Public Access
jaap-jan
c2c860b4f2
ADR 0007 decision 4 refused this a commit ago, on the grounds that widening
access to the machine's TPM is a system-wide decision an application package
should not make for an administrator. That refusal is overturned here, and the
reasoning it is replaced with is narrower rather than louder — because the
refusal had treated "widen access" as one thing when it is two.
Adding a person to `tss` is what the refusal was really about, and it stays the
wrong answer: permanent, effective over SSH, applying whether or not they are at
the keyboard, and needing an administrator and a fresh login before the feature
appears at all.
Tagging /dev/tpmrm0 with `uaccess` is a different thing. It is systemd's own
mechanism for this exact question — the one /dev/dri, webcams, scanners and FIDO
keys already use — and logind grants the ACL to the user of the *active local
session*, then takes it back when that session ends. Not over SSH, not on an
inactive VT, not to daemons. The rule changes neither the mode nor the group, so
everything that worked before still works and the ACL is layered on top.
tpmrm and never tpm, which is what makes it defensible at all: /dev/tpm0 is the
raw device and is exclusive, where /dev/tpmrm0 is the kernel's resource manager,
which virtualises handles and isolates callers precisely so that more than one
unprivileged program can use the chip.
◆ THE COST IS A DENIAL OF SERVICE AND NOT A DISCLOSURE, AND IT IS WRITTEN DOWN.
TPM objects are protected by their own auth values, so this lets nobody read
anybody else's sealed data. What it allows is *failed* attempts — and a TPM
answers enough of those by locking out everyone, which is the same anti-hammering
this project relies on to make a four-digit PIN safe. On a machine that unlocks
its disk with the TPM at boot, somebody sitting at it can trip that and stop the
next boot from unlocking until it recovers.
An administrator who does not accept that masks the file:
sudo ln -s /dev/null /etc/udev/rules.d/71-dodossh-tpm.rules
which is the standard override, survives upgrades, and puts DodoSSH back to
behaving as though the machine had no TPM. That is in the rule's own comments,
because the person who needs it is reading the file rather than the ADR.
Numbered 71 because 60-tpm-udev.rules is what creates the node's ownership and
73-seat-late.rules is what turns the tag into an ACL; verified in that order on
Fedora 44, along with `udevadm verify` accepting the file and the mask being
reported as "masked by previous entry". %post reloads and replays it against the
tpmrm subsystem, because a rule that waits for a reboot is a feature that does
not work on the day it is installed.
Both channels install the same path deliberately: this is about the machine's
TPM rather than about either build, the file is byte-identical from both, and
giving each its own copy would mean removing one package silently revoked the
other's access.
Phase 19 grew a check that the ACL exists for the local session and does *not*
exist for an SSH one, and another that masking the rule really does put the
machine back — then tells you to undo it before the checks that follow.
Also recorded, after it cost five confusing failures: swtpm serves exactly one
client, so a shell readiness probe sitting on the port makes the whole device-key
suite fail with "The TPM closed the connection" and nothing pointing at why.
55 lines
3.1 KiB
Plaintext
55 lines
3.1 KiB
Plaintext
# Lets the person sitting at this machine use its TPM, without joining a group.
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#
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# ◆ WHAT THIS CHANGES, AND WHAT IT DELIBERATELY DOES NOT.
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#
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# Fedora's 60-tpm-udev.rules leaves /dev/tpmrm0 as root:tss mode 0660, so using it means being in
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# the tss group — which means an administrator, a usermod and a fresh login before DodoSSH will
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# offer to seal a device key. This adds the uaccess tag, and nothing else: the mode and the group
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# are untouched, so everything that worked before still works and an ACL is layered on top.
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#
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# uaccess is systemd's own mechanism for exactly this question and is what /dev/dri, webcams,
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# scanners and FIDO keys already use. logind grants a POSIX ACL to the user of the *active local
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# session* and takes it away again when that session ends. So:
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#
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# - somebody logged in at the physical machine gets access;
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# - somebody logged in over SSH does not;
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# - a user switched away to another VT does not;
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# - a system daemon running as its own user does not.
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#
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# That is a much narrower grant than adding a person to tss, which is permanent, applies over SSH,
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# and applies whether or not they are the one at the keyboard.
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#
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# ◆ tpmrm AND NOT tpm, WHICH IS THE WHOLE REASON THIS IS DEFENSIBLE.
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#
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# /dev/tpm0 is the raw device and access to it is exclusive — two users cannot share it, and a
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# process holding it can leave the chip in a state that breaks everything else. /dev/tpmrm0 is the
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# kernel's resource manager: it virtualises handles, swaps contexts and isolates callers from each
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# other, and it exists *because* the TPM is meant to be usable by more than one unprivileged
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# program. Granting a seat's user the manager is the use it was built for. Granting the raw device
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# would not be, and this rule never names it.
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#
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# ◆ THE COST, WHICH IS REAL AND IS NOT ABOUT SECRECY.
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#
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# TPM keys are protected by their own auth values, so this grants nobody the ability to read
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# anybody else's sealed data. What it does grant is the ability to make failed authorisation
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# attempts, and a TPM answers those by counting them and eventually refusing everyone — the same
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# dictionary-attack lockout DodoSSH relies on to make a four-digit PIN safe.
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#
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# On a machine whose disk is unlocked by the TPM at boot (systemd-cryptenroll, Clevis), a local user
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# who trips that lockout can stop the next boot from unlocking until it recovers. That is a denial
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# of service against the machine's owner by somebody already sitting at the machine, and it is worth
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# knowing rather than discovering. An administrator who does not accept it can mask this file:
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#
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# sudo ln -s /dev/null /etc/udev/rules.d/71-dodossh-tpm.rules
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#
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# which is the standard override and survives upgrades. DodoSSH then behaves exactly as it did
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# before this file existed: no device key is offered, and unlocking asks for the passphrase.
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#
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# Numbered 71 because 73-seat-late.rules is what turns the tag into an ACL, and 60-tpm-udev.rules
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# is what creates the device node's ownership in the first place. Anything outside that gap either
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# never runs or runs too late to matter.
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ACTION=="remove", GOTO="dodossh_tpm_end"
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SUBSYSTEM=="tpmrm", KERNEL=="tpmrm[0-9]*", TAG+="uaccess"
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LABEL="dodossh_tpm_end"
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