CVE-2026-43121

/* * Conceptual PoC for CVE-2026-43121 Race Condition * This code simulates the race between io_zcrx_scrub and refill path. */ #include <pthread.h> #include <stdatomic.h> #include <stdio.h> #include <unistd.h> // Simulating kernel structure struct io_zcrx_queue { atomic_int user_refs; int free_count; int nr_iovs; }; struct io_zcrx_queue q; // Simulating io_zcrx_scrub (CPU1: no lock) void *scrub_thread(void *arg) { // Window opens: atomic_xchg without rq_lock int old = atomic_exchange(&q.user_refs, 0); printf("[Scrub] Exchanged ref %d to 0\n", old); // Simulate pushing to freelist (PUSH #1) q.free_count++; printf("[Scrub] Free count: %d\n", q.free_count); return NULL; } // Simulating io_zcrx_put_niov_uref (CPU0: holds rq_lock) void *refill_thread(void *arg) { // Simulate lock acquisition // Non-atomic check-then-decrement int ref = atomic_load(&q.user_refs); // Race window here usleep(100); if (ref > 0) { atomic_fetch_sub(&q.user_refs, 1); printf("[Refill] Decremented ref.\n"); // Simulate pushing to freelist (PUSH #2: DOUBLE-FREE) q.free_count++; printf("[Refill] Free count: %d (Potential OOB)\n", q.free_count); if (q.free_count > q.nr_iovs) { printf("[!!!] Out-of-bounds write triggered!\n"); } } return NULL; } int main() { atomic_init(&q.user_refs, 1); q.nr_iovs = 1; q.free_count = 0; pthread_t t1, t2; // Trigger race pthread_create(&t1, NULL, refill_thread, NULL); pthread_create(&t2, NULL, scrub_thread, NULL); pthread_join(t1, NULL); pthread_join(t2, NULL); return 0; }

{"cve": {"id": "CVE-2026-43121", "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "published": "2026-05-06T12:16:28.950", "lastModified": "2026-05-12T21:17:31.950", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nio_uring/zcrx: fix user_ref race between scrub and refill paths\n\nThe io_zcrx_put_niov_uref() function uses a non-atomic\ncheck-then-decrement pattern (atomic_read followed by separate\natomic_dec) to manipulate user_refs. This is serialized against other\ncallers by rq_lock, but io_zcrx_scrub() modifies the same counter with\natomic_xchg() WITHOUT holding rq_lock.\n\nOn SMP systems, the following race exists:\n\n CPU0 (refill, holds rq_lock) CPU1 (scrub, no rq_lock)\n put_niov_uref:\n atomic_read(uref) - 1\n // window opens\n atomic_xchg(uref, 0) - 1\n return_niov_freelist(niov) [PUSH #1]\n // window closes\n atomic_dec(uref) - wraps to -1\n returns true\n return_niov(niov)\n return_niov_freelist(niov) [PUSH #2: DOUBLE-FREE]\n\nThe same niov is pushed to the freelist twice, causing free_count to\nexceed nr_iovs. Subsequent freelist pushes then perform an out-of-bounds\nwrite (a u32 value) past the kvmalloc'd freelist array into the adjacent\nslab object.\n\nFix this by replacing the non-atomic read-then-dec in\nio_zcrx_put_niov_uref() with an atomic_try_cmpxchg loop that atomically\ntests and decrements user_refs. This makes the operation safe against\nconcurrent atomic_xchg from scrub without requiring scrub to acquire\nrq_lock.\n\n[pavel: removed a warning and a comment]"}], "metrics": {"cvssMetricV31": [{"source": "[email protected]", "type": "Primary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:H", "baseScore": 4.7, "baseSeverity": "MEDIUM", "attackVector": "LOCAL", "attackComplexity": "HIGH", "privilegesRequired": "LOW", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "NONE", "integrityImpact": "NONE", "availabilityImpact": "HIGH"}, "exploitabilityScore": 1.0, "impactScore": 3.6}]}, "weaknesses": [{"source": "[email protected]", "type": "Primary", "description": [{"lang": "en", "value": "CWE-362"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.15", "versionEndExcluding": "6.18.16", "matchCriteriaId": "5B5F989F-E891-48E4-9EC9-3C5EFD3DB9DC"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.19", "versionEndExcluding": "6.19.6", "matchCriteriaId": "373EEEDA-FAA1-4FB4-B6ED-DB4DD99DBE67"}]}]}], "references": [{"url": "https://git.kernel.org/stable/c/003049b1c4fb8aabb93febb7d1e49004f6ad653b", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "tags": ["Patch"]}, {"url": "https://git.kernel.org/stable/c/485dc691257b96e6d3bdc25b0eff2daadcc5c46c", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "tags": ["Patch"]}, {"url": "https://git.kernel.org/stable/c/a94f096e28bfc7975163a6b80f1c8f323efe317a", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "tags": ["Patch"]}]}}