CVE-2023-52759

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

gfs2: ignore negated quota changes

When lots of quota changes are made, there may be cases in which an inode's quota information is increased and then decreased, such as when blocks are added to a file, then deleted from it. If the timing is right, function doqc can add pending quota changes to a transaction, then later, another call to doqc can negate those changes, resulting in a net gain of 0. The quotachange information is recorded in the qc buffer (and qd element of the inode as well). The buffer is added to the transaction by the first call to doqc, but a subsequent call changes the value from non-zero back to zero. At that point it's too late to remove the buffer_head from the transaction. Later, when the quota sync code is called, the zero-change qd element is discovered and flagged as an assert warning. If the fs is mounted with errors=panic, the kernel will panic.

This is usually seen when files are truncated and the quota changes are negated by punchhole/truncate which uses gfs2quotahold and gfs2quotaunhold rather than block allocations that use gfs2quotalock and gfs2quota_unlock which automatically do quota sync.

This patch solves the problem by adding a check to qdchecksync such that net-zero quota changes already added to the transaction are no longer deemed necessary to be synced, and skipped.

In this case references are taken for the qd and the slot from do_qc so those need to be put. The normal sequence of events for a normal non-zero quota change is as follows:

gfs2quotachange doqc qdhold slot_hold

Later, when the changes are to be synced:

gfs2quotasync qdfish qdchecksync gets qd ref via lockrefgetnotdead dosync doqc(QCSYNC) qdput lockrefputorlock qdunlock qdput lockrefputorlock

In the net-zero change case, we add a check to qdchecksync so it puts the qd and slot references acquired in gfs2quotachange and skip the unneeded sync.