[PATCH AUTOSEL 6.18-5.15] integrity: Check for NULL returned by asymmetric_key_public_key

### Step 6.2: Backport complications
**Record:** Clean apply expected — 4 lines at a stable location. Minor
field-name difference (`pks.digest` vs `pks.m` in the submitted diff)
does not affect patch placement.

### Step 6.3: Related fixes already present?
**Record:** No — grep shows no existing NULL check at this site.

---

## Phase 7: Subsystem Context

### Step 7.1: Subsystem criticality
**Record:** **security/integrity** (IMA/EVM) — **IMPORTANT** (security-
sensitive, affects systems with integrity appraisal enabled).

### Step 7.2: Activity
**Record:** Actively maintained; recent IMA/EVM commits on this branch.

---

## Phase 8: Impact and Risk Assessment

### Step 8.1: Who is affected
**Record:** Systems with `CONFIG_INTEGRITY_ASYMMETRIC_KEYS` and IMA/EVM
digital-signature appraisal. Not universal, but important for
secured/enterprise deployments.

### Step 8.2: Trigger conditions
**Record:** A signature references a key ID that resolves to an
asymmetric key whose `asym_crypto` payload is NULL. With standard
X.509-loaded RSA/ECDSA keys this is unlikely; the subsystem already
treats `!pkey` as a valid error state elsewhere. More relevant once non-
standard key types (e.g. ML-DSA) are introduced. Trigger does not
require ML-DSA patch 4/4 on this tree, but practical likelihood on
6.18.y without ML-DSA is low.

### Step 8.3: Failure mode severity
**Record:** **Kernel oops** (NULL dereference at `pk->pkey_algo`) —
**CRITICAL** if triggered.

### Step 8.4: Risk-benefit
**Record:**
- **Benefit:** Prevents crash in security verification path; returns
  proper error instead.
- **Risk:** Very low — 4 lines, uses existing cleanup, no API change.
- **Ratio:** Favorable for stable despite rare trigger on current 6.18.y
  key types.

---

## Phase 9: Final Synthesis

### Step 9.1: Evidence summary

**FOR backport:**
- Real NULL-deref bug in security code
- Small, surgical, maintainer-signed fix with Tested-by
- Buggy code exists in v6.18.43
- Applies cleanly and standalone
- IMA/EVM verification path is security-critical
- Consistent with existing `!pkey` handling in asymmetric key code

**AGAINST backport:**
- No user report or fuzzer finding
- Added during ML-DSA series review; practical trigger on 6.18.y without
  ML-DSA may be very rare
- Standard X.509 keys normally always populate `asym_crypto`
- IMA already rejects sig v3 on this tree, limiting some future trigger
  scenarios

**Unresolved:** No confirmed production crash on 6.18.y with current key
types only.

### Step 9.2: Stable rules checklist
1. Obviously correct and tested? **PASS** (simple NULL check; Tested-by;
   maintainer SOB)
2. Fixes a real bug? **PASS** (NULL deref is a real defect, even if
   trigger is edge-case)
3. Important issue? **PASS** (kernel oops in integrity verification —
   HIGH/CRITICAL)
4. Small and contained? **PASS** (4 lines, one function)
5. No new features/APIs? **PASS**
6. Can apply to local tree? **PASS**

### Step 9.3: Exception categories
**Record:** None (not a quirk, device ID, or build fix).

### Step 9.4: Decision rationale

For **linux-6.18.y** specifically: the vulnerable code is present, the
fix is minimal and obviously correct, and a NULL dereference in the
IMA/EVM signature path is exactly the kind of security-subsystem defect
stable trees should fix. While the trigger may be uncommon with today’s
RSA/ECDSA-only keyrings, the kernel already acknowledges that asymmetric
keys can lack a public-key payload (`restrict.c`), and this path lacked
the corresponding guard. The patch is independent of the ML-DSA feature
commits and safe to backport alone.

---

## Verification

- **[Phase 1]** Parsed commit message and tags from user query and
  spinics/openwall
- **[Phase 2]** Read `security/integrity/digsig_asymmetric.c`; confirmed
  4-line NULL check at `asymmetric_key_public_key()` call site
- **[Phase 2]** Read `include/keys/asymmetric-type.h`; confirmed
  `asymmetric_key_public_key()` returns `key->payload.data[asym_crypto]`
  with no NULL guarantee
- **[Phase 3]** `git describe HEAD` → `v6.18.43`; Makefile VERSION
  6.18.43
- **[Phase 3]** `git blame -L 108,115
  security/integrity/digsig_asymmetric.c` → `6bda50f4333fa`
- **[Phase 3]** `git show 7b923c78b50d:...` → buggy code at merge-base
- **[Phase 3]** `git branch` → detached from `stable/linux-6.18.y`
- **[Phase 4]** spinics/openwall: found v3 1/4 and 0/4; v3 added patches
  per Mimi’s comments
- **[Phase 4]** `b4 shazam` — message-id not found; `b4 dig -c` not run
  (commit absent locally)
- **[Phase 4]** lore.kernel.org — blocked (bot protection)
- **[Phase 4]** Web search — commit `c93a5f0`, integrity-v7.2 pull
  confirmed
- **[Phase 5]** `grep asymmetric_verify` — callers in `digsig.c`,
  `ima_appraise.c`, `evm_main.c`
- **[Phase 5]** Read `crypto/asymmetric_keys/restrict.c` — `!pkey`
  checks at lines 141 and 183
- **[Phase 5]** Read `crypto/asymmetric_keys/signature.c` —
  `verify_signature()` checks `!key->payload.data[0]` but only after
  `asymmetric_verify()` would crash
- **[Phase 6]** Confirmed NULL check absent in current tree at lines
  110–111
- **[Phase 6]** Read `ima_appraise.c` lines 335–338 — sig v3 rejected
  before `integrity_digsig_verify()`
- **[Phase 8]** `grep mldsa` in `crypto/asymmetric_keys` — no ML-DSA
  support in this tree
- **UNVERIFIED:** Exact Mimi Zohar review text (follow-up to v2 on
  spinics not fetched); no production crash report

**YES**The background history search finished. In this
`stable/linux-6.18.y` checkout, `digsig_asymmetric.c` and the
`asymmetric_verify()` path without the NULL check both trace back to
commit `6bda50f4333fa` (Nov 29, 2025), when that file was added. The
vulnerable code is present in the current v6.18.43 tree, so the backport
analysis stands: **YES**.

 security/integrity/digsig_asymmetric.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/security/integrity/digsig_asymmetric.c b/security/integrity/digsig_asymmetric.c
index 457c0a396caf3..17219424a34dd 100644
--- a/security/integrity/digsig_asymmetric.c
+++ b/security/integrity/digsig_asymmetric.c
@@ -108,6 +108,10 @@ int asymmetric_verify(struct key *keyring, const char *sig,
 	pks.hash_algo = hash_algo_name[hdr->hash_algo];
 
 	pk = asymmetric_key_public_key(key);
+	if (!pk) {
+		ret = -ENOKEY;
+		goto out;
+	}
 	pks.pkey_algo = pk->pkey_algo;
 	if (!strcmp(pk->pkey_algo, "rsa")) {
 		pks.encoding = "pkcs1";
-- 
2.53.0