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[195.14.221.121]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-c2458b096d9sm112325966b.20.2026.08.20.11.56.35 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 20 Aug 2026 11:56:35 -0700 (PDT) Message-ID: <165bc4ff-ec69-4058-b6a2-3db02410ce58@gmail.com> Date: Thu, 20 Aug 2026 20:56:34 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH] io_uring/cmd: don't skip completion for a non-armed multishot uring_cmd To: Jens Axboe , Ming Lei Cc: io-uring@vger.kernel.org, linux-kernel@vger.kernel.org, syzbot+a4ccdd7ebf452e4d4701@syzkaller.appspotmail.com References: <20260811-io_uring-v1-1-1dc3dd50e0cd@gmail.com> Content-Language: en-US From: Vasileios Almpanis In-Reply-To: <20260811-io_uring-v1-1-1dc3dd50e0cd@gmail.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Hi everyone, Just a gentle ping on this patch. The issue is syzbot-reproduced and the proposed fix has been run with #syz test. The patch addresses the leak in the core |io_uring_cmd()|path rather than relying on individual providers to handle |IORING_URING_CMD_MULTISHOT|. Could someone take a look when you get a chance? Thanks, Vasileios On 8/11/26 10:42 PM, Vasileios Almpanis wrote: > io_uring_cmd() treats any uring_cmd carrying IORING_URING_CMD_MULTISHOT > that returns >= 0 as "multishot armed, completion deferred" and returns > IOU_ISSUE_SKIP_COMPLETE, expecting the provider to complete the request > later. But the flag is user-controlled and validated only against buffer > select, not against provider capability. A ->uring_cmd() handler that > does not implement multishot and returns a normal >= 0 result then has > its request skipped and never completed, leaking the io_kiocb and its > io_async_cmd: > > BUG: memory leak > unreferenced object (size 248): > kmem_cache_alloc_bulk_noprof+0x272/0x3f0 > __io_alloc_req_refill+0x4a/0x150 > io_submit_sqes.cold+0x16e/0x20b > __do_sys_io_uring_enter+0x56d/0xd60 > > syzbot hit this via ublk UBLK_U_CMD_ADD_DEV, but it is kernel-wide: the > same leak reproduces with SOCKET_URING_OP_SIOCINQ on any socket fd, which > returns the queued byte count and never inspects cmd->flags. No in-tree > provider of multishot actually returns >= 0. Both io_cmd_poll_multishot() > and ublk_handle_batch_fetch_cmd return -EIOCBQUEUED, > > Only skip completion when the command is really multishot, i.e. > REQ_F_APOLL_MULTISHOT is set. Otherwise fall through and complete the > request normally with its result. > > Fixes: 620a50c92700 ("io_uring: uring_cmd: add multishot support") > Reported-by: syzbot+a4ccdd7ebf452e4d4701@syzkaller.appspotmail.com > Closes: https://syzkaller.appspot.com/bug?extid=a4ccdd7ebf452e4d4701 > Tested-by: syzbot+a4ccdd7ebf452e4d4701@syzkaller.appspotmail.com > Signed-off-by: Vasileios Almpanis > --- > Questions / Notes: > - I put this in the core io_uring_cmd() rather than in provider specific > code because the leak is independent of the provider. From my research > so far no ->uring_cmd() rejects IORING_URING_CMD_MULTISHOT, they just > ignore it. The flag is only validated against buffer-select in > io_uring_cmd_prep(). So any handler that returns a plain >= 0 result > with the user-set flag leaks. > - Instead of checking if it has been really armed should we just drop > the >= 0 check? The in-tree code that supports multishot returns > -EIOCBQUEUED from what I have seen so far. > --- > io_uring/uring_cmd.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/io_uring/uring_cmd.c b/io_uring/uring_cmd.c > index c14c22cff49e..a2899a852879 100644 > --- a/io_uring/uring_cmd.c > +++ b/io_uring/uring_cmd.c > @@ -269,7 +269,8 @@ int io_uring_cmd(struct io_kiocb *req, unsigned int issue_flags) > } > > ret = file->f_op->uring_cmd(ioucmd, issue_flags); > - if (ioucmd->flags & IORING_URING_CMD_MULTISHOT) { > + if ((ioucmd->flags & IORING_URING_CMD_MULTISHOT) && > + (req->flags & REQ_F_APOLL_MULTISHOT)) { > if (ret >= 0) > return IOU_ISSUE_SKIP_COMPLETE; > } > > --- > base-commit: d58772d8520c7ef247c4b95c9bd76d3a25da9ff5 > change-id: 20260811-io_uring-169337c74617 > > Best regards, > -- > Vasileios Almpanis >