Encrypted kernel crash dumps.

Konrad Witaszczyk def at FreeBSD.org
Sat Dec 26 14:49:34 UTC 2015

Dear FreeBSD Community,

I posted a review of a patch containing encrypted kernel crash dumps feature.
You can find the review here: https://reviews.freebsd.org/D4712 .

Kernel crash dumps contain information about currently running processes.
They can include sensitive data, for example passwords kept in memory by
a browser when a kernel panic occurred. A person who can read data from a dump
device or a crash directory can also extract these information from a core dump.
In order to prevent this situation the core dump should be encrypted before it
is stored on the dump device.

Changes include modifications in kernel crash dump routines, dumpon(8) and
savecore(8). A new tool called decryptcore(8) was added. Below you can find
details on the changes.

A new DIOCSKERNELDUMP I/O control was added to send a kernel crash dump
configuration in the diocskerneldump_arg structure to the kernel.
The old DIOCSKERNELDUMP I/O control was renamed to DIOCSKERNELDUMPOLD for
backward ABI compatibility.

dumpon(8) generates an one-time random symmetric key and encrypts it using
an RSA public key in capability mode. Currently only AES-256-CBC is supported
but EKCD was designed to implement support for other algorithms in the future.
The public key is chosen using the -k flag. The dumpon rc(8) script can do this
automatically during startup using the dumppubkey rc.conf(5) variable.  Once the
keys are calculated dumpon sends them to the kernel via DIOCSKERNELDUMP I/O

When the kernel receives the DIOCSKERNELDUMP I/O control it generates a random
IV and sets up the key schedule for the specified algorithm. Each time the
kernel tries to write a crash dump to the dump device, the IV is replaced by
a SHA-256 hash of the previous value. This is intended to make a possible
differential cryptanalysis harder.

A kernel dump key consists of an algorithm identifier, an IV and an encrypted
symmetric key. The kernel dump key size is included in a kernel dump header.
The size is an unsigned 32-bit integer and it is aligned to a block size.
The header structure has 512 bytes to match the block size so it was required to
make a panic string 4 bytes shorter to add a new field to the header structure.
If the kernel dump key size in the header is nonzero it is assumed that the
kernel dump key is placed after the first header on the dump device and the core
dump is encrypted.

Separate functions were implemented to write the kernel dump header and the
kernel dump key as they need to be unencrypted. The dump_write function encrypts
data if the kernel was compiled with the EKCD option. Encrypted kernel textdumps
are not supported due to the way they are constructed which makes it impossible
to use the CBC mode for encryption.

savecore(8) writes the kernel dump key to a key.# file if its size in the header
is nonzero. # is the number of the current core dump.

decryptcore(8) decrypts the core dump using a private RSA key and the kernel
dump key. This is performed by a child process in capability mode.
If the decryption was not successful the parent process removes a partially
decrypted core dump.

Description on how to encrypt crash dumps was added to the decryptcore(8),
dumpon(8), rc.conf(5) and savecore(8) manual pages.

I would like to thank Pawel Jakub Dawidek for the support and time spent on
discussing the design and implementation of this project.

Best regards,
Konrad Witaszczyk

More information about the freebsd-security mailing list