Re: [PATCH] io_uring: Don't set affinity on a dying sqpoll thread

[Pavel Begunkov]

...
> Jens,
>
> I'm not sure I got the whole story on this one, but it seems fairly
> trivial to reproduce and I can't see another way it could be
> triggered. What do you think?

Right, it can be null and that should be checked. I haven't taken
a look at the report but the patch looks good regardless, thanks


> -- >8 --
> Subject: [PATCH] io_uring: Don't set affinity on a dying sqpoll thread
>
> Syzbot reported a null-ptr-deref of sqd->thread inside
> io_sqpoll_wq_cpu_affinity.  It turns out the sqd->thread can go away
> from under us during io_uring_register, in case the process gets a
> fatal signal during io_uring_register.
>
> It is not particularly hard to hit the race, and while I am not sure
> this is the exact case hit by syzbot, it solves it.  Finally, checking
> ->thread is enough to close the race because we locked sqd while
> "parking" the thread, thus preventing it from going away.
>
> I reproduced it fairly consistently with a program that does:
>
> int main(void) {
>    ...
>    io_uring_queue_init(RING_LEN, &ring1, IORING_SETUP_SQPOLL);
>    while (1) {
>      io_uring_register_iowq_aff(ring, 1, &mask);
>    }
> }
>
> Executed in a loop with timeout to trigger SIGTERM:
>    while true; do timeout 1 /a.out ; done
>
> This will hit the following BUG() in very few attempts.
>
> BUG: kernel NULL pointer dereference, address: 00000000000007a8
> PGD 800000010e949067 P4D 800000010e949067 PUD 10e46e067 PMD 0
> Oops: 0000 [#1] PREEMPT SMP PTI
> CPU: 0 PID: 15715 Comm: dead-sqpoll Not tainted 6.5.0-rc7-next-20230825-g193296236fa0-dirty #23
> Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
> RIP: 0010:io_sqpoll_wq_cpu_affinity+0x27/0x70
> Code: 90 90 90 0f 1f 44 00 00 55 53 48 8b 9f 98 03 00 00 48 85 db 74 4f
> 48 89 df 48 89 f5 e8 e2 f8 ff ff 48 8b 43 38 48 85 c0 74 22 <48> 8b b8
> a8 07 00 00 48 89 ee e8 ba b1 00 00 48 89 df 89 c5 e8 70
> RSP: 0018:ffffb04040ea7e70 EFLAGS: 00010282
> RAX: 0000000000000000 RBX: ffff93c010749e40 RCX: 0000000000000001
> RDX: 0000000000000000 RSI: ffffffffa7653331 RDI: 00000000ffffffff
> RBP: ffffb04040ea7eb8 R08: 0000000000000000 R09: c0000000ffffdfff
> R10: ffff93c01141b600 R11: ffffb04040ea7d18 R12: ffff93c00ea74840
> R13: 0000000000000011 R14: 0000000000000000 R15: ffff93c00ea74800
> FS:  00007fb7c276ab80(0000) GS:ffff93c36f200000(0000) knlGS:0000000000000000
> CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 00000000000007a8 CR3: 0000000111634003 CR4: 0000000000370ef0
> DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
> DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
> Call Trace:
>   <TASK>
>   ? __die_body+0x1a/0x60
>   ? page_fault_oops+0x154/0x440
>   ? do_user_addr_fault+0x174/0x7b0
>   ? exc_page_fault+0x63/0x140
>   ? asm_exc_page_fault+0x22/0x30
>   ? io_sqpoll_wq_cpu_affinity+0x27/0x70
>   __io_register_iowq_aff+0x2b/0x60
>   __io_uring_register+0x614/0xa70
>   __x64_sys_io_uring_register+0xaa/0x1a0
>   do_syscall_64+0x3a/0x90
>   entry_SYSCALL_64_after_hwframe+0x6e/0xd8
> RIP: 0033:0x7fb7c226fec9
> Code: 2e 00 b8 ca 00 00 00 0f 05 eb a5 66 0f 1f 44 00 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 8b 0d 97 7f 2d 00 f7 d8 64 89 01 48
> RSP: 002b:00007ffe2c0674f8 EFLAGS: 00000246 ORIG_RAX: 00000000000001ab
> RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007fb7c226fec9
> RDX: 00007ffe2c067530 RSI: 0000000000000011 RDI: 0000000000000003
> RBP: 00007ffe2c0675d0 R08: 00007ffe2c067550 R09: 00007ffe2c067550
> R10: 0000000000000001 R11: 0000000000000246 R12: 0000000000000000
> R13: 00007ffe2c067750 R14: 0000000000000000 R15: 0000000000000000
>   </TASK>
> Modules linked in:
> CR2: 00000000000007a8
> ---[ end trace 0000000000000000 ]---
>
> Reported-by: syzbot+c74fea926a78b8a91042@xxxxxxxxxxxxxxxxxxxxxxxxx
> Fixes: ebdfefc09c6d ("io_uring/sqpoll: fix io-wq affinity when IORING_SETUP_SQPOLL is used")
> Signed-off-by: Gabriel Krisman Bertazi <krisman@xxxxxxx>
> ---
>   io_uring/sqpoll.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/io_uring/sqpoll.c b/io_uring/sqpoll.c
> index ee2d2c687fda..bd6c2c7959a5 100644
> --- a/io_uring/sqpoll.c
> +++ b/io_uring/sqpoll.c
> @@ -430,7 +430,9 @@ __cold int io_sqpoll_wq_cpu_affinity(struct io_ring_ctx *ctx,
>   
>   	if (sqd) {
>   		io_sq_thread_park(sqd);
> -		ret = io_wq_cpu_affinity(sqd->thread->io_uring, mask);
> +		/* Don't set affinity for a dying thread */
> +		if (sqd->thread)
> +			ret = io_wq_cpu_affinity(sqd->thread->io_uring, mask);
>   		io_sq_thread_unpark(sqd);
>   	}
>   

--
Pavel Begunkov