tech@,

the Open Firmware getprop service returns the full property size while
copying at most buflen bytes. macppc redirects the firmware write through
OF_buf, but copied the returned size into the caller's buffer, allowing an
oversized property to overwrite adjacent memory.

Here, I limit the second copy to buflen while preserving getprop's return
value.

This fixes stack corruption when QEMU OpenBIOS returns the 60 byte e1000 reg
property to pci_intr_map(), which requests one 20 byte PCI record. With this
change, em0 attaches and the ramdisk reaches the installer.

Ok?

Index: sys/arch/macppc/macppc/openfirm.c
===================================================================
RCS file: /home/cvs/src/sys/arch/macppc/macppc/openfirm.c,v
diff -u -p -r1.13 openfirm.c
--- sys/arch/macppc/macppc/openfirm.c	2 Apr 2020 19:27:51 -0000	1.13
+++ sys/arch/macppc/macppc/openfirm.c	27 Jul 2026 10:01:58 -0000
@@ -183,7 +183,7 @@ OF_getprop(int handle, char *prop, void 
 		ret = -1;
 	else {
 		if (args.size > 0)
-			ofbcopy(OF_buf, buf, args.size);
+			ofbcopy(OF_buf, buf, MIN(args.size, buflen));
 		ret = args.size;
 	}
 	ppc_mtmsr(s);

-- 
wbr, Kirill