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

[Baokun Li]

On 2024/5/6 11:24, Jingbo Xu wrote:
>
> On 4/24/24 11:39 AM, libaokun at huaweicloud.com wrote:
>> 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.
> Good catch!
>
>> To avoid this, we will make the anonymous fd accessible to the userland by
>> executing fd_install() after copy_to_user() has succeeded, and by this
>> point we must have already grabbed the reference count of the cache.
> Why we must execute fd_install() after copy_to_user() has succeeded?
> Why not grab a reference to the cache before fd_install()?
Two things are actually done here:
1) Grab a reference to the cache before fd_install()
2) By kernel convention, fd is taken over by the user land after
fd_install() is executed, and the kernel does not call close_fd() after
this, so fd_install() is called after everything is ready.

Thanks,
Baokun
>
>> 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 0cf63bfedc9e..7c2d43104120 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))
>> @@ -244,14 +249,14 @@ 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);
>> @@ -263,16 +268,16 @@ 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;
>>   	}
>>   
>> @@ -281,15 +286,14 @@ static int cachefiles_ondemand_get_fd(struct cachefiles_req *req)
>>   		spin_unlock(&object->ondemand->lock);
>>   		ret = -EEXIST;
>>   		/* Avoid performing cachefiles_ondemand_fd_release(). */
>> -		file->private_data = NULL;
>> +		anon_file->file->private_data = NULL;
>>   		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);
>>   
>> @@ -298,9 +302,11 @@ 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:
>> @@ -357,6 +363,7 @@ 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);
>> @@ -390,7 +397,7 @@ 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;
>>   	}
>> @@ -398,10 +405,16 @@ 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);