NFSD: check truncate permission under inode lock

Summary

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.

Timeline

1. Sep 11, 2026, 7:46 PM UTCCVE published
2. Oct 16, 2026, 7:26 PM UTCCISA SSVC assessment

   Exploitation: none · Automatable: yes · Technical impact: partial

3. Oct 30, 2026, 12:32 PM UTCRecord last updated

Affected versions

• Linux Linux

  From 3.2.89 · fixed in 3.3; From 3.16.44 · fixed in 3.17; From 4.4.53 · fixed in 4.5; From 4.9.14 · fixed in 4.10; From 4.10.2 · fixed in 4.11

• Linux Linux

  4.11

• KernelLinux

  From 4.11.0 · fixed in 6.6.160; From 6.7.0 · fixed in 6.12.109; From 6.13.0 · fixed in 6.18.50; From 6.19.0 · fixed in 7.2.4

References 6