{"dataType":"CVE_RECORD","dataVersion":"5.2","cveMetadata":{"cveId":"CVE-2026-89678","assignerOrgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","state":"PUBLISHED","assignerShortName":"Linux","dateReserved":"2026-09-11T19:38:34.747Z","datePublished":"2026-09-11T19:46:02.184Z","dateUpdated":"2026-09-13T06:33:01.153Z"},"containers":{"cna":{"providerMetadata":{"orgId":"416baaa9-dc9f-4396-8d5f-8c081fb06d67","shortName":"Linux","dateUpdated":"2026-09-13T06:33:01.153Z"},"descriptions":[{"lang":"en","value":"In the Linux kernel, the following vulnerability has been resolved:\n\nnfsd: fix partial-write detection in nfsd_direct_write\n\nnfsd_direct_write() walks a list of write segments and, after each\nvfs_iocb_iter_write(), tries to detect a short write so the loop can\nstop before placing the next segment at a wrong file offset:\n\n    host_err = vfs_iocb_iter_write(file, kiocb, &segments[i].iter);\n    if (host_err < 0)\n            return host_err;\n    *cnt += host_err;\n    if (host_err < segments[i].iter.count)\n            break;\t/* partial write */\n\nvfs_iocb_iter_write() runs the iter through ->write_iter(), which\nadvances the iter by the number of bytes written. By the time the\ncheck runs, segments[i].iter.count is the residual, not the original\nrequest length:\n\n    before write_iter: iter.count == original_len\n    after  write_iter: iter.count == original_len - host_err\n\nThe condition then reduces to host_err < original_len - host_err, so\nthe break fires only when less than half of the segment was written.\nAny short write completing between 50% and 99% of the segment slips\nthrough; the loop advances to the next segment with kiocb->ki_pos\nonly bumped by the short amount, writing the next segment's payload\nat the wrong offset and over-reporting *cnt to the NFS client.\n\nSnapshot the segment's byte count before the write and compare\nhost_err against that snapshot so any short write breaks the loop."}],"metrics":[{"cvssV3_1":{"version":"3.1","vectorString":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N","baseScore":7.5,"baseSeverity":"HIGH"},"scenarios":[{"lang":"en","value":"AV:N - nfsd_direct_write() is reached from NFSv2/v3 WRITE (nfsd_proc_write/nfsd3_proc_write) and NFSv4 WRITE (nfsd4_write) in the in-kernel NFS server via svc_recv/svc_process over TCP/UDP 2049, so a remote client supplies the payload, offset, and length on the wire.\nAC:L - On nfsd hosts with io_cache_write=DIRECT, the attacker chooses WRITE offset and size to force a prefix/middle/suffix split (nsegs>=2) and can induce a 50-99% short write via quota, ENOSPC, or a concurrent truncate they also send; no race or layout outside their control is required.\nPR:N - Typical nfsd deployments accept AUTH_SYS/AUTH_NULL with IP-based export ACLs, so a network peer can MOUNT/LOOKUP, spoof an owner UID, and issue WRITE without cryptographic authentication or a local account on the NFS server.\nUI:N - The attacking NFS client sends the WRITE RPC itself; no separate victim user or administrator action on the server is required at attack time beyond nfsd already exporting a writable filesystem with direct writes enabled.\nS:U - Misplaced writes and the over-reported WRITE count affect only file data on the host running nfsd, remaining in that kernel/filesystem security authority without crossing a VM, IOMMU, or sandbox boundary.\nC:N - The bug only writes the client's own payload at a shifted file offset and over-reports the WRITE byte count; it does not read kernel memory or other inodes and is not a use-after-free or out-of-bounds read.\nI:H - After a 50-99% short write the loop continues and places the next segment at ki_pos advanced only by the short count, silently storing payload at the wrong file offset and over-reporting *cnt so clients treat corrupted server data as fully committed; when uncertain versus Low, High is required.\nA:N - The defect corrupts file contents and the NFS WRITE count without oops, panic, BUG_ON, deadlock, or hang; files remain accessible and nfsd workers continue, so there is no kernel or service crash."}]}],"affected":[{"product":"Linux","vendor":"Linux","defaultStatus":"unaffected","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["fs/nfsd/vfs.c"],"versions":[{"version":"06c5c97293e3fca99ce15da157068edf45a7c6e4","lessThan":"fa6590dfd16ab55f03b658b079072ace3504825e","status":"affected","versionType":"git"},{"version":"06c5c97293e3fca99ce15da157068edf45a7c6e4","lessThan":"250ec14932d5cfe102f68a57892bb566eee7f83e","status":"affected","versionType":"git"}]},{"product":"Linux","vendor":"Linux","defaultStatus":"affected","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","programFiles":["fs/nfsd/vfs.c"],"versions":[{"version":"6.19","status":"affected"},{"version":"0","lessThan":"6.19","status":"unaffected","versionType":"semver"},{"version":"7.2.4","lessThanOrEqual":"7.2.*","status":"unaffected","versionType":"semver"},{"version":"7.3-rc1","lessThanOrEqual":"*","status":"unaffected","versionType":"original_commit_for_fix"}]}],"cpeApplicability":[{"nodes":[{"operator":"OR","negate":false,"cpeMatch":[{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"6.19","versionEndExcluding":"7.2.4"},{"vulnerable":true,"criteria":"cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*","versionStartIncluding":"6.19","versionEndExcluding":"7.3-rc1"}]}]}],"references":[{"url":"https://git.kernel.org/stable/c/fa6590dfd16ab55f03b658b079072ace3504825e"},{"url":"https://git.kernel.org/stable/c/250ec14932d5cfe102f68a57892bb566eee7f83e"}],"title":"nfsd: fix partial-write detection in nfsd_direct_write","x_generator":{"engine":"bippy-1.2.0"}}}}