[PATCH 0/2] fuse: Fix possible memleak at startup with immediate teardown

[PATCH 0/2] fuse: Fix possible memleak at startup with immediate teardown

[Bernd Schubert]

Do not merge yet, the current series has not been tested yet.
The race is only easily reproducible with additional patches that
pin pages during FUSE_IO_URING_CMD_REGISTER - slows it down and then
xfstest's generic/001 triggers it reliably. However, I need to update
these pin patches for linux master.

Signed-off-by: Bernd Schubert <bschubert@ddn.com>
---
Bernd Schubert (1):
      fuse: Move ring queues_refs decrement

Jian Huang Li (1):
      fs/fuse: fix potential memory leak from fuse_uring_cancel

 fs/fuse/dev_uring.c | 33 ++++++++++++++-------------------
 1 file changed, 14 insertions(+), 19 deletions(-)
---
base-commit: 6548d364a3e850326831799d7e3ea2d7bb97ba08
change-id: 20251021-io-uring-fixes-cancel-mem-leak-820642677c37

Best regards,
-- 
Bernd Schubert <bschubert@ddn.com>

[PATCH 2/2] fs/fuse: fix potential memory leak from fuse_uring_cancel

[Bernd Schubert]

From: Jian Huang Li <ali@ddn.com>

This issue could be observed sometimes during libfuse xfstests, from
dmseg prints some like "kernel: WARNING: CPU: 4 PID: 0 at
fs/fuse/dev_uring.c:204 fuse_uring_destruct+0x1f5/0x200 [fuse]".

The cause is, if when fuse daemon just submitted
FUSE_IO_URING_CMD_REGISTER SQEs, then umount or fuse daemon quits at
this very early stage. After all uring queues stopped, might have one or
more unprocessed FUSE_IO_URING_CMD_REGISTER SQEs get processed then some
new ring entities are created and added to ent_avail_queue, and
immediately fuse_uring_cancel moved them to ent_in_userspace after SQEs
get canceled. These ring entities were not moved to ent_released, and
stayed in ent_in_userspace when fuse_uring_destruct was called.

One way to solve it would be to also free 'ent_in_userspace' in
fuse_uring_destruct(), but from code point of view it is hard to see why
it is needed. As suggested by Joanne, another solution is to avoid moving
entries in fuse_uring_cancel() to the 'ent_in_userspace' list and just
releasing them directly.

Fixes: b6236c8407cb ("fuse: {io-uring} Prevent mount point hang on fuse-server termination")
Cc: Joanne Koong <joannelkoong@gmail.com>
Cc: <stable@vger.kernel.org> # v6.14
Signed-off-by: Jian Huang Li <ali@ddn.com>
Signed-off-by: Bernd Schubert <bschubert@ddn.com>
---
 fs/fuse/dev_uring.c | 21 +++++++++------------
 1 file changed, 9 insertions(+), 12 deletions(-)

diff --git a/fs/fuse/dev_uring.c b/fs/fuse/dev_uring.c
index e7c1095b83b11fe46080c24f539df17e70969e21..d88a0c05434a04668241f09f123d5e3a9cc1621d 100644
--- a/fs/fuse/dev_uring.c
+++ b/fs/fuse/dev_uring.c
@@ -324,7 +324,7 @@ static void fuse_uring_stop_fuse_req_end(struct fuse_req *req)
 /*
  * Release a request/entry on connection tear down
  */
-static void fuse_uring_entry_teardown(struct fuse_ring_ent *ent)
+static void fuse_uring_entry_teardown(struct fuse_ring_ent *ent, int issue_flags)
 {
 	struct fuse_req *req;
 	struct io_uring_cmd *cmd;
@@ -352,7 +352,7 @@ static void fuse_uring_entry_teardown(struct fuse_ring_ent *ent)
 	spin_unlock(&queue->lock);
 
 	if (cmd)
-		io_uring_cmd_done(cmd, -ENOTCONN, IO_URING_F_UNLOCKED);
+		io_uring_cmd_done(cmd, -ENOTCONN, issue_flags);
 
 	if (req)
 		fuse_uring_stop_fuse_req_end(req);
@@ -383,7 +383,7 @@ static void fuse_uring_stop_list_entries(struct list_head *head,
 
