Re: [PATCH v5 4/5] vfio: Add vfio_copy_user_data()

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

On Fri,  8 Nov 2024 04:17:41 -0800
Yi Liu <yi.l.liu@intel.com> wrote:

> This generalizes the logic of copying user data when the user struct
> Have new fields introduced. The helpers can be used by the vfio uapis
> that have the argsz and flags fields in the beginning 8 bytes.
> 
> As an example, the vfio_device_{at|de}tach_iommufd_pt paths are updated
> to use the helpers.
> 
> The flags may be defined to mark a new field in the structure, reuse
> reserved fields, or special handling of an existing field. The extended
> size would differ for different flags. Each user API that wants to use
> the generalized helpers should define an array to store the corresponding
> extended sizes for each defined flag.
> 
> For example, we start out with the below, minsz is 12.
> 
>   struct vfio_foo_struct {
>   	__u32   argsz;
>   	__u32   flags;
>   	__u32   pt_id;
>   };
> 
> And then here it becomes:
> 
>   struct vfio_foo_struct {
>   	__u32   argsz;
>   	__u32   flags;
>   #define VFIO_FOO_STRUCT_PASID   (1 << 0)
>   	__u32   pt_id;
>   	__u32   pasid;
>   };
> 
> The array is { 16 }.
> 
> If the next flag is simply related to the processing of @pt_id and
> doesn't require @pasid, then the extended size of the new flag is
> 12. The array become { 16, 12 }
> 
>   struct vfio_foo_struct {
>   	__u32   argsz;
>   	__u32   flags;
>   #define VFIO_FOO_STRUCT_PASID   (1 << 0)
>   #define VFIO_FOO_STRUCT_SPECICAL_PTID   (1 << 1)
>   	__u32   pt_id;
>   	__u32   pasid;
>   };
> 
> Similarly, rather than adding new field, we might have reused a previously
> reserved field, for instance what if we already expanded the structure
> as the below, array is already { 24 }.
> 
>   struct vfio_foo_struct {
>   	__u32   argsz;
>   	__u32   flags;
>   #define VFIO_FOO_STRUCT_XXX     (1 << 0)
>   	__u32   pt_id;
>   	__u32   reserved;
>   	__u64   xxx;
>   };
> 
> If we then want to add @pasid, we might really prefer to take advantage
> of that reserved field and the array becomes { 24, 16 }.
> 
>   struct vfio_foo_struct {
>   	__u32   argsz;
>   	__u32   flags;
>   #define VFIO_FOO_STRUCT_XXX     (1 << 0)
>   #define VFIO_FOO_STRUCT_PASID   (1 << 1)
>   	__u32   pt_id;
>   	__u32   reserved;

