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

[Alex Williamson <alex.williamson@redhat.com>]

Hi,

On Sun,  7 Oct 2018 09:44:25 -0500
Wenwen Wang <wang6495@umn.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@umn.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,

Alex