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d937426306
On machines without a TPM, we'd still like some way for the BIOS to attest that it has not been modified. With a Librem Key, we can have the BIOS use its own ROM measurement converted to a SHA256sum and truncated so it fits within an HOTP secret. Like with a TPM, a malicious BIOS with access to the correct measurements can send pre-known good measurements to the Librem Key. This approach provides one big drawback in that we have to truncate the SHA256sum to 20 characters so that it fits within the limitations of HOTP secrets. This means the possibility of collisions is much higher but again, an attacker could also capture and spoof an existing ROM's measurements if they have prior access to it, either with this approach or with a TPM. Signed-off-by: Kyle Rankin <kyle.rankin@puri.sm>
69 lines
1.8 KiB
Bash
Executable File
69 lines
1.8 KiB
Bash
Executable File
#!/bin/bash
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# Retrieve the sealed file and counter from the NVRAM, unseal it and compute the hotp
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. /etc/functions
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HOTP_SECRET="/tmp/secret/hotp.key"
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HOTP_COUNTER="/boot/kexec_hotp_counter"
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mount_boot_or_die()
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{
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TRACE "Under /bin/unseal-hotp:mount_boot_or_die"
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# Mount local disk if it is not already mounted
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if ! grep -q /boot /proc/mounts ; then
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mount -o ro /boot \
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|| die "Unable to mount /boot"
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fi
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}
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TRACE "Under /bin/unseal-hotp"
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# Store counter in file instead of TPM for now, as it conflicts with Heads
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# config TPM counter as TPM 1.2 can only increment one counter between reboots
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# get current value of HOTP counter in TPM, create if absent
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mount_boot_or_die
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#check_tpm_counter $HOTP_COUNTER hotp \
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#|| die "Unable to find/create TPM counter"
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#counter="$TPM_COUNTER"
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#
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#counter_value=$(read_tpm_counter $counter | cut -f2 -d ' ' | awk 'gsub("^000e","")')
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#
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counter_value=$(cat $HOTP_COUNTER)
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if [ "$counter_value" == "" ]; then
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die "Unable to read HOTP counter"
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fi
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#counter_value=$(printf "%d" 0x${counter_value})
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if [ "$CONFIG_TPM" = "y" ]; then
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DEBUG "Unsealing HOTP secret reuses TOTP sealed secret..."
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tpmr unseal 4d47 0,1,2,3,4,7 312 "$HOTP_SECRET"
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else
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# without a TPM, use the first 20 characters of the ROM SHA256sum
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secret_from_rom_hash > "$HOTP_SECRET"
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fi
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if ! hotp $counter_value < "$HOTP_SECRET"; then
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shred -n 10 -z -u "$HOTP_SECRET" 2> /dev/null
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die 'Unable to compute HOTP hash?'
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fi
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shred -n 10 -z -u "$HOTP_SECRET" 2> /dev/null
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#increment_tpm_counter $counter > /dev/null \
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#|| die "Unable to increment tpm counter"
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mount -o remount,rw /boot
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counter_value=`expr $counter_value + 1`
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echo $counter_value > $HOTP_COUNTER \
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|| die "Unable to create hotp counter file"
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#sha256sum /tmp/counter-$counter > $HOTP_COUNTER \
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#|| die "Unable to create hotp counter file"
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mount -o remount,ro /boot
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exit 0
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