 	/* no queue lock to avoid lock order issues */
 	list_for_each_entry_safe(ent, next, &to_teardown, list)
-		fuse_uring_entry_teardown(ent);
+		fuse_uring_entry_teardown(ent, IO_URING_F_UNLOCKED);
 }
 
 static void fuse_uring_teardown_entries(struct fuse_ring_queue *queue)
@@ -499,7 +499,7 @@ static void fuse_uring_cancel(struct io_uring_cmd *cmd,
 {
 	struct fuse_ring_ent *ent = uring_cmd_to_ring_ent(cmd);
 	struct fuse_ring_queue *queue;
-	bool need_cmd_done = false;
+	bool teardown = false;
 
 	/*
 	 * direct access on ent - it must not be destructed as long as
@@ -508,17 +508,14 @@ static void fuse_uring_cancel(struct io_uring_cmd *cmd,
 	queue = ent->queue;
 	spin_lock(&queue->lock);
 	if (ent->state == FRRS_AVAILABLE) {
-		ent->state = FRRS_USERSPACE;
-		list_move_tail(&ent->list, &queue->ent_in_userspace);
-		need_cmd_done = true;
-		ent->cmd = NULL;
+		ent->state = FRRS_TEARDOWN;
+		list_del_init(&ent->list);
+		teardown = true;
 	}
 	spin_unlock(&queue->lock);
 
-	if (need_cmd_done) {
-		/* no queue lock to avoid lock order issues */
-		io_uring_cmd_done(cmd, -ENOTCONN, issue_flags);
-	}
+	if (teardown)
+		fuse_uring_entry_teardown(ent, issue_flags);
 }
 
 static void fuse_uring_prepare_cancel(struct io_uring_cmd *cmd, int issue_flags,

-- 
2.43.0

Re: [PATCH 0/2] fuse: Fix possible memleak at startup with immediate teardown

[Bernd Schubert]

On 10/21/25 23:33, Bernd Schubert wrote:
> Do not merge yet, the current series has not been tested yet.

I'm glad that that I was hesitating to apply it, the DDN branch had it
for ages and this patch actually introduced a possible fc->num_waiting
issue, because fc->uring->queue_refs might go down to 0 though
fuse_uring_cancel() and then fuse_uring_abort() would never stop and
flush the queues without another addition.

Thanks,
Bernd

> The race is only easily reproducible with additional patches that
> pin pages during FUSE_IO_URING_CMD_REGISTER - slows it down and then
> xfstest's generic/001 triggers it reliably. However, I need to update
> these pin patches for linux master.
> 
> Signed-off-by: Bernd Schubert <bschubert@ddn.com>
> ---
> Bernd Schubert (1):
>       fuse: Move ring queues_refs decrement
> 
> Jian Huang Li (1):
>       fs/fuse: fix potential memory leak from fuse_uring_cancel
> 
>  fs/fuse/dev_uring.c | 33 ++++++++++++++-------------------
>  1 file changed, 14 insertions(+), 19 deletions(-)
> ---
> base-commit: 6548d364a3e850326831799d7e3ea2d7bb97ba08
> change-id: 20251021-io-uring-fixes-cancel-mem-leak-820642677c37
> 
> Best regards,

Re: [PATCH 0/2] fuse: Fix possible memleak at startup with immediate teardown

[Joanne Koong]

On Thu, Apr 9, 2026 at 4:02 AM Bernd Schubert <bernd@bsbernd.com> wrote:
>
>
>
> On 10/21/25 23:33, Bernd Schubert wrote:
> > Do not merge yet, the current series has not been tested yet.
>
> I'm glad that that I was hesitating to apply it, the DDN branch had it
> for ages and this patch actually introduced a possible fc->num_waiting
> issue, because fc->uring->queue_refs might go down to 0 though
> fuse_uring_cancel() and then fuse_uring_abort() would never stop and
> flush the queues without another addition.
>

Hi Bernd and Jian,

For some reason the "[PATCH 2/2] fs/fuse: fix potential memory leak
from fuse_uring_cancel" email was never delivered to my inbox, so I am
just going to write my reply to that patch here instead, hope that's
ok.

