by Gao Xiang

[permalink] [raw]

Subject: Re: [PATCH v2 4/5] cachefiles: cyclic allocation of msg_id to avoid reuse

On 2024/5/21 10:36, Baokun Li wrote:
> On 2024/5/20 22:56, Gao Xiang wrote:
>> Hi Baokun,
>>
>> On 2024/5/20 21:24, Baokun Li wrote:
>>> On 2024/5/20 20:54, Gao Xiang wrote:
>>>>
>>>>
>>>> On 2024/5/20 20:42, Baokun Li wrote:
>>>>> On 2024/5/20 18:04, Jeff Layton wrote:
>>>>>> On Mon, 2024-05-20 at 12:06 +0800, Baokun Li wrote:
>>>>>>> Hi Jeff,
>>>>>>>
>>>>>>> Thank you very much for your review!
>>>>>>>
>>>>>>> On 2024/5/19 19:11, Jeff Layton wrote:
>>>>>>>> On Wed, 2024-05-15 at 20:51 +0800, [email protected] wrote:
>>>>>>>>> From: Baokun Li <[email protected]>
>>>>>>>>>
>>>>>>>>> Reusing the msg_id after a maliciously completed reopen request may cause
>>>>>>>>> a read request to remain unprocessed and result in a hung, as shown below:
>>>>>>>>>
>>>>>>>>>          t1       |      t2       |      t3
>>>>>>>>> -------------------------------------------------
>>>>>>>>> cachefiles_ondemand_select_req
>>>>>>>>>    cachefiles_ondemand_object_is_close(A)
>>>>>>>>>    cachefiles_ondemand_set_object_reopening(A)
>>>>>>>>>    queue_work(fscache_object_wq, &info->work)
>>>>>>>>>                   ondemand_object_worker
>>>>>>>>> cachefiles_ondemand_init_object(A)
>>>>>>>>> cachefiles_ondemand_send_req(OPEN)
>>>>>>>>>                       // get msg_id 6
>>>>>>>>> wait_for_completion(&req_A->done)
>>>>>>>>> cachefiles_ondemand_daemon_read
>>>>>>>>>    // read msg_id 6 req_A
>>>>>>>>>    cachefiles_ondemand_get_fd
>>>>>>>>>    copy_to_user
>>>>>>>>>                                   // Malicious completion msg_id 6
>>>>>>>>>                                   copen 6,-1
>>>>>>>>> cachefiles_ondemand_copen
>>>>>>>>> complete(&req_A->done)
>>>>>>>>>                                    // will not set the object to close
>>>>>>>>>                                    // because ondemand_id && fd is valid.
>>>>>>>>>
>>>>>>>>>                   // ondemand_object_worker() is done
>>>>>>>>>                   // but the object is still reopening.
>>>>>>>>>
>>>>>>>>>                                   // new open req_B
>>>>>>>>> cachefiles_ondemand_init_object(B)
>>>>>>>>> cachefiles_ondemand_send_req(OPEN)
>>>>>>>>>                                    // reuse msg_id 6
>>>>>>>>> process_open_req
>>>>>>>>>    copen 6,A.size
>>>>>>>>>    // The expected failed copen was executed successfully
>>>>>>>>>
>>>>>>>>> Expect copen to fail, and when it does, it closes fd, which sets the
>>>>>>>>> object to close, and then close triggers reopen again. However, due to
>>>>>>>>> msg_id reuse resulting in a successful copen, the anonymous fd is not
>>>>>>>>> closed until the daemon exits. Therefore read requests waiting for reopen
>>>>>>>>> to complete may trigger hung task.
>>>>>>>>>
>>>>>>>>> To avoid this issue, allocate the msg_id cyclically to avoid reusing the
>>>>>>>>> msg_id for a very short duration of time.
>>>>>>>>>
>>>>>>>>> Fixes: c8383054506c ("cachefiles: notify the user daemon when looking up cookie")
>>>>>>>>> Signed-off-by: Baokun Li <[email protected]>
>>>>>>>>> ---
>>>>>>>>>    fs/cachefiles/internal.h | 1 +
>>>>>>>>>    fs/cachefiles/ondemand.c | 20 ++++++++++++++++----
>>>>>>>>>    2 files changed, 17 insertions(+), 4 deletions(-)
>>>>>>>>>
>>>>>>>>> diff --git a/fs/cachefiles/internal.h b/fs/cachefiles/internal.h
>>>>>>>>> index 8ecd296cc1c4..9200c00f3e98 100644
>>>>>>>>> --- a/fs/cachefiles/internal.h
>>>>>>>>> +++ b/fs/cachefiles/internal.h
>>>>>>>>> @@ -128,6 +128,7 @@ struct cachefiles_cache {
>>>>>>>>>        unsigned long            req_id_next;
>>>>>>>>>        struct xarray            ondemand_ids;    /* xarray for ondemand_id allocation */
>>>>>>>>>        u32                ondemand_id_next;
>>>>>>>>> +    u32                msg_id_next;
