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 do_qc can add pending quota changes to a
transaction, then later, another call to do_qc can negate those changes,
resulting in a net gain of 0. The quota_change 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 do_qc, 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 punch_hole/truncate which uses gfs2_quota_hold and
gfs2_quota_unhold rather than block allocations that use gfs2_quota_lock
and gfs2_quota_unlock which automatically do quota sync. This patch solves
the problem by adding a check to qd_check_sync 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: gfs2_quota_change do_qc
qd_hold slot_hold Later, when the changes are to be synced: gfs2_quota_sync
qd_fish qd_check_sync gets qd ref via lockref_get_not_dead do_sync
do_qc(QC_SYNC) qd_put lockref_put_or_lock qd_unlock qd_put
lockref_put_or_lock In the net-zero change case, we add a check to
qd_check_sync so it puts the qd and slot references acquired in
gfs2_quota_change and skip the unneeded sync.