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

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

On Mon,  8 Oct 2018 13:06:20 -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 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@umn.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,

Alex