audit,io_uring: io_uring openat triggers audit reference count underflow

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@vger.kernel.org
Link: https://lore.kernel.org/lkml/MW2PR2101MB1033FFF044A258F84AEAA584F1C9A@MW2PR2101MB1033.namprd21.prod.outlook.com/


Fixes: 5bd2182d ("audit,io_uring,io-wq: add some basic audit support to io_uring")
Signed-off-by: Dan Clash <daclash@linux.microsoft.com>
Link: https://lore.kernel.org/r/20231012215518.GA4048@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net


Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Christian Brauner <brauner@kernel.org>