CVE-2026-89713

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.

Affected Software

Weaknesses

References

• https://git.kernel.org/stable/c/3afa17d93ba8c925f49370c816c6dae5112d8c24
• https://git.kernel.org/stable/c/d8352da196349182e1afd5a93308256cddc0a97d
• https://git.kernel.org/stable/c/44086254479035de42ca3d286ecf25521d4e6325
• https://git.kernel.org/stable/c/b778e0e0a16759f22a70579c3cf8d254a40d4a7f

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