drm/vmwgfx: validate external BO copy bounds for both stride paths

drm/vmwgfx: validate external BO copy bounds for both stride paths

vmw_external_bo_copy() trusts caller-supplied offsets, strides, and
heights and operates on imported dma-buf vmaps:

  - The equal-stride memcpy() bound was clamped after subtracting the
    offsets from dst_size and src_size; an offset larger than the BO
    size wraps the unsigned subtraction to a huge value and the
    resulting memcpy() runs off the end of the vmap.  dst_stride *
    height is also a u32 multiplication that can overflow.
  - The non-equal-stride row-by-row path had no bound at all.  The
    loop touches bytes through offset + (height - 1) * stride +
    width_in_bytes, with only a WARN_ON(dst_stride < width_in_bytes),
    and could likewise step past the end of either mapping.

The offsets and strides are derived from STDU/SOU plane state, so a
configured CRTC submitting a crafted atomic commit on an imported
framebuffer can reach this path.

Validate the exact row-copy endpoint against each BO's size up front
using check_mul_overflow() and check_add_overflow().  Use the bulk
memcpy() path only when width_in_bytes covers the whole stride;
otherwise copy one row at a time so partial-row updates near the bottom
of a framebuffer remain valid.  Also reject zero strides and stride <
width_in_bytes, both of which the row-by-row path cannot represent
safely.

Fixes: 50f119925091 ("drm/vmwgfx: Fix prime with external buffers")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4.7
Signed-off-by: Zack Rusin <zack.rusin@broadcom.com>
Reviewed-by: Ian Forbes <ian.forbes@broadcom.com>
Link: https://patch.msgid.link/20260505222728.519626-13-zack.rusin@broadcom.com