>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
>Quarter:
>Keywords:
>Date-Required:
>Class: sw-bug
>Submitter-Id: current-users
>Arrival-Date: Fri Mar 08 13:40:01 UTC 2013
>Closed-Date:
>Last-Modified:
>Originator: Arrigo Marchiori
>Release: 9-STABLE
>Organization:
>Environment:
FreeBSD myhost 9.1-STABLE FreeBSD 9.1-STABLE #19 r247772: Mon Mar 4 10:47:30
CET 2013 root@myhost:/usr/obj/usr/src/sys/GENERIC i386
>Description:
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.
>How-To-Repeat:
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.
>Fix:
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 @@
v86int();
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;
>Release-Note:
>Audit-Trail:
>Unformatted:
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