linux-kernel - Re: [PATCH v2] drivers/vfio: Fix a redundant copy bug

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Message-ID: <CAAa=b7d5a07o8b9Xkn_dqpiLrm43DYQujro2w=SNev702q0zkA@mail.gmail.com>
Date:   Wed, 17 Oct 2018 09:20:50 -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,
        Wenwen Wang <wang6495@....edu>
Subject: Re: [PATCH v2] drivers/vfio: Fix a redundant copy bug

On Mon, Oct 8, 2018 at 1:47 PM Alex Williamson
<alex.williamson@...hat.com> wrote:
>
> On Mon,  8 Oct 2018 13:06:20 -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.
> >
> > Signed-off-by: Wenwen Wang <wang6495@....edu>
> > ---
> >  drivers/vfio/vfio_spapr_eeh.c | 9 ++++++---
> >  1 file changed, 6 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);
>
> We already have this in minsz, offsetofend(,op) == offsetof(,err.type).
> That can't change without breaking userspace.
>
> > +                     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))
>
> So we trade 12 bytes of redundant copy for an extra stack variable and
> an arithmetic operation, not necessarily an obvious win, but more
> correct I guess.
>
> Alexey, I also notice that these 12 bytes means that the u64 fields in
> struct vfio_eeh_pe_err are not 8-byte aligned which could lead to
> compiler dependent packing interpretation issues with userspace.
> Should there be a 4-byte reserved field in there to make it explicit
> (so long as it matches the current interpretation)?  Thanks,

It sounds reasonable. I can add such a field in struct vfio_eeh_pe_op. Thanks!

Wenwen