# CVE-2026-63825: gcov: use atomic counter updates to fix concurrent access crashes

> Concurrent GCOV counter updates in the Linux kernel can cause out-of-bounds writes and crashes during IPComp processing, requiring atomic counter updates.

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 19 Jul 2026
- **Updated:** 21 Oct 2026
- **Also known as:** AZL-92516, BELL-CVE-2026-63825, DEBIAN-CVE-2026-63825, GHSA-xc4c-7263-hq9r, ROOT-OS-DEBIAN-11-CVE-2026-63825, ROOT-OS-DEBIAN-12-CVE-2026-63825, ROOT-OS-DEBIAN-13-CVE-2026-63825, ROOT-OS-UBUNTU-2204-CVE-2026-63825, UBUNTU-CVE-2026-63825

## What is CVE-2026-63825?

CVE-2026-63825 is a critical vulnerability in Linux, published on 19 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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

gcov: use atomic counter updates to fix concurrent access crashes

GCC's GCOV instrumentation can merge global branch counters with loop
induction variables as an optimization.  In inflate_fast(), the inner copy
loops get transformed so that the GCOV counter value is loaded multiple
times to compute the loop base address, start index, and end bound.  Since
GCOV counters are global (not per-CPU), concurrent execution on different
CPUs causes the counter to change between loads, producing inconsistent
values and out-of-bounds memory writes.

The crash manifests during IPComp (IP Payload Compression) processing when
inflate_fast() runs concurrently on multiple CPUs:

  BUG: unable to handle page fault for address: ffffd0a3c0902ffa
  RIP: inflate_fast+1431
  Call Trace:
   zlib_inflate
   __deflate_decompress
   crypto_comp_decompress
   ipcomp_decompress [xfrm_ipcomp]
   ipcomp_input [xfrm_ipcomp]
   xfrm_input

At the crash point, the compiler generated three loads from the same
global GCOV counter (__gcov0.inflate_fast+216) to compute base, start, and
end for an indexed loop.  Another CPU modified the counter between loads,
making the values inconsistent - the write went 3.4 MB past a 65 KB
buffer.

Add -fprofile-update=prefer-atomic to CFLAGS_GCOV at the global level in
the top-level Makefile, guarded by a try-run compile test.  The test
compiles a minimal program with and without -fprofile-update=prefer-atomic
using the full KBUILD_CFLAGS, then compares undefined symbols in the
resulting object files.  If prefer-atomic introduces new undefined
references (such as __atomic_fetch_add_8 on i386 or __aarch64_ldadd8_relax
on arm64 with outline-atomics), the flag is not added -- the kernel does
not link against libatomic.

On architectures where GCC inlines 64-bit atomic counter updates (x86_64,
s390, ...) the test passes and the flag is enabled, preventing the
compiler from merging counters with loop induction variables and fixing
the observed concurrent-access crash.

On architectures where the flag would introduce libatomic dependencies, it
is silently omitted and behaviour is no worse than before this patch.

Move the CFLAGS_GCOV block from its original position (before the arch
Makefile include) to after the core KBUILD_CFLAGS assignments but before
the scripts/Makefile.gcc-plugins include.  This placement ensures the
try-run test sees arch-specific flags (-m32, -march=,
-mno-outline-atomics) while avoiding GCC plugin flags (-fplugin=) that
would break the test on clean builds when plugin shared objects do not yet
exist.

## Which versions are affected by CVE-2026-63825?


## How do I fix CVE-2026-63825?

Apply the vendor's patch.

## Is CVE-2026-63825 being exploited?

There is no known exploitation of CVE-2026-63825 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-63825 and fix it if it is affected.

What it is: CVE-2026-63825 is a critical vulnerability in Linux, published on 19 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-63825 in the wild and no public exploit code.

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-63825/ (Markdown: https://insights.sbomify.com/CVE-2026-63825/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-63825
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-63825
- OSV: https://osv.dev/vulnerability/CVE-2026-63825

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-63825/
Summaries are generated and may contain mistakes; the official description above is the record.