NFSD: check truncate permission under inode lock

Overview

In the Linux kernel, the following vulnerability has been resolved:

NFSD: check truncate permission under inode lock

nfsd_setattr() checks whether a size update needs NFSD_MAY_TRUNC before it takes inode_lock(). The comparison uses the file size sampled by that unlocked read, but the actual ATTR_SIZE update is applied later under inode_lock() by notify_change().

This leaves a TOCTOU window for append-only files. If a client sends a SETATTR that does not shrink the file at the time of the unlocked sample, a concurrent append can extend the file before nfsd_setattr() takes inode_lock(). notify_change() then applies a real truncation without the NFSD_MAY_TRUNC check that rejects IS_APPEND(inode). The VFS truncate syscall paths perform their own append-only checks before calling notify_change(), so NFSD must make this decision against the locked size it is about to change.

Split the write-count acquisition from the truncation permission check. Keep get_write_access() before the locked setattr work, then recheck whether the requested size is below i_size_read(inode) after inode_lock() has been acquired and before notify_change(ATTR_SIZE). This also avoids the plain unlocked inode->i_size load.

Affected

• Linux >= 783112f7401ff449d979530209b3f6c2594fdb4e < 52d0c5bdfe232c9974d4b11f97078b9c7814f05d
• Linux >= 783112f7401ff449d979530209b3f6c2594fdb4e < 3afa17d93ba8c925f49370c816c6dae5112d8c24
• Linux >= 783112f7401ff449d979530209b3f6c2594fdb4e < d8352da196349182e1afd5a93308256cddc0a97d
• Linux >= 783112f7401ff449d979530209b3f6c2594fdb4e < 44086254479035de42ca3d286ecf25521d4e6325
• Linux >= 783112f7401ff449d979530209b3f6c2594fdb4e < b778e0e0a16759f22a70579c3cf8d254a40d4a7f
• Linux 604a3c407026d6162d15300478e63f901e435efc
• Linux cc4d5dc73841b98d33cdfb9822d70b0aac4beca5
• Linux 3ee4f442e5b37a537297b812557b1163f96b5399
• Linux a3c6cbc4eac4473ed5461d5faae2794d3e5c0e44
• Linux 982898d7f97a35447403c3fcecc0d96c646ce101
• Linux >= 3.2.89 < 3.3
• Linux >= 3.16.44 < 3.17
• Linux >= 4.4.53 < 4.5
• Linux >= 4.9.14 < 4.10
• Linux >= 4.10.2 < 4.11
• Linux 4.11

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

Vendor advisories

• Red Hat VEX · Important · affected: Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 7, Red Hat Enterprise Linux 8, Red Hat Enterprise Linux 9, Red Hat OpenShift Container Platform 4, Red Hat Enterprise Linux 6 · no fix planned: Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 9, Red Hat OpenShift Container Platform 4, Red Hat Enterprise Linux 7, … · updated 2026-09-21 · vex