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

[Yi Liu <yi.l.liu@xxxxxxxxx>]

On 2024/11/12 08:03, Alex Williamson wrote:
> On Fri,  8 Nov 2024 04:17:41 -0800
> Yi Liu <yi.l.liu@xxxxxxxxx> 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/

you are right.

> >    	__u64   xxx;
> >    };
> >
> > Suggested-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
> > Signed-off-by: Yi Liu <yi.l.liu@xxxxxxxxx>
> > ---
> >   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.

got it.

> > +			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;

Perhaps we should return error if the header->flags has any bit set. Such
cases require a valid xend_array.

> (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,

yes. I would add a BUILD_BUG like the below.

BUILD_BUG_ON(ARRAY_SIZE(_xend_array) < ilog2(_flags_mask));

--
Regards,
Yi Liu