linux-kernel - Re: [PATCH v3] drivers/vfio: Fix a redundant copy bug

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Date:   Wed, 17 Oct 2018 09:45:06 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Wenwen Wang <wang6495@....edu>
Cc:     Kangjie Lu <kjlu@....edu>,
        kvm@...r.kernel.org (open list:VFIO DRIVER),
        linux-kernel@...r.kernel.org (open list), <aik@...abs.ru>,
        David Gibson <david@...son.dropbear.id.au>
Subject: Re: [PATCH v3] drivers/vfio: Fix a redundant copy bug

On Wed, 17 Oct 2018 09:32:04 -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.
> 
> This patch also adds a 4-byte reserved field in the structure
> vfio_eeh_pe_op to make sure that the u64 fields in the structure
> vfio_eeh_pe_err are 8-byte aligned.
> 
> Signed-off-by: Wenwen Wang <wang6495@....edu>
> ---
>  drivers/vfio/vfio_spapr_eeh.c | 9 ++++++---
>  include/uapi/linux/vfio.h     | 1 +
>  2 files changed, 7 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);

I noted in the previous version that we already have this in minsz, so
you're fixing a redundant copy with a redundant operation.

> +			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))
>  				return -EFAULT;
>  
>  			ret = eeh_pe_inject_err(pe, op.err.type, op.err.func,
> diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h
> index 1aa7b82..d904c42 100644
> --- a/include/uapi/linux/vfio.h
> +++ b/include/uapi/linux/vfio.h
> @@ -729,6 +729,7 @@ struct vfio_eeh_pe_op {
>  	__u32 argsz;
>  	__u32 flags;
>  	__u32 op;
> +	__u32 __resv;
>  	union {
>  		struct vfio_eeh_pe_err err;
>  	};

Please don't include two separate issues in the same patch.  Am I also
correct in assuming that this is untested?  Thanks,

Alex