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A canonical event log structure for IMA is a TCG-defined outer record format that wraps IMA measurements in a common, portable envelope for collection and verification. It does not replace Linux IMA’s native log or change what IMA measures: it preserves IMA-specific content inside a shared record structure.
Why wrap IMA events in a canonical log?
Linux IMA records measurements using its own event-log format and templates. A verifier that receives those records must understand that format and the template rules used to interpret each payload. The Trusted Computing Group’s Canonical Event Log (CEL) provides a common outer model for verifiers handling events from different content sources, including IMA.
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The distinction is important: CEL is an encapsulation format, not a replacement for the format defined by IMA. TCG’s Canonical Event Log Format, Version 1.1, Revision 10 (2024 public-review document) explicitly describes CEL as not replacing existing content-custodian formats such as IMA. The IMA template and payload remain the authority for the meaning of the inner event.
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A CEL log is a sequence of records. Each record has four logical parts:
- Record number (
recnum): a sequence number maintained separately for each PCR or NV index. It starts at zero and increases monotonically. - PCR or NV index: identifies the register or index associated with the record.
- Digest list: contains one or more digest values supplied to the TPM Extend operation, with details depending on the TPM operation.
- Typed event content: identifies the content family and carries content interpreted according to that type’s rules.
The CEL specification defines common content-type identifiers, including ima_template and ima_tlv. These identify the form of the enclosed content; they do not make IMA’s payload semantics generic.
Why the record number matters
Sequence numbers are security-relevant structure, not display labels. Because they advance for measured and unmeasured events, a verifier can use them to detect missing records even where an event did not cause a PCR extension. Explicit numbering also helps identify ordering problems when logs are exported or moved between systems.
What belongs inside the IMA payload?
IMA’s native binary log records a PCR index, a hash of the template data, the template name, and the template data itself. The selected template determines the fields in that data. It can depend on compile-time defaults, boot-time settings, or policy rules.
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Common templates illustrate why CEL must retain the IMA payload rather than flattening it into a generic event description:
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ima-ngcarries a digest and filename.ima-sigcarries a digest, filename, and signature.ima-bufcarries a digest, filename, and buffer.
In a CEL-wrapped record, the outer content type says how to classify or parse the content family; the enclosed IMA template name and data preserve the source format’s event-specific meaning.
How native IMA records and CEL-wrapped records differ
| Aspect | Native IMA log | CEL representation |
|---|---|---|
| Purpose | IMA’s measurement list and template data. | Common encapsulation for verifier input across event sources. |
| Ordering | Native log order. | Explicit sequence number per PCR or NV index. |
| Content meaning | The selected IMA template defines the payload. | The content type identifies the content family; IMA still defines the payload semantics. |
| Interoperability | Requires parsing IMA’s format and templates. | Adds a shared outer information model and encoding choices. |
| Verification | Replay the native measurements against attestation state. | Preserve the critical source data so the converted record can still be verified using TPM quote information. |
How the log relates to PCRs and attestation
IMA measurement events are appended to the log and may extend a TPM PCR. PCR 10 is common, but policy can direct events to other PCRs, and an event appearing in the log is not proof that it was extended. A verifier replays the applicable measurements and compares its calculated PCR value with the value authenticated by the TPM quote.
For a conversion to CEL to preserve that verification path, the converter must retain the original digest values supplied to the Extend operation, not just a final PCR value. It must also preserve the critical source information needed to verify the event. Under CEL, the custodian for a content type defines what content is hashed to derive the extend value; for IMA content, the IMA template rules remain essential.
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Preserve sequence, digests, and content type
- Keep the per-index record number explicit and in order; do not infer it later from file position alone.
- Retain the digest values used for the TPM extension, rather than storing only the resulting PCR state.
- Keep the CEL content type and the complete IMA template representation so a verifier can apply the appropriate IMA parsing and hashing rules.
Account for IMA byte order
IMA multi-byte values default to the creating host’s byte order unless otherwise specified. The ima_canonical_fmt option forces little-endian encoding. A verifier must know the byte order for any values included in hashes; interpreting the same bytes differently can produce a different measurement.
Match replay to the TPM bank and quote
Event-log replay requires a hash algorithm that matches an enabled TPM PCR bank. The available bank and operating-system defaults vary by deployment, so there is no single algorithm that applies to every system. Intel’s IMA log guidance discusses this operational dependency.
TPM 2.0 quote generation and PCR reads are separate operations. Runtime events can be appended between them. IMA verification guidance therefore recommends replaying events until the calculated PCR matches the quoted value and treating later appended events as expected concurrency, rather than treating a mismatch with a separate PCR read as automatic proof of tampering.
What the CEL design history does—and does not—establish
A 2017 Linux Foundation presentation on measurement and attestation proposed making record number, PCR, digest, and content explicit, with sequence numbers useful for synchronization and compression and timestamps useful for correlation. Those points help explain the motivation for a structured envelope. The current information model described here is the TCG CEL specification, not the presentation’s historical proposal.
The available IMA event-log documentation describes the binary record format substantially, but marks its ASCII serialization section as a FIXME. Do not assume that this reference fully specifies a canonical ASCII representation of IMA records.
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