linux-kernel - Re: [PATCH 07/12] cachefiles: add spin_lock for cachefiles_ondemand

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Date: Mon, 6 May 2024 11:23:36 +0800
From: Baokun Li <libaokun@...weicloud.com>
To: Jingbo Xu <jefflexu@...ux.alibaba.com>, netfs@...ts.linux.dev
Cc: dhowells@...hat.com, jlayton@...nel.org, zhujia.zj@...edance.com,
 linux-erofs@...ts.ozlabs.org, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org, libaokun@...weicloud.com,
 yangerkun <yangerkun@...wei.com>
Subject: Re: [PATCH 07/12] cachefiles: add spin_lock for
 cachefiles_ondemand_info

On 2024/5/6 10:55, Jingbo Xu wrote:
>
> On 4/24/24 11:39 AM, libaokun@...weicloud.com wrote:
>> From: Baokun Li <libaokun1@...wei.com>
>>
>> The following concurrency may cause a read request to fail to be completed
>> and result in a hung:
>>
>>             t1             |             t2
>> ---------------------------------------------------------
>>                              cachefiles_ondemand_copen
>>                                req = xa_erase(&cache->reqs, id)
>> // Anon fd is maliciously closed.
>> cachefiles_ondemand_fd_release
>>    xa_lock(&cache->reqs)
>>    cachefiles_ondemand_set_object_close(object)
>>    xa_unlock(&cache->reqs)
>>                                cachefiles_ondemand_set_object_open
>>                                // No one will ever close it again.
>> cachefiles_ondemand_daemon_read
>>    cachefiles_ondemand_select_req
>>    // Get a read req but its fd is already closed.
>>    // The daemon can't issue a cread ioctl with an closed fd, then hung.
>>
>> So add spin_lock for cachefiles_ondemand_info to protect ondemand_id and
>> state, thus we can avoid the above problem in cachefiles_ondemand_copen()
>> by using ondemand_id to determine if fd has been released.
>>
>> Signed-off-by: Baokun Li <libaokun1@...wei.com>
> This indeed looks like a reasonable scenario where the kernel side
> should fix, as a non-malicious daemon could also run into this.
>
> How about reusing &cache->reqs spinlock rather than introducing a new
> spinlock, as &cache->reqs spinlock is already held when setting object
> to close state in cachefiles_ondemand_fd_release()?
We've considered reusing &cache->reqs spinlock before, but their
uses don't exactly overlap, and there are patches coming that will
use the new spin_lock,. In addition, this reduces competition for
&cache->reqs spinlock.
>> ---
>>   fs/cachefiles/internal.h |  1 +
>>   fs/cachefiles/ondemand.c | 16 +++++++++++++++-
>>   2 files changed, 16 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/cachefiles/internal.h b/fs/cachefiles/internal.h
>> index 7745b8abc3aa..45c8bed60538 100644
>> --- a/fs/cachefiles/internal.h
>> +++ b/fs/cachefiles/internal.h
>> @@ -55,6 +55,7 @@ struct cachefiles_ondemand_info {
>>   	int				ondemand_id;
>>   	enum cachefiles_object_state	state;
>>   	struct cachefiles_object	*object;
>> +	spinlock_t			lock;
>>   };
>>   
>>   /*
>> diff --git a/fs/cachefiles/ondemand.c b/fs/cachefiles/ondemand.c
>> index 898fab68332b..b5e6a851ef04 100644
>> --- a/fs/cachefiles/ondemand.c
>> +++ b/fs/cachefiles/ondemand.c
>> @@ -16,13 +16,16 @@ static int cachefiles_ondemand_fd_release(struct inode *inode,
>>   	struct cachefiles_object *object = file->private_data;
>>   	struct cachefiles_cache *cache = object->volume->cache;
>>   	struct cachefiles_ondemand_info *info = object->ondemand;
>> -	int object_id = info->ondemand_id;
>> +	int object_id;
>>   	struct cachefiles_req *req;
>>   	XA_STATE(xas, &cache->reqs, 0);
>>   
>>   	xa_lock(&cache->reqs);
>> +	spin_lock(&info->lock);
>> +	object_id = info->ondemand_id;
>>   	info->ondemand_id = CACHEFILES_ONDEMAND_ID_CLOSED;
>>   	cachefiles_ondemand_set_object_close(object);
>> +	spin_unlock(&info->lock);
>>   
>>   	/* Only flush CACHEFILES_REQ_NEW marked req to avoid race with daemon_read */
>>   	xas_for_each_marked(&xas, req, ULONG_MAX, CACHEFILES_REQ_NEW) {
>> @@ -127,6 +130,7 @@ int cachefiles_ondemand_copen(struct cachefiles_cache *cache, char *args)
>>   {
>>   	struct cachefiles_req *req;
>>   	struct fscache_cookie *cookie;
>> +	struct cachefiles_ondemand_info *info;
>>   	char *pid, *psize;
>>   	unsigned long id;
>>   	long size;
>> @@ -185,6 +189,14 @@ int cachefiles_ondemand_copen(struct cachefiles_cache *cache, char *args)
>>   		goto out;
>>   	}
>>   
>> +	info = req->object->ondemand;
>> +	spin_lock(&info->lock);
>> +	/* The anonymous fd was closed before copen ? */
> I would like describe more details in the comment, e.g. put the time
> sequence described in the commit message here.
OK, thanks for your suggestion, I will describe it in more detail
in the next revision.

Thanks,
Baokun
>
>> +	if (info->ondemand_id == CACHEFILES_ONDEMAND_ID_CLOSED) {
>> +		spin_unlock(&info->lock);
>> +		req->error = -EBADFD;
>> +		goto out;
>> +	}
>>   	cookie = req->object->cookie;
>>   	cookie->object_size = size;
>>   	if (size)
>> @@ -194,6 +206,7 @@ int cachefiles_ondemand_copen(struct cachefiles_cache *cache, char *args)
>>   	trace_cachefiles_ondemand_copen(req->object, id, size);
>>   
>>   	cachefiles_ondemand_set_object_open(req->object);
>> +	spin_unlock(&info->lock);
>>   	wake_up_all(&cache->daemon_pollwq);
>>   
>>   out:
>> @@ -596,6 +609,7 @@ int cachefiles_ondemand_init_obj_info(struct cachefiles_object *object,
>>   		return -ENOMEM;
>>   
>>   	object->ondemand->object = object;
>> +	spin_lock_init(&object->ondemand->lock);
>>   	INIT_WORK(&object->ondemand->ondemand_work, ondemand_object_worker);
>>   	return 0;
>>   }