Just to summarize, the race is that during unmount, fuse_abort() ->
fuse_uring_abort() -> ... -> fuse_uring_teardown_entries() -> ... ->
fuse_uring_entry_teardown() gets run but there may still be sqes that
are being registered, which results in new ents that are created (and
leaked) after the teardown logic has finished and the queues are
stopped/dead. The async teardown work (fuse_uring_async_stop_queues())
never gets scheduled because at the time of teardown, queue->refs is 0
as those sqes have not fully created the ents and grabbed refs yet.
fuse_uring_destruct() runs during unmount, but this doesn't clean up
the created ents because those registered ents got put on the
ent_in_userspace list which fuse_uring_destruct() doesn't go through
to free, resulting in those ents being leaked.

The root cause of the race is that ents are being registered even when
the queue is already stopped/dead. I think if we at registration time
check the queue state before calling fuse_uring_prepare_cancel(), we
eliminate the race altogether. If we see that the abort path has
already triggered (eg queue->stopped == true), we manually free the
ent and return an error instead of adding it to a list, eg

diff --git a/fs/fuse/dev_uring.c b/fs/fuse/dev_uring.c
index d88a0c05434a..351c19150aae 100644
--- a/fs/fuse/dev_uring.c
+++ b/fs/fuse/dev_uring.c
@@ -969,7 +969,7 @@ static bool is_ring_ready(struct fuse_ring *ring,
int current_qid)
 /*
  * fuse_uring_req_fetch command handling
  */
-static void fuse_uring_do_register(struct fuse_ring_ent *ent,
+static int fuse_uring_do_register(struct fuse_ring_ent *ent,
                                   struct io_uring_cmd *cmd,
                                   unsigned int issue_flags)
 {
@@ -978,6 +978,16 @@ static void fuse_uring_do_register(struct
fuse_ring_ent *ent,
        struct fuse_conn *fc = ring->fc;
        struct fuse_iqueue *fiq = &fc->iq;

+       spin_lock(&queue->lock);
+       /* abort teardown path is running or has run */
+       if (queue->stopped) {
+               spin_unlock(&queue->lock);
+               atomic_dec(&ring->queue_refs);
+               kfree(ent);
+               return -ECONNABORTED;
+       }
+       spin_unlock(&queue->lock);
+
        fuse_uring_prepare_cancel(cmd, issue_flags, ent);

        spin_lock(&queue->lock);
@@ -994,6 +1004,7 @@ static void fuse_uring_do_register(struct
fuse_ring_ent *ent,
                        wake_up_all(&fc->blocked_waitq);
                }
        }
+       return 0;
 }

 /*
@@ -1109,9 +1120,7 @@ static int fuse_uring_register(struct io_uring_cmd *cmd,
        if (IS_ERR(ent))
                return PTR_ERR(ent);

-       fuse_uring_do_register(ent, cmd, issue_flags);
-
-       return 0;
+       return fuse_uring_do_register(ent, cmd, issue_flags);
 }

There's the scenario where the abort path's "queue->stopped = true"
gets set right between when we drop the queue lock and before we call
fuse_uring_prepare_cancel(), but the fuse_uring_create_ring_ent()
logic that was called before fuse_uring_do_register() has already
grabbed the ref on ring->queue_refs, which means in the abort path,
the async teardown (fuse_uring_async_stop_queues()) work is guaranteed
to run and clean up / free the entry.

