Re: [PATCH] io_uring: Fix a null-ptr-deref in io_tctx_exit_cb()

[Harshit Mogalapalli]

On 07/12/22 2:45 am, Jens Axboe wrote:
> On 12/6/22 2:38?AM, Harshit Mogalapalli wrote:
> > Syzkaller reports a NULL deref bug as follows:
> >
> >   BUG: KASAN: null-ptr-deref in io_tctx_exit_cb+0x53/0xd3
> >   Read of size 4 at addr 0000000000000138 by task file1/1955
> >
> >   CPU: 1 PID: 1955 Comm: file1 Not tainted 6.1.0-rc7-00103-gef4d3ea40565 #75
> >   Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.0-2.el7 04/01/2014
> >   Call Trace:
> >    <TASK>
> >    dump_stack_lvl+0xcd/0x134
> >    ? io_tctx_exit_cb+0x53/0xd3
> >    kasan_report+0xbb/0x1f0
> >    ? io_tctx_exit_cb+0x53/0xd3
> >    kasan_check_range+0x140/0x190
> >    io_tctx_exit_cb+0x53/0xd3
> >    task_work_run+0x164/0x250
> >    ? task_work_cancel+0x30/0x30
> >    get_signal+0x1c3/0x2440
> >    ? lock_downgrade+0x6e0/0x6e0
> >    ? lock_downgrade+0x6e0/0x6e0
> >    ? exit_signals+0x8b0/0x8b0
> >    ? do_raw_read_unlock+0x3b/0x70
> >    ? do_raw_spin_unlock+0x50/0x230
> >    arch_do_signal_or_restart+0x82/0x2470
> >    ? kmem_cache_free+0x260/0x4b0
> >    ? putname+0xfe/0x140
> >    ? get_sigframe_size+0x10/0x10
> >    ? do_execveat_common.isra.0+0x226/0x710
> >    ? lockdep_hardirqs_on+0x79/0x100
> >    ? putname+0xfe/0x140
> >    ? do_execveat_common.isra.0+0x238/0x710
> >    exit_to_user_mode_prepare+0x15f/0x250
> >    syscall_exit_to_user_mode+0x19/0x50
> >    do_syscall_64+0x42/0xb0
> >    entry_SYSCALL_64_after_hwframe+0x63/0xcd
> >   RIP: 0023:0x0
> >   Code: Unable to access opcode bytes at 0xffffffffffffffd6.
> >   RSP: 002b:00000000fffb7790 EFLAGS: 00000200 ORIG_RAX: 000000000000000b
> >   RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
> >   RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
> >   RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
> >   R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
> >   R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
> >    </TASK>
> >   Kernel panic - not syncing: panic_on_warn set ...
> >
> > Add a NULL check on tctx to prevent this.
>
> I agree with Vegard that I don't think this is fixing the core of
> the issue. I think what is happening here is that we don't run the
> task_work in io_uring_cancel_generic() unconditionally, if we don't
> need to in the loop above. But we do need to ensure we run it before
> we clear current->io_uring.
>
> Do you have a reproducer? If so, can you try the below? I _think_
> this is all we need. We can't be hitting the delayed fput path as
> the task isn't exiting, and we're dealing with current here.
Thanks Jens and Vegard for the suggestions and analysis.

Yes, the below patch silences the reproducer.

> diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
> index 36cb63e4174f..4791d94c88f5 100644
> --- a/io_uring/io_uring.c
> +++ b/io_uring/io_uring.c
> @@ -3125,6 +3125,15 @@ __cold void io_uring_cancel_generic(bool cancel_all, struct io_sq_data *sqd)
>   
>   	io_uring_clean_tctx(tctx);
>   	if (cancel_all) {
> +		/*
> +		 * If we didn't run task_work in the loop above, ensure we
> +		 * do so here. If an fput() queued up exit task_work for the
> +		 * ring descriptor before we started the exec that led to this
> +		 * cancelation, then we need to have that run before we proceed
> +		 * with tearing down current->io_uring.
> +		 */
> +		io_run_task_work();
> +
>   		/*
>   		 * We shouldn't run task_works after cancel, so just leave
>   		 * ->in_idle set for normal exit.

Thanks,
Harshit