linux-kernel - Re: [PATCH] audit,io_uring: io_uring openat triggers audit reference count underflow

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Message-ID: <eb150179-c328-4058-80e3-f517d45310c4@kernel.dk>
Date:   Thu, 12 Oct 2023 16:07:46 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Dan Clash <daclash@...ux.microsoft.com>, audit@...r.kernel.org,
        io-uring@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, paul@...l-moore.com,
        linux-fsdevel@...r.kernel.org, brauner@...nel.org,
        dan.clash@...rosoft.com
Subject: Re: [PATCH] audit,io_uring: io_uring openat triggers audit reference
 count underflow

On 10/12/23 3:55 PM, Dan Clash wrote:
> An io_uring openat operation can update an audit reference count
> from multiple threads resulting in the call trace below.
> 
> A call to io_uring_submit() with a single openat op with a flag of
> IOSQE_ASYNC results in the following reference count updates.
> 
> These first part of the system call performs two increments that do not race.
> 
> do_syscall_64()
>   __do_sys_io_uring_enter()
>     io_submit_sqes()
>       io_openat_prep()
>         __io_openat_prep()
>           getname()
>             getname_flags()       /* update 1 (increment) */
>               __audit_getname()   /* update 2 (increment) */
> 
> The openat op is queued to an io_uring worker thread which starts the
> opportunity for a race.  The system call exit performs one decrement.
> 
> do_syscall_64()
>   syscall_exit_to_user_mode()
>     syscall_exit_to_user_mode_prepare()
>       __audit_syscall_exit()
>         audit_reset_context()
>            putname()              /* update 3 (decrement) */
> 
> The io_uring worker thread performs one increment and two decrements.
> These updates can race with the system call decrement.
> 
> io_wqe_worker()
>   io_worker_handle_work()
>     io_wq_submit_work()
>       io_issue_sqe()
>         io_openat()
>           io_openat2()
>             do_filp_open()
>               path_openat()
>                 __audit_inode()   /* update 4 (increment) */
>             putname()             /* update 5 (decrement) */
>         __audit_uring_exit()
>           audit_reset_context()
>             putname()             /* update 6 (decrement) */
> 
> The fix is to change the refcnt member of struct audit_names
> from int to atomic_t.
> 
> kernel BUG at fs/namei.c:262!
> Call Trace:
> ...
>  ? putname+0x68/0x70
>  audit_reset_context.part.0.constprop.0+0xe1/0x300
>  __audit_uring_exit+0xda/0x1c0
>  io_issue_sqe+0x1f3/0x450
>  ? lock_timer_base+0x3b/0xd0
>  io_wq_submit_work+0x8d/0x2b0
>  ? __try_to_del_timer_sync+0x67/0xa0
>  io_worker_handle_work+0x17c/0x2b0
>  io_wqe_worker+0x10a/0x350
> 
> Cc: <stable@...r.kernel.org>
> Link: https://lore.kernel.org/lkml/MW2PR2101MB1033FFF044A258F84AEAA584F1C9A@MW2PR2101MB1033.namprd21.prod.outlook.com/
> Fixes: 5bd2182d58e9 ("audit,io_uring,io-wq: add some basic audit support to io_uring")
> Signed-off-by: Dan Clash <daclash@...ux.microsoft.com>

Reviewed-by: Jens Axboe <axboe@...nel.dk>

-- 
Jens Axboe