>>>>>>>>>    };
>>>>>>>>>    static inline bool cachefiles_in_ondemand_mode(struct cachefiles_cache *cache)
>>>>>>>>> diff --git a/fs/cachefiles/ondemand.c b/fs/cachefiles/ondemand.c
>>>>>>>>> index f6440b3e7368..b10952f77472 100644
>>>>>>>>> --- a/fs/cachefiles/ondemand.c
>>>>>>>>> +++ b/fs/cachefiles/ondemand.c
>>>>>>>>> @@ -433,20 +433,32 @@ static int cachefiles_ondemand_send_req(struct cachefiles_object *object,
>>>>>>>>>            smp_mb();
>>>>>>>>>            if (opcode == CACHEFILES_OP_CLOSE &&
>>>>>>>>> - !cachefiles_ondemand_object_is_open(object)) {
>>>>>>>>> + !cachefiles_ondemand_object_is_open(object)) {
>>>>>>>>> WARN_ON_ONCE(object->ondemand->ondemand_id == 0);
>>>>>>>>>                xas_unlock(&xas);
>>>>>>>>>                ret = -EIO;
>>>>>>>>>                goto out;
>>>>>>>>>            }
>>>>>>>>> -        xas.xa_index = 0;
>>>>>>>>> +        /*
>>>>>>>>> +         * Cyclically find a free xas to avoid msg_id reuse that would
>>>>>>>>> +         * cause the daemon to successfully copen a stale msg_id.
>>>>>>>>> +         */
>>>>>>>>> +        xas.xa_index = cache->msg_id_next;
>>>>>>>>>            xas_find_marked(&xas, UINT_MAX, XA_FREE_MARK);
>>>>>>>>> +        if (xas.xa_node == XAS_RESTART) {
>>>>>>>>> +            xas.xa_index = 0;
>>>>>>>>> +            xas_find_marked(&xas, cache->msg_id_next - 1, XA_FREE_MARK);
>>>>>>>>> +        }
>>>>>>>>>            if (xas.xa_node == XAS_RESTART)
>>>>>>>>>                xas_set_err(&xas, -EBUSY);
>>>>>>>>> +
>>>>>>>>>            xas_store(&xas, req);
>>>>>>>>> -        xas_clear_mark(&xas, XA_FREE_MARK);
>>>>>>>>> -        xas_set_mark(&xas, CACHEFILES_REQ_NEW);
>>>>>>>>> +        if (xas_valid(&xas)) {
>>>>>>>>> +            cache->msg_id_next = xas.xa_index + 1;
>>>>>>>> If you have a long-standing stuck request, could this counter wrap
>>>>>>>> around and you still end up with reuse?
>>>>>>> Yes, msg_id_next is declared to be of type u32 in the hope that when
>>>>>>> xa_index == UINT_MAX, a wrap around occurs so that msg_id_next
>>>>>>> goes to zero. Limiting xa_index to no more than UINT_MAX is to avoid
>>>>>>> the xarry being too deep.
>>>>>>>
>>>>>>> If msg_id_next is equal to the id of a long-standing stuck request
>>>>>>> after the wrap-around, it is true that the reuse in the above problem
>>>>>>> may also occur.
>>>>>>>
>>>>>>> But I feel that a long stuck request is problematic in itself, it means
>>>>>>> that after we have sent 4294967295 requests, the first one has not
>>>>>>> been processed yet, and even if we send a million requests per
>>>>>>> second, this one hasn't been completed for more than an hour.
>>>>>>>
>>>>>>> We have a keep-alive process that pulls the daemon back up as
>>>>>>> soon as it exits, and there is a timeout mechanism for requests in
>>>>>>> the daemon to prevent the kernel from waiting for long periods
>>>>>>> of time. In other words, we should avoid the situation where
>>>>>>> a request is stuck for a long period of time.
>>>>>>>
>>>>>>> If you think UINT_MAX is not enough, perhaps we could raise
>>>>>>> the maximum value of msg_id_next to ULONG_MAX?
>>>>>>>> Maybe this should be using
>>>>>>>> ida_alloc/free instead, which would prevent that too?
>>>>>>>>
>>>>>>> The id reuse here is that the kernel has finished the open request
>>>>>>> req_A and freed its id_A and used it again when sending the open
>>>>>>> request req_B, but the daemon is still working on req_A, so the
>>>>>>> copen id_A succeeds but operates on req_B.
>>>>>>>
>>>>>>> The id that is being used by the kernel will not be allocated here
>>>>>>> so it seems that ida _alloc/free does not prevent reuse either,
>>>>>>> could you elaborate a bit more how this works?
>>>>>>>
>>>>>> ida_alloc and free absolutely prevent reuse while the id is in use.
