S4 sig-check-lpc-20130918

Signature verification of
hibernate snapshot
September, 2013, LPC 2013, New Orleans

Joey Lee

Problem
●

●

On a multi-boot machine, hacker use any hole in
another UEFI trusted OS to modify the hibernate
snapshot image in swap partition.
Through uswsusp, userspace can take the
snapshot of memory then modify it. Upload it
back to memory then trigger the restore.

© SUSE, All rights reserved.

Idea
●

●

●

Jiri Kosina: Let EFI bootloader generates keypair then pass to kernel for sign hibernate
image.
Fundamental point: Trust the boot time
variable is secure when UEFI secure boot
enabled.
Attempt to protect snapshot image integrity.


Steps (when hibernate)
●

●

●

●

shim bootloader geneates key-pair and put keys
to non-volatile boot time varaibles.
EFI stub kernel loads private key before
ExitBootServices().
Hibernate subsystem copy the private key to a
empty page to keep it for sign snapshot when
hibernate launched.
Kernel generates signature of snapshot image
then put the signature to snapshot header.
Current reserved max size of signature is 512
bytes.


Steps (when hibernate restore)
●

●

After hibernate loaded snapshot image from
swap to temporary memory space, kernel uses
the public key from runtime volatile variable to
verify the signature that's stored in snapshot
header.
Then depend on sig_enforce
●

●

OFF: taint kernel and produce complain log
when signature check fail
ON: fail the hiberntae restore, then finish
boot process when signature check fail.


How to enable sig_enforce?
●

●

Use snapshot_sig_enforce kernel parameter.
Set kernel config then enable UEFI secure boot:
EFI_SECURE_BOOT_SNAPSHOT_SIG_ENFO
RCE


EFI variable name and GUID
●

GUID:
●

●

S4SignKey [BT][NV]→ private key
●

●

fe141863-c070-478e-b8a3-878a5dc9ef21
PKCS#8 _uncompressed_ private key format

S4WakeKey [RT][V] → public key
●

X.509 format


When shim should generate keys?
●

●

When system boot, and shim didn't find key-pair
When shim found GenS4Key EFI variable from
kernel:
●

●

●

GenS4Key-fe141863-c070-478e-b8a3878a5dc9ef21
[RT][NV]
Kernel or userspace write GenS4Key
variable to '1' when hibernate launched.
Kernel will delete GenS4Key in system boot.


Implementation Parts
●

Key-pair generator in shim
●

●

●

Author: Gary Lin
https://github.com/lcp/shim/tree/s4-key-upstream

Asymmetric Keys in Kernel:
●

●

●

Implemented PKCS#8 and PKCS#1 RSA private key parser
Add signature generation API and implement signature
generation logic in PKCS#1 (RFC3447 sec 8.2.2)

Hibernate in Kernel:
●

CONFIG_SNAPSHOT_VERIFICATION=y

●

Maintain and forward private key.

●

Avoid private key included in snapshot image.s

●

Sign snapshot image: generate signature then put it to snapshot
header.


Performance of hash (machine 1)
●

CPU:
●

●
●

Intel(R) Core(TM) i5 CPU
x86_64, ssse3

Normal
●

SHA1: 150.80 MB/s

●

SHA256: 59.19 MB/s

●
●

650 @ 3.20GHz

SHA512: 78.44 MB/s

Builded ssse3 support (v3.10 later)
●

SHA1: 195.60 MB/s

●

SHA256: 82.76 MB/s

●

SHA512: 120.60 MB/s


Performance of hash (machine 2)
●

CPU:
●

●
●

Intel(R) CPU @ 2.60GHz
x86_64, ssse3, avx, avx2

Normal
●

●

SHA256: 163.23 MB/s

●
●

SHA1: 436.42 MB/s
SHA512: 228.67 MB/s

Builded ssse3, avx, avx2 support (v3.10 later)
●

SHA1: 609.66 MB/s <=== fastest

●

SHA256: 242.03 MB/s

●

SHA512: 344.87 MB/s <=== more secure


Performance of hash (summary)
●

Speed between SHA1, SHA256, SHA512
●

●

SHA1 is 1.8 times of SHA512

●
●


Enabled ssse3
●

●

39% improved on SHA256

●
●


Enabled ssse3, avx, avx2
●


●


●



Performance of hash (summary)
●

Machine 1:
●

Best performance
–
–

●

SHA1: 195.60 MB/s on ssse3, avx, avx2
snapshot image grown to 3GB, then need 15.7 seconds for hash

SHA512's best performance
–
–

●

120.60 MB/s on ssse3, avx, avx2

Machine 2:
●

Best performance
–
–

●

SHA1: 609.66 MB/s on ssse3, avx, avx2
snapshot image grown to 3GB, then need 5 seconds for hash

SHA512's best performasnce
–

344.87 MB/s on ssse3, avx, avx2

–



Patch status
●

V4 RFC patches sent to kernel upstream and openSUSE kernel for
reviewing:
●

●
●

[RFC V4 PATCH 00/15] Signature verification of hibernate snapshot
https://lkml.org/lkml/2013/9/14/183

Following kernel experts gave suggestions:
●

Hibernate

●

Matt Fleming <matt@console-pimps.org>

EFI

●
●

Pavel Machek <pavel@ucw.cz>
Dmitry Kasatkin <dmitry.kasatkin@gmail.com>

Asymmetric keys

Followed Pavel and Matt's suggestions, already fix in V2, V3 patches


TODO
●

V5 patches:
●

●

●

Implement Dmitry Kasatkin's suggestions to
Asymmetric keys.
Should we remove the kernel config to user
for select hash algorithms?

Function add:
●

●

Kernel pass random number seed by EFI
variable to shim.
Encript snapshot image before sign it?


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S4 sig-check-lpc-20130918

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