[PATCH] io_uring/kbuf: ensure ring ctx is held locked over io_put_kbuf()

[PATCH] io_uring/kbuf: ensure ring ctx is held locked over io_put_kbuf()

[Jens Axboe]

The issue_flags will tell us if this is needed or not, however a
previous commit was a bit too eager with the cleanups and removed the
required locking in case IO_URING_F_UNLOCKED is set in the issue_flags.

Cc: stable@vger.kernel.org
Fixes: e150e70fce42 ("io_uring/kbuf: open code __io_put_kbuf()")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

---

diff --git a/io_uring/kbuf.h b/io_uring/kbuf.h
index 723d0361898e..7f17e87d8617 100644
--- a/io_uring/kbuf.h
+++ b/io_uring/kbuf.h
@@ -4,6 +4,7 @@
 
 #include <uapi/linux/io_uring.h>
 #include <linux/io_uring_types.h>
+#include "io_uring.h"
 
 enum {
 	/* ring mapped provided buffers */
@@ -124,9 +125,14 @@ static inline bool io_kbuf_recycle(struct io_kiocb *req, unsigned issue_flags)
 static inline unsigned int io_put_kbuf(struct io_kiocb *req, int len,
 				       unsigned issue_flags)
 {
+	int ret;
+
 	if (!(req->flags & (REQ_F_BUFFER_RING | REQ_F_BUFFER_SELECTED)))
 		return 0;
-	return __io_put_kbufs(req, len, 1);
+	io_ring_submit_lock(req->ctx, issue_flags);
+	ret = __io_put_kbufs(req, len, 1);
+	io_ring_submit_unlock(req->ctx, issue_flags);
+	return ret;
 }
 
 static inline unsigned int io_put_kbufs(struct io_kiocb *req, int len,

-- 
Jens Axboe

Re: [PATCH] io_uring/kbuf: ensure ring ctx is held locked over io_put_kbuf()

[Jens Axboe]

On 8/20/25 5:16 PM, Jens Axboe wrote:
> The issue_flags will tell us if this is needed or not, however a
> previous commit was a bit too eager with the cleanups and removed the
> required locking in case IO_URING_F_UNLOCKED is set in the issue_flags.

Actually, disregard that one - on closer inspection, it doesn't matter
anymore. On the legacy side, it's just being freed. For ring buffers,
they would already have been committed.

-- 
Jens Axboe

[syzbot ci] Re: io_uring/kbuf: ensure ring ctx is held locked over io_put_kbuf()

[syzbot ci]

syzbot ci has tested the following series

[v1] io_uring/kbuf: ensure ring ctx is held locked over io_put_kbuf()
https://lore.kernel.org/all/e2f14b20-2ad4-4e59-9966-26dd6aa70f31@kernel.dk
* [PATCH] io_uring/kbuf: ensure ring ctx is held locked over io_put_kbuf()

and found the following issue:
possible deadlock in io_req_defer_failed

Full report is available here:
https://ci.syzbot.org/series/fd113bf0-fc76-46b7-8c0b-08fa3c8bda14

***

possible deadlock in io_req_defer_failed

tree:      torvalds
URL:       https://kernel.googlesource.com/pub/scm/linux/kernel/git/torvalds/linux
base:      dfc0f6373094dd88e1eaf76c44f2ff01b65db851
arch:      amd64
compiler:  Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7
config:    https://ci.syzbot.org/builds/e832762c-b6ee-4bb3-b01f-da0e8f70d1d0/config
C repro:   https://ci.syzbot.org/findings/57691bf2-072c-42f3-8742-80414426104c/c_repro
syz repro: https://ci.syzbot.org/findings/57691bf2-072c-42f3-8742-80414426104c/syz_repro

============================================
WARNING: possible recursive locking detected
syzkaller #0 Not tainted
--------------------------------------------
syz.0.17/6012 is trying to acquire lock:
ffff88802aa620a8 (&ctx->uring_lock){+.+.}-{4:4}, at: io_ring_submit_lock io_uring/io_uring.h:287 [inline]
ffff88802aa620a8 (&ctx->uring_lock){+.+.}-{4:4}, at: io_put_kbuf io_uring/kbuf.h:132 [inline]
ffff88802aa620a8 (&ctx->uring_lock){+.+.}-{4:4}, at: io_req_defer_failed+0x166/0x550 io_uring/io_uring.c:988

but task is already holding lock:
ffff88802aa620a8 (&ctx->uring_lock){+.+.}-{4:4}, at: __do_sys_io_uring_enter io_uring/io_uring.c:3463 [inline]
ffff88802aa620a8 (&ctx->uring_lock){+.+.}-{4:4}, at: __se_sys_io_uring_enter+0x2d4/0x2b20 io_uring/io_uring.c:3398

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(&ctx->uring_lock);
  lock(&ctx->uring_lock);

 *** DEADLOCK ***

 May be due to missing lock nesting notation

1 lock held by syz.0.17/6012:
 #0: ffff88802aa620a8 (&ctx->uring_lock){+.+.}-{4:4}, at: __do_sys_io_uring_enter io_uring/io_uring.c:3463 [inline]
 #0: ffff88802aa620a8 (&ctx->uring_lock){+.+.}-{4:4}, at: __se_sys_io_uring_enter+0x2d4/0x2b20 io_uring/io_uring.c:3398

stack backtrace:
CPU: 0 UID: 0 PID: 6012 Comm: syz.0.17 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
Call Trace:
 <TASK>
 dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
 print_deadlock_bug+0x28b/0x2a0 kernel/locking/lockdep.c:3041
 check_deadlock kernel/locking/lockdep.c:3093 [inline]
 validate_chain+0x1a3f/0x2140 kernel/locking/lockdep.c:3895
 __lock_acquire+0xab9/0xd20 kernel/locking/lockdep.c:5237
 lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5868
 __mutex_lock_common kernel/locking/mutex.c:598 [inline]
 __mutex_lock+0x187/0x1360 kernel/locking/mutex.c:760
 io_ring_submit_lock io_uring/io_uring.h:287 [inline]
 io_put_kbuf io_uring/kbuf.h:132 [inline]
 io_req_defer_failed+0x166/0x550 io_uring/io_uring.c:988
 io_queue_sqe io_uring/io_uring.c:2032 [inline]
 io_submit_sqe io_uring/io_uring.c:2284 [inline]
 io_submit_sqes+0xe28/0x1d10 io_uring/io_uring.c:2397
 __do_sys_io_uring_enter io_uring/io_uring.c:3464 [inline]
 __se_sys_io_uring_enter+0x2df/0x2b20 io_uring/io_uring.c:3398
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f36d418ebe9
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffc1d32a978 EFLAGS: 00000246 ORIG_RAX: 00000000000001aa
RAX: ffffffffffffffda RBX: 00007f36d43b5fa0 RCX: 00007f36d418ebe9
RDX: 0000000000000000 RSI: 0000000000003516 RDI: 0000000000000003
RBP: 00007f36d4211e19 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f36d43b5fa0 R14: 00007f36d43b5fa0 R15: 0000000000000006
 </TASK>


***

If these findings have caused you to resend the series or submit a
separate fix, please add the following tag to your commit message:
  Tested-by: syzbot@syzkaller.appspotmail.com

---
This report is generated by a bot. It may contain errors.
syzbot ci engineers can be reached at syzkaller@googlegroups.com.