libfdt: check for potential overrun in _fdt_splice()

This patch catches the conditions where:
 - 'splicepoint' is set to a point outside of [ fdt, fdt_totalsize(fdt) )
 - 'newlen' is negative, or 'splicepoint' plus 'newlen' results in overflow

Either of these cases can be caused by math which overflows in calling
functions, or by sizes specified through dynamic means.

Signed-off-by: Courtney Cavin <courtney.cavin@sonymobile.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>

libfdt/fdt_rw.c

@@ -101,6 +101,8 @@ static int _fdt_splice(void *fdt, void *splicepoint, int oldlen, int newlen)
 
 	if (((p + oldlen) < p) || ((p + oldlen) > end))
 		return -FDT_ERR_BADOFFSET;
+	if ((p < (char *)fdt) || ((end - oldlen + newlen) < (char *)fdt))
+		return -FDT_ERR_BADOFFSET;
 	if ((end - oldlen + newlen) > ((char *)fdt + fdt_totalsize(fdt)))
 		return -FDT_ERR_NOSPACE;
 	memmove(p + newlen, p + oldlen, end - p - oldlen);