I think this was supposed to be s/reserved/pasid/

>   	__u64   xxx;
>   };
> 
> Suggested-by: Alex Williamson <alex.williamson@redhat.com>
> Signed-off-by: Yi Liu <yi.l.liu@intel.com>
> ---
>  drivers/vfio/device_cdev.c | 81 +++++++++++++-------------------------
>  drivers/vfio/vfio.h        | 18 +++++++++
>  drivers/vfio/vfio_main.c   | 55 ++++++++++++++++++++++++++
>  3 files changed, 100 insertions(+), 54 deletions(-)
>
> diff --git a/drivers/vfio/device_cdev.c b/drivers/vfio/device_cdev.c
> index 4519f482e212..35c7664b9a97 100644
> --- a/drivers/vfio/device_cdev.c
> +++ b/drivers/vfio/device_cdev.c
> @@ -159,40 +159,33 @@ void vfio_df_unbind_iommufd(struct vfio_device_file *df)
>  	vfio_device_unblock_group(device);
>  }
>  
> +#define VFIO_ATTACH_FLAGS_MASK VFIO_DEVICE_ATTACH_PASID
> +static unsigned long
> +vfio_attach_xends[ilog2(VFIO_ATTACH_FLAGS_MASK) + 1] = {
> +	XEND_SIZE(VFIO_DEVICE_ATTACH_PASID,
> +		  struct vfio_device_attach_iommufd_pt, pasid),
> +};
> +
> +#define VFIO_DETACH_FLAGS_MASK VFIO_DEVICE_DETACH_PASID
> +static unsigned long
> +vfio_detach_xends[ilog2(VFIO_DETACH_FLAGS_MASK) + 1] = {
> +	XEND_SIZE(VFIO_DEVICE_DETACH_PASID,
> +		  struct vfio_device_detach_iommufd_pt, pasid),
> +};
> +
>  int vfio_df_ioctl_attach_pt(struct vfio_device_file *df,
>  			    struct vfio_device_attach_iommufd_pt __user *arg)
>  {
>  	struct vfio_device_attach_iommufd_pt attach;
>  	struct vfio_device *device = df->device;
> -	unsigned long minsz, xend = 0;
>  	int ret;
>  
> -	minsz = offsetofend(struct vfio_device_attach_iommufd_pt, pt_id);
> -
> -	if (copy_from_user(&attach, arg, minsz))
> -		return -EFAULT;
> -
> -	if (attach.argsz < minsz)
> -		return -EINVAL;
> -
> -	if (attach.flags & (~VFIO_DEVICE_ATTACH_PASID))
> -		return -EINVAL;
> -
> -	if (attach.flags & VFIO_DEVICE_ATTACH_PASID)
> -		xend = offsetofend(struct vfio_device_attach_iommufd_pt, pasid);
> -
> -	/*
> -	 * xend may be equal to minsz if a flag is defined for reusing a
> -	 * reserved field or a special usage of an existing field.
> -	 */
> -	if (xend > minsz) {
> -		if (attach.argsz < xend)
> -			return -EINVAL;
> -
> -		if (copy_from_user((void *)&attach + minsz,
> -				   (void __user *)arg + minsz, xend - minsz))
> -			return -EFAULT;
> -	}
> +	ret = vfio_copy_user_data((void __user *)arg, &attach,
> +				  struct vfio_device_attach_iommufd_pt,
> +				  pt_id, VFIO_ATTACH_FLAGS_MASK,
> +				  vfio_attach_xends);
> +	if (ret)
> +		return ret;
>  
>  	if ((attach.flags & VFIO_DEVICE_ATTACH_PASID) &&
>  	    !device->ops->pasid_attach_ioas)
> @@ -227,34 +220,14 @@ int vfio_df_ioctl_detach_pt(struct vfio_device_file *df,
>  {
>  	struct vfio_device_detach_iommufd_pt detach;
>  	struct vfio_device *device = df->device;
> -	unsigned long minsz, xend = 0;
> -
> -	minsz = offsetofend(struct vfio_device_detach_iommufd_pt, flags);
> -
> -	if (copy_from_user(&detach, arg, minsz))
> -		return -EFAULT;
> -
> -	if (detach.argsz < minsz)
> -		return -EINVAL;
> -
> -	if (detach.flags & (~VFIO_DEVICE_DETACH_PASID))
> -		return -EINVAL;
> -
> -	if (detach.flags & VFIO_DEVICE_DETACH_PASID)
> -		xend = offsetofend(struct vfio_device_detach_iommufd_pt, pasid);
> -
> -	/*
> -	 * xend may be equal to minsz if a flag is defined for reusing a
> -	 * reserved field or a special usage of an existing field.
> -	 */
> -	if (xend > minsz) {
> -		if (detach.argsz < xend)
> -			return -EINVAL;
> +	int ret;
>  
> -		if (copy_from_user((void *)&detach + minsz,
> -				   (void __user *)arg + minsz, xend - minsz))
> -			return -EFAULT;
> -	}
> +	ret = vfio_copy_user_data((void __user *)arg, &detach,
> +				  struct vfio_device_detach_iommufd_pt,
> +				  flags, VFIO_DETACH_FLAGS_MASK,
> +				  vfio_detach_xends);
> +	if (ret)
> +		return ret;
>  
>  	if ((detach.flags & VFIO_DEVICE_DETACH_PASID) &&
>  	    !device->ops->pasid_detach_ioas)
> diff --git a/drivers/vfio/vfio.h b/drivers/vfio/vfio.h
> index 50128da18bca..87bed550c46e 100644
> --- a/drivers/vfio/vfio.h
> +++ b/drivers/vfio/vfio.h
> @@ -34,6 +34,24 @@ void vfio_df_close(struct vfio_device_file *df);
>  struct vfio_device_file *
>  vfio_allocate_device_file(struct vfio_device *device);
>  
> +int vfio_copy_from_user(void *buffer, void __user *arg,
> +			unsigned long minsz, u32 flags_mask,
> +			unsigned long *xend_array);
> +
> +#define vfio_copy_user_data(_arg, _local_buffer, _struct, _min_last,          \
> +			    _flags_mask, _xend_array)                         \
> +	vfio_copy_from_user(_local_buffer, _arg,                              \
> +			    offsetofend(_struct, _min_last) +                \
> +			    BUILD_BUG_ON_ZERO(offsetof(_struct, argsz) !=     \
> +					      0) +                            \
> +			    BUILD_BUG_ON_ZERO(offsetof(_struct, flags) !=     \
> +					      sizeof(u32)),                   \
> +			    _flags_mask, _xend_array)
> +
> +#define XEND_SIZE(_flag, _struct, _xlast)                                    \
> +	[ilog2(_flag)] = offsetofend(_struct, _xlast) +                      \
> +			 BUILD_BUG_ON_ZERO(_flag == 0)                       \
> +
>  extern const struct file_operations vfio_device_fops;
>  
>  #ifdef CONFIG_VFIO_NOIOMMU
> diff --git a/drivers/vfio/vfio_main.c b/drivers/vfio/vfio_main.c
> index a5a62d9d963f..c61336ea5123 100644
> --- a/drivers/vfio/vfio_main.c
> +++ b/drivers/vfio/vfio_main.c
> @@ -1694,6 +1694,61 @@ int vfio_dma_rw(struct vfio_device *device, dma_addr_t iova, void *data,
>  }
>  EXPORT_SYMBOL(vfio_dma_rw);
>  
> +/**
> + * vfio_copy_from_user - Copy the user struct that may have extended fields
> + *
> + * @buffer: The local buffer to store the data copied from user
> + * @arg: The user buffer pointer
> + * @minsz: The minimum size of the user struct
> + * @flags_mask: The combination of all the falgs defined
> + * @xend_array: The array that stores the xend size for set flags.
> + *
> + * This helper requires the user struct put the argsz and flags fields in
> + * the first 8 bytes.
> + *
> + * Return 0 for success, otherwise -errno
> + */
> +int vfio_copy_from_user(void *buffer, void __user *arg,