>>>>>> That's sort of the point of those functions. Basically it uses a set of
>>>>>> bitmaps in an xarray to track which IDs are in use, so ida_alloc only
>>>>>> hands out values which are not in use. See the comments over
>>>>>> ida_alloc_range() in lib/idr.c.
>>>>>>
>>>>> Thank you for the explanation!
>>>>>
>>>>> The logic now provides the same guarantees as ida_alloc/free.
>>>>> The "reused" id, indeed, is no longer in use in the kernel, but it is still
>>>>> in use in the userland, so a multi-threaded daemon could be handling
>>>>> two different requests for the same msg_id at the same time.
>>>>>
>>>>> Previously, the logic for allocating msg_ids was to start at 0 and look
>>>>> for a free xas.index, so it was possible for an id to be allocated to a
>>>>> new request just as the id was being freed.
>>>>>
>>>>> With the change to cyclic allocation, the kernel will not use the same
>>>>> id again until INT_MAX requests have been sent, and during the time
>>>>> it takes to send requests, the daemon has enough time to process
>>>>> requests whose ids are still in use by the daemon, but have already
>>>>> been freed in the kernel.
>>>>
>>>> Again, If I understand correctly, I think the main point
>>>> here is
>>>>
>>>> wait_for_completion(&req_A->done)
>>>>
>>>> which could hang due to some malicious deamon. But I think it
>>>> should be switched to wait_for_completion_killable() instead. *
>>>> It's up to users to kill the mount instance if there is a
>>>> malicious user daemon.
>>>>
>>>> So in that case, hung task will not be triggered anymore, and
>>>> you don't need to care about cyclic allocation too.
>>>>
>>>> Thanks,
>>>> Gao Xiang
>>> Hi Xiang,
>>>
>>> The problem is not as simple as you think.
>>>
>>> If you make it killable, it just won't trigger a hung task in
>>> cachefiles_ondemand_send_req(), and the process waiting for the
>>> resource in question will also be hung.
>>>
>>> * When the open/read request in the mount process gets stuck,
>>>    the sync/drop cache will trigger a hung task panic in iterate_supers()
>>>    as it waits for sb->umount to be unlocked.
>>> * After umount, anonymous fd is not closed causing a hung task panic
>>>    in fscache_hash_cookie() because of waiting for cookie unhash.
>>> * The dentry is in a loop up state, because the read request is not being
>>>    processed, another process looking for the same dentry is waiting for
>>>    the previous lookup to finish, which triggers a hung task panic in
>>>    d_alloc_parallel().
>>
>>
>> As for your sb->umount, d_alloc_parallel() or even i_rwsem,
>> which are all currently unkillable, also see some previous
>> threads like:
>>
>> https://lore.kernel.org/linux-fsdevel/CAJfpegu6v1fRAyLvFLOPUSAhx5aAGvPGjBWv-TDQjugqjUA_hQ@mail.gmail.com/T/#u
>>
>> I don't think it's the issue of on-demand cachefiles, even
>> NVMe or virtio-blk or networking can be stuck in
>> .lookup, fill_sb or whatever.
>>
>> Which can makes sb->umount, d_alloc_parallel() or even
>> i_rwsem unkillable.
>>
> Everyone and every company has different requirements for quality,
> and if you don't think these hung_task_panic are problems, I respect
> your opinion.
>
> But the company I work for has much higher requirements for quality,
> and it's not acceptable to leave these issues that have been tested out
> unresolved.
>>>
>>> Can all this be made killable?
>>
>> I can understand your hung_task_panic concern but it
>> sounds like a workaround to me anyway.
>>
>> Thanks,
>> Gao Xiang
>>
> From the current perspective, cyclic allocation is a valid solution to
> the current msg_id collision problem, and it also makes it fairer to
> copy out requests than it was before.

Okay, for this patch, I agree it's better than none and it can
indeed cause fairer requests, so

Reviewed-by: Gao Xiang <[email protected]>

Thanks,
Gao Xiang

>
> Thanks!
>