Re: [syzbot ci] Re: io_uring: avoid uring_lock for IORING_SETUP_SINGLE_ISSUER

Caleb Sander Mateos
2025-09-04T16:50:50Z

On Thu, Sep 4, 2025 at 9:46 AM Caleb Sander Mateos
<csander@purestorage.com> wrote:
>
> On Thu, Sep 4, 2025 at 7:52 AM Caleb Sander Mateos
> <csander@purestorage.com> wrote:
> >
> > On Wed, Sep 3, 2025 at 4:30 PM Jens Axboe <axboe@kernel.dk> wrote:
> > >
> > > On 9/3/25 3:55 PM, syzbot ci wrote:
> > > > syzbot ci has tested the following series
> > > >
> > > > [v1] io_uring: avoid uring_lock for IORING_SETUP_SINGLE_ISSUER
> > > > https://lore.kernel.org/all/20250903032656.2012337-1-csander@purestorage.com
> > > > * [PATCH 1/4] io_uring: don't include filetable.h in io_uring.h
> > > > * [PATCH 2/4] io_uring/rsrc: respect submitter_task in io_register_clone_buffers()
> > > > * [PATCH 3/4] io_uring: factor out uring_lock helpers
> > > > * [PATCH 4/4] io_uring: avoid uring_lock for IORING_SETUP_SINGLE_ISSUER
> > > >
> > > > and found the following issue:
> > > > WARNING in io_handle_tw_list
> > > >
> > > > Full report is available here:
> > > > https://ci.syzbot.org/series/54ae0eae-5e47-4cfe-9ae7-9eaaf959b5ae
> > > >
> > > > ***
> > > >
> > > > WARNING in io_handle_tw_list
> > > >
> > > > tree: linux-next
> > > > URL: https://kernel.googlesource.com/pub/scm/linux/kernel/git/next/linux-next
> > > > base: 5d50cf9f7cf20a17ac469c20a2e07c29c1f6aab7
> > > > arch: amd64
> > > > compiler: Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
> > > > config: https://ci.syzbot.org/builds/1de646dd-4ee2-418d-9c62-617d88ed4fd2/config
> > > > syz repro: https://ci.syzbot.org/findings/e229a878-375f-4286-89fe-b6724c23addd/syz_repro
> > > >
> > > > ------------[ cut here ]------------
> > > > WARNING: io_uring/io_uring.h:127 at io_ring_ctx_lock io_uring/io_uring.h:127 [inline], CPU#1: iou-sqp-6294/6297
> > > > WARNING: io_uring/io_uring.h:127 at io_handle_tw_list+0x234/0x2e0 io_uring/io_uring.c:1155, CPU#1: iou-sqp-6294/6297
> > > > Modules linked in:
> > > > CPU: 1 UID: 0 PID: 6297 Comm: iou-sqp-6294 Not tainted syzkaller #0 PREEMPT(full)
> > > > Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
> > > > RIP: 0010:io_ring_ctx_lock io_uring/io_uring.h:127 [inline]
> > > > RIP: 0010:io_handle_tw_list+0x234/0x2e0 io_uring/io_uring.c:1155
> > > > Code: 00 00 48 c7 c7 e0 90 02 8c be 8e 04 00 00 31 d2 e8 01 e5 d2 fc 2e 2e 2e 31 c0 45 31 e4 4d 85 ff 75 89 eb 7c e8 ad fb 00 fd 90 <0f> 0b 90 e9 cf fe ff ff 89 e9 80 e1 07 80 c1 03 38 c1 0f 8c 22 ff
> > > > RSP: 0018:ffffc900032cf938 EFLAGS: 00010293
> > > > RAX: ffffffff84bfcba3 RBX: dffffc0000000000 RCX: ffff888107f61cc0
> > > > RDX: 0000000000000000 RSI: 0000000000001000 RDI: 0000000000000000
> > > > RBP: ffff8881119a8008 R08: ffff888110bb69c7 R09: 1ffff11022176d38
> > > > R10: dffffc0000000000 R11: ffffed1022176d39 R12: ffff8881119a8000
> > > > R13: ffff888108441e90 R14: ffff888107f61cc0 R15: 0000000000000000
> > > > FS: 00007f81f25716c0(0000) GS:ffff8881a39f5000(0000) knlGS:0000000000000000
> > > > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > > > CR2: 0000001b31b63fff CR3: 000000010f24c000 CR4: 00000000000006f0
> > > > Call Trace:
> > > > <TASK>
> > > > tctx_task_work_run+0x99/0x370 io_uring/io_uring.c:1223
> > > > io_sq_tw io_uring/sqpoll.c:244 [inline]
> > > > io_sq_thread+0xed1/0x1e50 io_uring/sqpoll.c:327
> > > > ret_from_fork+0x47f/0x820 arch/x86/kernel/process.c:148
> > > > ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
> > > > </TASK>
> > >
> > > Probably the sanest thing to do here is to clear
> > > IORING_SETUP_SINGLE_ISSUER if it's set with IORING_SETUP_SQPOLL. If we
> > > allow it, it'll be impossible to uphold the locking criteria on both the
> > > issue and register side.
> >
> > Yup, I was thinking the same thing. Thanks for taking a look.
>
> On further thought, IORING_SETUP_SQPOLL actually does guarantee a
> single issuer. io_uring_enter() already avoids taking the uring_lock
> in the IORING_SETUP_SQPOLL case because it doesn't issue any SQEs
> itself. Only the SQ thread does that, so it *is* the single issuer.
> The assertions I added in io_ring_ctx_lock()/io_ring_ctx_unlock() is
> just unnecessarily strict. It should expect current ==
> ctx->sq_data->thread in the IORING_SETUP_SQPOLL case.

Oh, but you are totally correct about needing the mutex to synchronize
between issue on the SQ thread and io_uring_register() on other
threads. Yeah, I don't see an easy way to avoid taking the mutex on
the SQ thread unless we disallowed io_uring_register() completely.
Clearing IORING_SETUP_SINGLE_ISSUER seems like the best option for
now.

Best,
Caleb