CVE-2026-89713

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

NFSD: check truncate permission under inode lock

nfsdsetattr() checks whether a size update needs NFSDMAY_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 inodelock() by notifychange().

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 inodelock(). notifychange() then applies a real truncation

without the NFSDMAYTRUNC 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 getwriteaccess() before the locked setattr work, then recheck

whether the requested size is below isizeread(inode) after inode_lock()

has been acquired and before notifychange(ATTRSIZE). This also avoids

the plain unlocked inode->i_size load.