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

[Paul Moore <paul@paul-moore.com>]

On Fri, Oct 13, 2023 at 10:21 AM Jens Axboe <axboe@kernel.dk> wrote:
> On 10/13/23 2:24 AM, Christian Brauner wrote:
> > On Thu, Oct 12, 2023 at 02:55:18PM -0700, 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@vger.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@linux.microsoft.com>
> >> ---
> >>  fs/namei.c         | 9 +++++----
> >>  include/linux/fs.h | 2 +-
> >>  kernel/auditsc.c   | 8 ++++----
> >>  3 files changed, 10 insertions(+), 9 deletions(-)
> >>
> >> diff --git a/fs/namei.c b/fs/namei.c
> >> index 567ee547492b..94565bd7e73f 100644
> >> --- a/fs/namei.c
> >> +++ b/fs/namei.c
> >> @@ -188,7 +188,7 @@ getname_flags(const char __user *filename, int flags, int *empty)
> >>              }
> >>      }
> >>
> >> -    result->refcnt = 1;
> >> +    atomic_set(&result->refcnt, 1);
> >>      /* The empty path is special. */
> >>      if (unlikely(!len)) {
> >>              if (empty)
> >> @@ -249,7 +249,7 @@ getname_kernel(const char * filename)
> >>      memcpy((char *)result->name, filename, len);
> >>      result->uptr = NULL;
> >>      result->aname = NULL;
> >> -    result->refcnt = 1;
> >> +    atomic_set(&result->refcnt, 1);
> >>      audit_getname(result);
> >>
> >>      return result;
> >> @@ -261,9 +261,10 @@ void putname(struct filename *name)
> >>      if (IS_ERR(name))
> >>              return;
> >>
> >> -    BUG_ON(name->refcnt <= 0);
> >> +    if (WARN_ON_ONCE(!atomic_read(&name->refcnt)))
> >> +            return;
> >>
> >> -    if (--name->refcnt > 0)
> >> +    if (!atomic_dec_and_test(&name->refcnt))
> >>              return;
> >
> > Fine by me. I'd write this as:
> >
> > count = atomic_dec_if_positive(&name->refcnt);
> > if (WARN_ON_ONCE(unlikely(count < 0))
> >       return;
> > if (count > 0)
> >       return;
>
> Would be fine too, my suspicion was that most archs don't implement a
> primitive for that, and hence it might be more expensive than
> atomic_read()/atomic_dec_and_test() which do. But I haven't looked at
> the code generation. The dec_if_positive degenerates to a atomic cmpxchg
> for most cases.

I'm not too concerned, either approach works for me, the important bit
is moving to an atomic_t/refcount_t so we can protect ourselves
against the race.  The patch looks good to me and I'd like to get this
fix merged.

Dan, barring any further back-and-forth on the putname() change, I
would say to go ahead and make the change Christian suggested and
repost the patch.  Based on Jens comment above it seems safe to
preserve his 'Reviewed-by:' tag on the next revision.  Assuming there
are no objections posted in the meantime, I'll plan to merge the next
revision into the audit/stable-6.6 branch and get that up to Linus
(likely next week since it's Friday).

Thanks everyone!

-- 
paul-moore.com