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Subject: bug#35714: Parted destroys mbr_signature (Disk Identifier) when adding MBR bootcode
Resent-From: Matthijs Kooijman <matthijs@HIDDEN>
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Date: Mon, 13 May 2019 10:52:12 +0200
From: Matthijs Kooijman <matthijs@HIDDEN>
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Hi folks,

I believe I found a bug in the dos partition table writer. Let me start
with the cause. There's this bit of code:

	if (!table->boot_code[0]) {
		memset (table->boot_code, 0, 512);
		memcpy (table->boot_code, MBR_BOOT_CODE, sizeof (MBR_BOOT_CODE));
	}

When a disk has no MBR bootcode, this adds some. However, it also starts
with clearing the *entire* MBR sector. I believe this should just clear
the bootcode, e.g.

	- memset (table->boot_code, 0, 512);
	+ memset (table->boot_code, 0, sizeof(table->boot_code));

The current code clears the mbr_signature, partition table and magic at
the end. The partition table is rewritten from the table in memory, the
magic value is simply replaced with a fixed value, but the mbr_signature
is replaced by newly randomly generated value.

Effectively, this means that when modifying a partition table without
any boot code, the mbr_signature will be changed.

I've noticed this problem on a Raspberry pi. The default raspbian images
are generated using fdisk, without any boot code (which is not needed on
a pi). Modifying the partition table using parted then generates a new
mbr_signature. Since this signature (called Disk Identifier by fdisk) is
used to generate the PARTUUID for partitions [1], and the PARTUUID is used
to locate the root filesystem, this effectively makes the system
unbootable.

It seems this has been known for a while, since a workaround [2] has
been (silently, without mentioning it in the commend message) introduced
two years ago [3] in the raspberry pi resizing code (probably at the
same time they started using PARTUUID I suspect).

Also note that the disk identifier is only changes once, since then the
MBR has bootcode and parted no longer needs to clear things.

Gr.

Matthijs

[1]: https://raspberrypi.stackexchange.com/a/95436/70022
[2]: https://github.com/RPi-Distro/raspi-config/blob/master/init_resize.sh#L77-L82
[3]: https://github.com/RPi-Distro/raspi-config/commit/44a2e6188c004de50b6c25c8bace2872d672cdd1

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From: help-debbugs@HIDDEN (GNU bug Tracking System)
To: Matthijs Kooijman <matthijs@HIDDEN>
Subject: bug#35714: Acknowledgement (Parted destroys mbr_signature (Disk
 Identifier) when adding MBR bootcode)
Message-ID: <handler.35714.B.155775725912962.ack <at> debbugs.gnu.org>
References: <20190513085212.gtsiqnazw3jx5kvk@HIDDEN>
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Date: Mon, 13 May 2019 14:21:03 +0000

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Message sent to bug-parted@HIDDEN:

Subject: bug#35714: Parted destroys mbr_signature (Disk Identifier) when adding MBR bootcode
Resent-From: Matthijs Kooijman <matthijs@HIDDEN>
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Date: Mon, 13 May 2019 16:30:32 +0200
From: Matthijs Kooijman <matthijs@HIDDEN>
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Hi folks,

one more addition: Below is an example of reproducing this problem on an
Ubuntu Disco system running parted 3.2-25. I haven't tried git master,
but looking at the code, I'm pretty sure the problem stil exists there.

Gr.

Matthijs


# Use fdisk to create an empty partition table without any boot code
matthijs@grubby:~$ truncate -s 8M tmp.img
matthijs@grubby:~$ echo -e 'o\nw' | fdisk tmp.img

Welcome to fdisk (util-linux 2.33.1).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.

Device does not contain a recognized partition table.
Created a new DOS disklabel with disk identifier 0x0298967f.

Command (m for help): Created a new DOS disklabel with disk identifier 0x94bfb6ca.

Command (m for help): The partition table has been altered.
Syncing disks.

matthijs@grubby:~$ fdisk -l tmp.img | grep identifier
Disk identifier: 0x94bfb6ca
matthijs@grubby:~$ head -c 512 tmp.img | hd
00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
000001b0  00 00 00 00 00 00 00 00  ca b6 bf 94 00 00 00 00  |................|
000001c0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
000001f0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 55 aa  |..............U.|
00000200


# Then use parted to add a partition
matthijs@grubby:~$ parted tmp.img mkpart p fat32 4096s 8191s
/sys/firmware/dmi/tables/smbios_entry_point: Permission denied
/dev/mem: Permission denied
WARNING: You are not superuser.  Watch out for permissions.

# This has changed the disk identifier (and added bootcode)
matthijs@grubby:~$ fdisk -l tmp.img | grep identifier
Disk identifier: 0x5d7de418
matthijs@grubby:~$ head -c 512 tmp.img | hd
00000000  fa b8 00 10 8e d0 bc 00  b0 b8 00 00 8e d8 8e c0  |................|
00000010  fb be 00 7c bf 00 06 b9  00 02 f3 a4 ea 21 06 00  |...|.........!..|
00000020  00 be be 07 38 04 75 0b  83 c6 10 81 fe fe 07 75  |....8.u........u|
00000030  f3 eb 16 b4 02 b0 01 bb  00 7c b2 80 8a 74 01 8b  |.........|...t..|
00000040  4c 02 cd 13 ea 00 7c 00  00 eb fe 00 00 00 00 00  |L.....|.........|
00000050  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
000001b0  00 00 00 00 00 00 00 00  18 e4 7d 5d 00 00 00 00  |..........}]....|
000001c0  01 20 0c 03 20 3f 00 10  00 00 00 10 00 00 00 00  |. .. ?..........|
000001d0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
*
000001f0  00 00 00 00 00 00 00 00  00 00 00 00 00 00 55 aa  |..............U.|
00000200

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Message sent to bug-parted@HIDDEN:

Subject: bug#35714: Parted destroys mbr_signature (Disk Identifier) when adding MBR bootcode
Resent-From: Phillip Susi <phill@HIDDEN>
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Matthijs Kooijman writes:

> Hi folks,
>
> I believe I found a bug in the dos partition table writer. Let me start
> with the cause. There's this bit of code:
>
> 	if (!table->boot_code[0]) {
> 		memset (table->boot_code, 0, 512);
> 		memcpy (table->boot_code, MBR_BOOT_CODE, sizeof (MBR_BOOT_CODE));
> 	}

Yikes! Clearly this should have been sizeof( table->boot_code ) rather
than 512.

Last modified: Mon, 25 Nov 2019 12:00:02 UTC

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