linux-kernel - Re: [PATCH 06/12] cachefiles: add consistency check for copen/cread

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Date: Mon, 6 May 2024 11:12:31 +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 06/12] cachefiles: add consistency check for copen/cread

Hi Jingbo,

Thank you very much for the review!

On 2024/5/6 10:31, Jingbo Xu wrote:
> Hi Baokun,
>
> Thanks for improving on this!
>
> On 4/24/24 11:39 AM, libaokun@...weicloud.com wrote:
>> From: Baokun Li <libaokun1@...wei.com>
>>
>> This prevents malicious processes from completing random copen/cread
>> requests and crashing the system. Added checks are listed below:
>>
>>    * Generic, copen can only complete open requests, and cread can only
>>      complete read requests.
>>    * For copen, ondemand_id must not be 0, because this indicates that the
>>      request has not been read by the daemon.
>>    * For cread, the object corresponding to fd and req should be the same.
>>
>> Signed-off-by: Baokun Li <libaokun1@...wei.com>
>> ---
>>   fs/cachefiles/ondemand.c | 27 ++++++++++++++++++++-------
>>   1 file changed, 20 insertions(+), 7 deletions(-)
>>
>> diff --git a/fs/cachefiles/ondemand.c b/fs/cachefiles/ondemand.c
>> index bb94ef6a6f61..898fab68332b 100644
>> --- a/fs/cachefiles/ondemand.c
>> +++ b/fs/cachefiles/ondemand.c
>> @@ -82,12 +82,12 @@ static loff_t cachefiles_ondemand_fd_llseek(struct file *filp, loff_t pos,
>>   }
>>   
>>   static long cachefiles_ondemand_fd_ioctl(struct file *filp, unsigned int ioctl,
>> -					 unsigned long arg)
>> +					 unsigned long id)
>>   {
>>   	struct cachefiles_object *object = filp->private_data;
>>   	struct cachefiles_cache *cache = object->volume->cache;
>>   	struct cachefiles_req *req;
>> -	unsigned long id;
>> +	XA_STATE(xas, &cache->reqs, id);
>>   
>>   	if (ioctl != CACHEFILES_IOC_READ_COMPLETE)
>>   		return -EINVAL;
>> @@ -95,10 +95,15 @@ static long cachefiles_ondemand_fd_ioctl(struct file *filp, unsigned int ioctl,
>>   	if (!test_bit(CACHEFILES_ONDEMAND_MODE, &cache->flags))
>>   		return -EOPNOTSUPP;
>>   
>> -	id = arg;
>> -	req = xa_erase(&cache->reqs, id);
>> -	if (!req)
>> +	xa_lock(&cache->reqs);
>> +	req = xas_load(&xas);
>> +	if (!req || req->msg.opcode != CACHEFILES_OP_READ ||
>> +	    req->object != object) {
>> +		xa_unlock(&cache->reqs);
>>   		return -EINVAL;
>> +	}
>> +	xas_store(&xas, NULL);
>> +	xa_unlock(&cache->reqs);
>>   
>>   	trace_cachefiles_ondemand_cread(object, id);
>>   	complete(&req->done);
>> @@ -126,6 +131,7 @@ int cachefiles_ondemand_copen(struct cachefiles_cache *cache, char *args)
>>   	unsigned long id;
>>   	long size;
>>   	int ret;
>> +	XA_STATE(xas, &cache->reqs, 0);
>>   
>>   	if (!test_bit(CACHEFILES_ONDEMAND_MODE, &cache->flags))
>>   		return -EOPNOTSUPP;
>> @@ -149,9 +155,16 @@ int cachefiles_ondemand_copen(struct cachefiles_cache *cache, char *args)
>>   	if (ret)
>>   		return ret;
>>   
>> -	req = xa_erase(&cache->reqs, id);
>> -	if (!req)
>> +	xa_lock(&cache->reqs);
>> +	xas.xa_index = id;
>> +	req = xas_load(&xas);
>> +	if (!req || req->msg.opcode != CACHEFILES_OP_OPEN ||
>> +	    !req->object->ondemand->ondemand_id) {
>> +		xa_unlock(&cache->reqs);
>>   		return -EINVAL;
>> +	}
>> +	xas_store(&xas, NULL);
>> +	xa_unlock(&cache->reqs);
>>   
>>   	/* fail OPEN request if copen format is invalid */
>>   	ret = kstrtol(psize, 0, &size);
> The code looks good to me, but I still have some questions.
>
> First, what's the worst consequence if the daemon misbehaves like
> completing random copen/cread requests? I mean, does that affect other
> processes on the system besides the direct users of the ondemand mode,
> e.g. will the misbehavior cause system crash?
This can lead to system crashes, which can lead to a lot of problems.
For example, on reopen, to finish the read request, we might UAF in
ondemand_object_worker();
Or we might UAF in cachefiles_ondemand_daemon_read() when we
haven't added reference counts to the req yet.
Even though these issues are completely resolved in other ways,
I think some basic consistency checks are still necessary.
>
> Besides, it seems that the above security improvement is only "best
> effort".  It can not completely prevent a malicious misbehaved daemon
> from completing random copen/cread requests, right?
>
Yes, this doesn't solve the problem completely, we still can't
distinguish between the following cases:

1) different read reqs of the same object reusing the req id.
2) open reqs of different objects.

Ideally, we would calculate a checksum from
timestamps + struct cachefiles_msg to check if the requests
are consistent, but this breaks the uapi.

Thanks,
Baokun