drm/vmwgfx: validate DRAW_PRIMITIVES header size before division

drm/vmwgfx: validate DRAW_PRIMITIVES header size before division

vmw_cmd_draw() computes

	maxnum = (header->size - sizeof(cmd->body)) / sizeof(*decl);

where header->size is u32 and is taken straight from the user-supplied
command stream.  When header->size is less than sizeof(cmd->body) the
unsigned subtraction wraps to nearly 4 GiB, producing a huge maxnum.
Any user-controlled cmd->body.numVertexDecls then passes the bound and
the loop dereferences decl[i] far past the end of the kernel command
bounce buffer, producing an out-of-bounds read of kernel memory.

Reject undersized headers up front.

Fixes: 7a73ba7469cb ("drm/vmwgfx: Use TTM handles instead of SIDs as user-space surface handles.")
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/20260505222728.519626-7-zack.rusin@broadcom.com