This should probably be prefixed with an underscore and note that
callers should use the wrapper function to impose the parameter
checking.

> +			unsigned long minsz, u32 flags_mask,
> +			unsigned long *xend_array)
> +{
> +	unsigned long xend = minsz;
> +	struct user_header {
> +		u32 argsz;
> +		u32 flags;
> +	} *header;
> +	unsigned long flags;
> +	u32 flag;
> +
> +	if (copy_from_user(buffer, arg, minsz))
> +		return -EFAULT;
> +
> +	header = (struct user_header *)buffer;
> +	if (header->argsz < minsz)
> +		return -EINVAL;
> +
> +	if (header->flags & ~flags_mask)
> +		return -EINVAL;

I'm already wrestling with whether this is an over engineered solution
to remove a couple dozen lines of mostly duplicate logic between attach
and detach, but a couple points that could make it more versatile:

(1) Test xend_array here:

	if (!xend_array)
		return 0;

(2) Return ssize_t/-errno for the caller to know the resulting copy
size.

> +
> +	/* Loop each set flag to decide the xend */
> +	flags = header->flags;
> +	for_each_set_bit(flag, &flags, BITS_PER_TYPE(u32)) {
> +		if (xend_array[flag] > xend)
> +			xend = xend_array[flag];

Can we craft a BUILD_BUG in the wrapper to test that xend_array is at
least long enough to match the highest bit in flags?  Thanks,

Alex

> +	}
> +
> +	if (xend > minsz) {
> +		if (header->argsz < xend)
> +			return -EINVAL;
> +
> +		if (copy_from_user(buffer + minsz,
> +				   arg + minsz, xend - minsz))
> +			return -EFAULT;
> +	}
> +
> +	return 0;
> +}
> +
>  /*
>   * Module/class support
>   */