CVE-2026-89713
CriticalCVSS 9.1Exploitation Probability (EPSS)
Low risk10th percentile - higher than 10% of all known CVEs
Summary
In the Linux kernel, nfsd_setattr() checked the NFSD_MAY_TRUNC permission using a file size sampled without the inode lock, while the actual size update was applied later under inode_lock(). This created a TOCTOU window for append-only files where a concurrent append could extend the file before a real truncation was applied without the required check.
Risk Assessment
An authenticated NFS client can bypass the append-only permission check and cause file truncation, violating data integrity.
Recommendation
Update the Linux kernel to a version containing the fix that rechecks the size under inode_lock() before notify_change(ATTR_SIZE).
Other vulnerabilities in Linux kernel
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Original NVD description (English source)
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.