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Message-ID: <e522702477bed6e73c1e365bd8bd77a4250955c2.camel@kernel.org>
Date: Mon, 20 May 2024 09:24:27 -0400
From: Jeff Layton <jlayton@...nel.org>
To: Baokun Li <libaokun@...weicloud.com>, netfs@...ts.linux.dev,
dhowells@...hat.com
Cc: hsiangkao@...ux.alibaba.com, jefflexu@...ux.alibaba.com,
zhujia.zj@...edance.com, linux-erofs@...ts.ozlabs.org,
linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
yangerkun@...wei.com, houtao1@...wei.com, yukuai3@...wei.com,
wozizhi@...wei.com, Baokun Li <libaokun1@...wei.com>
Subject: Re: [PATCH v2 4/5] cachefiles: cyclic allocation of msg_id to avoid
reuse
On Mon, 2024-05-20 at 20:42 +0800, 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,
> > > > libaokun@...weicloud.com wrote:
> > > > > From: Baokun Li <libaokun1@...wei.com>
> > > > >
> > > > > 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 <libaokun1@...wei.com>
> > > > > ---
> > > > > 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.
>
>
If you're checking for collisions somewhere else, then this should be
fine:
Acked-by: Jeff Layton <jlayton@...nel.org>
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