Re: [RFC PATCH v3 2/5] io_uring/cmd: zero-init pdu in io_uring_cmd_prep() to avoid UB

[Sidong Yang <sidong.yang@furiosa.ai>]

On Tue, Sep 02, 2025 at 08:31:00AM -0700, Caleb Sander Mateos wrote:
> On Tue, Sep 2, 2025 at 3:23 AM Sidong Yang <sidong.yang@furiosa.ai> wrote:
> >
> > On Mon, Sep 01, 2025 at 05:34:28PM -0700, Caleb Sander Mateos wrote:
> > > On Fri, Aug 22, 2025 at 5:56 AM Sidong Yang <sidong.yang@furiosa.ai> wrote:
> > > >
> > > > The pdu field in io_uring_cmd may contain stale data when a request
> > > > object is recycled from the slab cache. Accessing uninitialized or
> > > > garbage memory can lead to undefined behavior in users of the pdu.
> > > >
> > > > Ensure the pdu buffer is cleared during io_uring_cmd_prep() so that
> > > > each command starts from a well-defined state. This avoids exposing
> > > > uninitialized memory and prevents potential misinterpretation of data
> > > > from previous requests.
> > > >
> > > > No functional change is intended other than guaranteeing that pdu is
> > > > always zero-initialized before use.
> > > >
> > > > Signed-off-by: Sidong Yang <sidong.yang@furiosa.ai>
> > > > ---
> > > >  io_uring/uring_cmd.c | 1 +
> > > >  1 file changed, 1 insertion(+)
> > > >
> > > > diff --git a/io_uring/uring_cmd.c b/io_uring/uring_cmd.c
> > > > index 053bac89b6c0..2492525d4e43 100644
> > > > --- a/io_uring/uring_cmd.c
> > > > +++ b/io_uring/uring_cmd.c
> > > > @@ -203,6 +203,7 @@ int io_uring_cmd_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
> > > >         if (!ac)
> > > >                 return -ENOMEM;
> > > >         ioucmd->sqe = sqe;
> > > > +       memset(&ioucmd->pdu, 0, sizeof(ioucmd->pdu));
> > >
> > > Adding this overhead to every existing uring_cmd() implementation is
> > > unfortunate. Could we instead track the initialized/uninitialized
> > > state by using different types on the Rust side? The io_uring_cmd
> > > could start as an IoUringCmd, where the PDU field is MaybeUninit,
> > > write_pdu<T>() could return a new IoUringCmdPdu<T> that guarantees the
> > > PDU has been initialized.
> >
> > I've found a flag IORING_URING_CMD_REISSUE that we could initialize
> > the pdu. In uring_cmd callback, we can fill zero when it's not reissued.
> > But I don't know that we could call T::default() in miscdevice. If we
> > make IoUringCmdPdu<T>, MiscDevice also should be MiscDevice<T>.
> >
> > How about assign a byte in pdu for checking initialized? In uring_cmd(),
> > We could set a byte flag that it's not initialized. And we could return
> > error that it's not initialized in read_pdu().
> 
> Could we do the zero-initialization (or T::default()) in
> MiscdeviceVTable::uring_cmd() if the IORING_URING_CMD_REISSUE flag
> isn't set (i.e. on the initial issue)? That way, we avoid any
> performance penalty for the existing C uring_cmd() implementations.
> I'm not quite sure what you mean by "assign a byte in pdu for checking
> initialized".

Sure, we could fill zero when it's the first time uring_cmd called with
checking the flag. I would remove this commit for next version. I also
suggests that we would provide the method that read_pdu() and write_pdu().
In read_pdu() I want to check write_pdu() is called before. So along the
20 bytes for pdu, maybe we could use a bytes for the flag that pdu is
initialized?

But maybe I would introduce a new struct that has Pin<&mut IoUringCmd> and
issue_flags. How about some additional field for pdu is initialized like below?

struct IoUringCmdArgs {
  ioucmd: Pin<&mut IoUringCmd>,
  issue_flags: u32,
  pdu_initialized: bool,
}

> 
> Best,
> Caleb