CVE-2026-99235: tracing: Save the event file instead of the compatible histogram

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

tracing: Save the event file instead of the compatible histogram

createfieldvarhist() creates a histogram on the matched event to supply a field variable, and records it on the target so it can be torn down later:

histdata = findcompatiblehist(targethistdata, file); ... / Save the compatible histogram information / varhist->histdata = histdata;

histdata is not the histogram it creates. It is one that was already there, whose keys the new one copies. The saved pointer has a single user, unregisterfieldvarhists(), which runs when the target is removed:

file = histdata->fieldvarhists[i]->histdata->eventfile;

findcompatiblehist() matches on keys alone, so it can pick one with no variables of its own. checkvarrefs() then returns false and the user can remove it while the target still points at it:

BUG: KASAN: slab-use-after-free in eventhisttriggerfree+0x2b2/0x320 Read of size 8 at addr ffff8880093b90e0 by task init/1

Freed by task 1: kfree+0x154/0x420 eventhisttriggerfree+0x1d5/0x320 eventhisttriggerparse+0x35f3/0x69e0 triggerprocessregex+0x1a6/0x250

Save the event file instead. It is all the dereference ever wanted, and it does not go away when the user removes a trigger.