RE: Early kernel panic in dmi_decode when running 32-bit kernel on Hyper-V on Windows 11

[Michael Kelley]

From: Michael Kelley <mhklinux@xxxxxxxxxxx> Sent: Wednesday, April 17, 2024 3:35 PM
> 
> From: Michael Schierl <schierlm@xxxxxx> Sent: Wednesday, April 17, 2024 2:08 PM
> >
> > > Don't let the type 10 distract you. It is entirely possible that the
> > > byte corresponding to type == 10 is already part of the corrupted
> > > memory area. Can you check if the DMI table generated by Hyper-V is
> > > supposed to contain type 10 records at all?
> >
> > How? Hyper-V is not open source :-)
> 
> I think that request from Jean is targeted to me or the Microsoft
> people on the thread.  :-)
> 
> >
> > My best guess to get Linux out of the equation would be to boot my
> > trusted MS-DOS 6.2 floppy and use debug.com to dump the DMI:
> >
> > > | A:\>debug
> > > | -df000:93d0 [to inspect]
> > > | -nfromdos.dmi
> > > | -rcx
> > > | CX 0000
> > > | :439B
> > > | -w f000:93d0
> > > | -q
> >
> >
> > The result is byte-for-byte identical to the DMI dump I created from
> > sysfs and pasted earlier in this thread. Of course, it does not have to
> > be identical to the memory situation while it was parsed.
> 
> I've been looking at the details of the DMI blob in a Linux VM on my
> local Windows 11 laptop, as well as in a Generation 1 VM in the Azure
> public cloud, which uses Hyper-V.   The overall size and layout
> of the DMI blob appears to be the same in both cases.  The blob is
> corrupted in the VM on the local laptop, but good in the Azure VM.
> 
> I was wondering how to check if the Linux bootloaders and grub
> were somehow corrupting the DMI blob, but now you've
> answered the question by running MS-DOS and dumping the
> contents.  Excellent experiment!
> 
> I still want to understand why 32-bit Linux is taking an oops during
> boot while 64-bit Linux does not.  

The difference is in this statement in dmi_save_devices():

	count = (dm->length - sizeof(struct dmi_header)) / 2;

On a 64-bit system, count is 0xFFFFFFFE.  That's seen as a
negative value, and the "for" loop does not do any iterations. So
nothing bad happens.

But on a 32-bit system, count is 0x7FFFFFFE. That's a big
positive number, and the "for" loop iterates to non-existent
memory as Michael Schierl originally described.

I don't know the "C" rules for mixed signed and unsigned
expressions, and how they differ on 32-bit and 64-bit systems.
But that's the cause of the different behavior.

Regardless of the 32-bit vs. 64-bit behavior, the DMI blob is malformed,
almost certainly as created by Hyper-V.  I'll see if I can bring this to
the attention of one of my previous contacts on the Hyper-V team.

Michael

> During boot, I can see that 64-bit
> Linux wanders through the corrupted part of the DMI blob and
> looks at a lot of bogus entries before it gets back on track again.
> But the bogus entries don't cause an oops.  Once I figure out
> those details, we still have the corrupted DMI blob, and based on
> your MS-DOS experiment, it's looking like Hyper-V created the
> corrupted form.   I want to think more about how to debug that.
> 
> FWIW, in comparing the Azure VM with my local VM, it looks like
> the corrupted entry is the first type 4 entry describing a CPU.
> 
> Michael Kelley
> 
> >
> > > You should also check the memory map (as displayed early at boot, so
> > > near the top of dmesg) and verify that the DMI table is located in a
> > > "reserved" memory area, so that area can't be used for memory
> > > allocation.
> >
> > The e820 memory map was included in the early printk output I posted
> > earlier:
> >
> > > [    0.000000] BIOS-provided physical RAM map:
> > > [    0.000000] BIOS-e820: [mem 0x0000000000000000-0x000000000009fbff] usable
> > > [    0.000000] BIOS-e820: [mem 0x000000000009fc00-0x000000000009ffff] reserved
> > > [    0.000000] BIOS-e820: [mem 0x00000000000e0000-0x00000000000fffff] reserved
> > > [    0.000000] BIOS-e820: [mem 0x0000000000100000-0x000000007ffeffff] usable
> > > [    0.000000] BIOS-e820: [mem 0x000000007fff0000-0x000000007fffefff] ACPI data
> > > [    0.000000] BIOS-e820: [mem 0x000000007ffff000-0x000000007fffffff] ACPI NVS
> >
> > And from the dmidecode I pasted earlier:
> >
> > > Table at 0x000F93D0.
> >
> > The size is 0x0000439B, so the last byte should be at 0x000FD76A, well
> > inside the third i820 entry (the second reserved one) - and accessible
> > even from DOS without requiring any extra effort.
> >
> > > So the table starts at physical address 0xba135000, which is in the
> > > following memory map segment:
> > >
> > > reserve setup_data: [mem 0x00000000b87b0000-0x00000000bb77dfff] reserved
> >
> > Looks like UEFI, and well outside the 1MB range :-)
> >
> > > If the whole DMI table IS located in a "reserved" memory area, it can
> > > still get corrupted, but only by code which itself operates on data
> > > located in a reserved memory area.
> >
> >
> > > Both DMI tables are corrupted, but are they corrupted in the exact same
> > > way?
> >
> > At least the dumped tables are byte-for-byte identical on both OS
> > flavors. And (as I tested above) byte-for-byte identical to a version
> > dumped from MS-DOS.
> >
> >
> > Regards,
> >
> >
> > Michael
>