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

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Date:   Mon, 8 Oct 2018 12:42:40 -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>,
        Wenwen Wang <wang6495@....edu>
Subject: Re: [PATCH] drivers/vfio: Fix a redundant copy bug

On Mon, Oct 8, 2018 at 11:43 AM Alex Williamson
<alex.williamson@...hat.com> wrote:
>
> Hi,
>
> On Sun,  7 Oct 2018 09:44:25 -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 and security reasons, the redundant part of the second copy
> > should be removed.
>
> Redundant, yes.  Performance-wise it's 12 bytes on a non-performance
> path, so theoretically yes, but in practice maybe it's a simplicity
> trade-off.  Security?  I don't see it, please explain.
>
> > This patch removes such a part in the second copy. It only copies the 'err'
> > information from the buffer 'arg'.
> >
> > Signed-off-by: Wenwen Wang <wang6495@....edu>
> > ---
> >  drivers/vfio/vfio_spapr_eeh.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/vfio/vfio_spapr_eeh.c b/drivers/vfio/vfio_spapr_eeh.c
> > index 38edeb4..5bc4b60 100644
> > --- a/drivers/vfio/vfio_spapr_eeh.c
> > +++ b/drivers/vfio/vfio_spapr_eeh.c
> > @@ -86,10 +86,10 @@ 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)
> > +                     if (op.argsz < sizeof(op))
> >                               return -EINVAL;
>
> The original code is written such that new operations can be added,
> possibly with new entries in the struct vfio_eeh_pe_op union, which
> might change sizeof(op) to be more than necessary for a
> VFIO_EEH_PE_INJECT_ERR op.  Existing userspace suddenly wouldn't work
> without effectively reverting this change.  This is a subtle dependency
> that is not worth the above code change, imo.
>
> > -                     if (copy_from_user(&op, (void __user *)arg, minsz))
> > +                     if (copy_from_user(&op.err, (char __user *)arg +
> > +                                             minsz, sizeof(op.err)))
> >                               return -EFAULT;
>
> Please rework with the assumption that the union in struct
> vfio_eeh_pe_op can be expanded and must not break existing userspace.

Thanks for your suggestion. I will resubmit the patch.

Wenwen

> Thanks,
>
> Alex