On Mon, Oct 8, 2018 at 1:47 PM Alex Williamson
<alex.williamson@xxxxxxxxxx> wrote:
>
> On Mon, 8 Oct 2018 13:06:20 -0500
> Wenwen Wang <wang6495@xxxxxxx> 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@xxxxxxx>
> > ---
> > 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
