
 
 	if (msg->opcode == CACHEFILES_OP_OPEN) {
-		ret = cachefiles_ondemand_get_fd(req);
+		ret = cachefiles_ondemand_get_fd(req, &anon_file);
 		if (ret)
 			goto out;
 	}
@@ -417,10 +424,16 @@ ssize_t cachefiles_ondemand_daemon_read(struct cachefiles_cache *cache,
 	msg->msg_id = xas.xa_index;
 	msg->object_id = req->object->ondemand->ondemand_id;
 
-	if (copy_to_user(_buffer, msg, n) != 0) {
+	if (copy_to_user(_buffer, msg, n) != 0)
 		ret = -EFAULT;
-		if (msg->opcode == CACHEFILES_OP_OPEN)
-			close_fd(((struct cachefiles_open *)msg->data)->fd);
+
+	if (msg->opcode == CACHEFILES_OP_OPEN) {
+		if (ret < 0) {
+			fput(anon_file.file);
+			put_unused_fd(anon_file.fd);
+			goto out;
+		}
+		fd_install(anon_file.fd, anon_file.file);
 	}
 out:
 	cachefiles_put_object(req->object, cachefiles_obj_put_read_req);
-- 
2.39.2