Thanks,
Joanne

> Thanks,
> Bernd
>
> > The race is only easily reproducible with additional patches that
> > pin pages during FUSE_IO_URING_CMD_REGISTER - slows it down and then
> > xfstest's generic/001 triggers it reliably. However, I need to update
> > these pin patches for linux master.
> >
> > Signed-off-by: Bernd Schubert <bschubert@ddn.com>
> > ---
> > Bernd Schubert (1):
> >       fuse: Move ring queues_refs decrement
> >
> > Jian Huang Li (1):
> >       fs/fuse: fix potential memory leak from fuse_uring_cancel
> >
> >  fs/fuse/dev_uring.c | 33 ++++++++++++++-------------------
> >  1 file changed, 14 insertions(+), 19 deletions(-)
> > ---
> > base-commit: 6548d364a3e850326831799d7e3ea2d7bb97ba08
> > change-id: 20251021-io-uring-fixes-cancel-mem-leak-820642677c37
> >
> > Best regards,
>

Re: Re: [PATCH 0/2] fuse: Fix possible memleak at startup with immediate teardown

[Horst Birthelmer]

On Thu, Apr 09, 2026 at 04:09:53PM -0700, Joanne Koong wrote:
> On Thu, Apr 9, 2026 at 4:02 AM Bernd Schubert <bernd@bsbernd.com> wrote:
> >
> >
> >
> > On 10/21/25 23:33, Bernd Schubert wrote:
> > > Do not merge yet, the current series has not been tested yet.
> >
> > I'm glad that that I was hesitating to apply it, the DDN branch had it
> > for ages and this patch actually introduced a possible fc->num_waiting
> > issue, because fc->uring->queue_refs might go down to 0 though
> > fuse_uring_cancel() and then fuse_uring_abort() would never stop and
> > flush the queues without another addition.
> >
> 
> Hi Bernd and Jian,
> 
> For some reason the "[PATCH 2/2] fs/fuse: fix potential memory leak
> from fuse_uring_cancel" email was never delivered to my inbox, so I am
> just going to write my reply to that patch here instead, hope that's
> ok.
> 
> Just to summarize, the race is that during unmount, fuse_abort() ->
> fuse_uring_abort() -> ... -> fuse_uring_teardown_entries() -> ... ->
> fuse_uring_entry_teardown() gets run but there may still be sqes that
> are being registered, which results in new ents that are created (and
> leaked) after the teardown logic has finished and the queues are
> stopped/dead. The async teardown work (fuse_uring_async_stop_queues())
> never gets scheduled because at the time of teardown, queue->refs is 0
> as those sqes have not fully created the ents and grabbed refs yet.
> fuse_uring_destruct() runs during unmount, but this doesn't clean up
> the created ents because those registered ents got put on the
> ent_in_userspace list which fuse_uring_destruct() doesn't go through
> to free, resulting in those ents being leaked.
> 
> The root cause of the race is that ents are being registered even when
> the queue is already stopped/dead. I think if we at registration time
> check the queue state before calling fuse_uring_prepare_cancel(), we
> eliminate the race altogether. If we see that the abort path has
> already triggered (eg queue->stopped == true), we manually free the
> ent and return an error instead of adding it to a list, eg

In my case (Bernd mentioned that I was investigating a hang during umount)
there were a lot of requests created during teardown, so what happened
was very similar, but for exact the opposite reason.
In fuse_uring_abort() queue_refs was already 0 due to an optimization
where the ring teardown ran before fuse_abort_conn(). 
Thus the queue->stopped was never set.

How do we make sure that fuse_uring_teardown_entries() has not been
called by fuse_uring_async_stop_queues()?

Maybe I'm missing something?

My fix was to remove the check for queue_refs > 0 in fuse_uring_abort()
and make sure that even if the teardown was complete nothing bad happens
in fuse_uring_abort_end_requests() and fuse_uring_stop_queues().

Thanks,
Horst