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 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().

Thanks,
Sidong

> 
> Best,
> Caleb
> 
> >         return 0;
> >  }
> >
> > --
> > 2.43.0
> >