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

[Jens Axboe <axboe@kernel.dk>]

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.

-- 
Jens Axboe