kern/176748: BTX Loader i386 incorrectly probes EDD, possibly resulting division by zero

Arrigo Marchiori ardovm at
Fri Mar 8 13:40:02 UTC 2013

>Number:         176748
>Category:       kern
>Synopsis:       BTX Loader i386 incorrectly probes EDD, possibly resulting division by zero
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    freebsd-bugs
>State:          open
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Fri Mar 08 13:40:01 UTC 2013
>Originator:     Arrigo Marchiori
>Release:        9-STABLE
FreeBSD myhost 9.1-STABLE FreeBSD 9.1-STABLE #19 r247772: Mon Mar  4 10:47:30 CET 2013     root at myhost:/usr/obj/usr/src/sys/GENERIC  i386
BTX probes for disk drives using interrupt 0x13.
The "EDD" information is parsed incorrectly: the loader may incorrectly assume that the EDD information is present, and therefore overwrite the number of sectors and their size with incorrect values.

The error is in file /usr/src/sys/boot/i386/libi386/biosdisk.c, inside function bd_int13probe().

The call to interrupt 0x13, AH=0x41 returns values in the bits of the CX register. The bit EDD_INTERFACE_EDD is not checked, while it should be.

The effect on my configuration was that bd_int13probe() incorrectly assumed that EDD information was present; it requested for it with int 0x13, AH=0x48 and overwrote the number and size of sectors with invalid values (zero, in my case). This resulted in a zero by zero division in file
/usr/src/sys/boot/i386/common/disk.c, function disk_open().

My setup had some buildworld/buildkernel tunings through src.conf(5). For instance, ZFS support was not compiled in. I believe such customization should not influence this particular problem.
Find a system with a mass memory that does not provide EDD information.
This problem was detected on a AMD Geode GX CPU, using a USB drive for boot.
Please let me know if you need any more information about this.
The attached patch adds the check for the EDD_INTERFACE_EDD bit inside the data returned by the call to interrupt 0x13, AH=0x41.
It solves the problem on my hardware configuration.

Patch attached with submission follows:

Index: biosdisk.c
--- biosdisk.c	(revisione 247767)
+++ biosdisk.c	(copia locale)
@@ -231,7 +231,8 @@
 	if (V86_CY(v86.efl) ||	/* carry set */
 	    (v86.ebx & 0xffff) != 0xaa55 || /* signature */
-	    (v86.ecx & EDD_INTERFACE_FIXED_DISK) == 0)
+	    (v86.ecx & EDD_INTERFACE_FIXED_DISK) == 0 ||
+            (v86.ecx & EDD_INTERFACE_EDD) == 0)
 		return (1);
 	/* EDD supported */
 	bd->bd_flags |= BD_MODEEDD1;


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