linux-kernel - Re: [PATCH v3] drivers/vfio: Fix a redundant copy bug

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Message-ID: <CAAa=b7erQ-FwD6eQiAKanU1kb5fEoUGoEG4P6LQQP6YMDxfakg@mail.gmail.com>
Date:   Wed, 17 Oct 2018 12:58:26 -0500
From:   Wenwen Wang <wang6495@....edu>
To:     alex.williamson@...hat.com
Cc:     Kangjie Lu <kjlu@....edu>, kvm@...r.kernel.org,
        open list <linux-kernel@...r.kernel.org>, aik@...abs.ru,
        david@...son.dropbear.id.au, Wenwen Wang <wang6495@....edu>
Subject: Re: [PATCH v3] drivers/vfio: Fix a redundant copy bug

On Wed, Oct 17, 2018 at 10:45 AM Alex Williamson
<alex.williamson@...hat.com> wrote:
>
> On Wed, 17 Oct 2018 09:32:04 -0500
> Wenwen Wang <wang6495@....edu> wrote:
>
> > In vfio_spapr_iommu_eeh_ioctl(), if the ioctl command is VFIO_EEH_PE_OP,
> > the user-space buffer 'arg' is copied to the kernel object 'op' and the
> > 'argsz' and 'flags' fields of 'op' are checked. If the check fails, an
> > error code EINVAL is returned. Otherwise, 'op.op' is further checked
> > through a switch statement to invoke related handlers. If 'op.op' is
> > VFIO_EEH_PE_INJECT_ERR, the whole user-space buffer 'arg' is copied again
> > to 'op' to obtain the err information. However, in the following execution
> > of this case, the fields of 'op', except the field 'err', are actually not
> > used. That is, the second copy has a redundant part. Therefore, for both
> > performance consideration, the redundant part of the second copy should be
> > removed.
> >
> > This patch removes such a part in the second copy. It only copies from
> > 'err.type' to 'err.mask', which is exactly required by the
> > VFIO_EEH_PE_INJECT_ERR op.
> >
> > This patch also adds a 4-byte reserved field in the structure
> > vfio_eeh_pe_op to make sure that the u64 fields in the structure
> > vfio_eeh_pe_err are 8-byte aligned.
> >
> > Signed-off-by: Wenwen Wang <wang6495@....edu>
> > ---
> >  drivers/vfio/vfio_spapr_eeh.c | 9 ++++++---
> >  include/uapi/linux/vfio.h     | 1 +
> >  2 files changed, 7 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/vfio/vfio_spapr_eeh.c b/drivers/vfio/vfio_spapr_eeh.c
> > index 38edeb4..66634c6 100644
> > --- a/drivers/vfio/vfio_spapr_eeh.c
> > +++ b/drivers/vfio/vfio_spapr_eeh.c
> > @@ -37,6 +37,7 @@ long vfio_spapr_iommu_eeh_ioctl(struct iommu_group *group,
> >       struct eeh_pe *pe;
> >       struct vfio_eeh_pe_op op;
> >       unsigned long minsz;
> > +     unsigned long start, end;
> >       long ret = -EINVAL;
> >
> >       switch (cmd) {
> > @@ -86,10 +87,12 @@ long vfio_spapr_iommu_eeh_ioctl(struct iommu_group *group,
> >                       ret = eeh_pe_configure(pe);
> >                       break;
> >               case VFIO_EEH_PE_INJECT_ERR:
> > -                     minsz = offsetofend(struct vfio_eeh_pe_op, err.mask);
> > -                     if (op.argsz < minsz)
> > +                     start = offsetof(struct vfio_eeh_pe_op, err.type);
>
> I noted in the previous version that we already have this in minsz, so
> you're fixing a redundant copy with a redundant operation.

The value in start is different from the value in minsz. So why is
this a redundant operation?

> > +                     end = offsetofend(struct vfio_eeh_pe_op, err.mask);
> > +                     if (op.argsz < end)
> >                               return -EINVAL;
> > -                     if (copy_from_user(&op, (void __user *)arg, minsz))
> > +                     if (copy_from_user(&op.err, (char __user *)arg +
> > +                                             start, end - start))
> >                               return -EFAULT;
> >
> >                       ret = eeh_pe_inject_err(pe, op.err.type, op.err.func,
> > diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> > index 1aa7b82..d904c42 100644
> > --- a/include/uapi/linux/vfio.h
> > +++ b/include/uapi/linux/vfio.h
> > @@ -729,6 +729,7 @@ struct vfio_eeh_pe_op {
> >       __u32 argsz;
> >       __u32 flags;
> >       __u32 op;
> > +     __u32 __resv;
> >       union {
> >               struct vfio_eeh_pe_err err;
> >       };
>
> Please don't include two separate issues in the same patch.  Am I also
> correct in assuming that this is untested?  Thanks,

No problem. I will seperate these two patches. And yes, this is not tested.

Thanks,
Wenwen