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CVE-2026-63825
gcov: use atomic counter updates to fix concurrent access crashes
Published 2026-07-19 (3 months ago) by the Linux kernel CNA. Subsystem Makefile. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-63825 in every release this site tracks, and what each one means in practice. Every status below is the one the Debian Security Team publishes for this CVE. source
| Release | Status | Version | What this means |
|---|---|---|---|
| Debian 12bookworm | vulnerable | 6.1.190-1 | No fix published anywhere yet, neither in Debian nor upstream for the series bookworm tracks. Watch the Debian tracker page for this CVE. Advisories for bookworm have historically landed a median of 23 days after publication. |
| Debian 13trixie | vulnerable | 6.12.113-1 | No fix published anywhere yet, neither in Debian nor upstream for the series trixie tracks. Watch the Debian tracker page for this CVE. Advisories for trixie have historically landed a median of 11 days after publication. |
| Debian 14forky | fixed | 7.1.3-1 | Update the kernel and reboot. Fixed in 7.1.3-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.1.3-1 | Update the kernel and reboot. Fixed in 7.1.3-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
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.
Where it lives
The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.
Makefile
- What this is
- KERNEL BUILD + files below scripts/ (unless maintained elsewhere)MAINTAINERS section matched on Makefile · linux-kbuild@vger.kernel.org · Odd Fixes
Show whether Debian builds this, per release
Not established. This site could not find the build rule for the affected file in the kernel trees Debian ships, so it has no answer about whether Debian compiles this code. Read that as unknown, never as absent.
- bookworm, trixie, forky, sid?
- ?
- not established - no Makefile entry for the affected file
Where a release above reads "not established", no build rule for the affected file was found in that release's kernel tree. That is a gap in this lookup, not a finding that the code is absent.
From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS
Signals
- Published
- 2026-07-19 (3 months ago)source
- CVSS
- 9.8 critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.44%, higher than 36.6% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- Makefile
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 2.6.31 | 6.18.38 | 49d893b9cbcf |
| 2.6.31 | 7.1.3 | 5b959c1dbb45 |
| 2.6.31 | 7.2 | 56cb9b7d96b2 |
Why the CNA scored it this way
Quoted from the kernel CNA's own scoring file, word for word. A highlighted metric carries the most severe value CVSS v3.1 defines for it, which is what pushes the score up. source
- AV:NAttack vector, network
- The demonstrated crash path is inbound IPComp processing via xfrm_input on network-received packets (xfrm4_ipcomp_rcv → ipcomp_input → deflate_decompress → inflate_fast), reachable by any remote host that can send IP traffic to a system with a matching IPComp SA.
- AC:LAttack complexity, low
- Exploitation requires a concurrent GCOV counter race, and the attacker fully controls timing by flooding parallel IPComp/deflate decompression requests; per-CPU acompress streams allow inflate_fast() to run simultaneously on multiple CPUs.
- PR:NPrivileges required, none
- No local account or kernel capabilities are needed on the victim; a remote attacker only needs network reachability to an endpoint with IPComp configured and can trigger decompression by sending crafted IPComp packets matching an existing SA.
- UI:NUser interaction, none
- Exploitation is driven entirely by attacker-sent network packets and requires no action from any local user or administrator at attack time.
- S:UScope, unchanged
- The out-of-bounds write corrupts kernel memory within the same kernel security domain; it enables local privilege escalation but does not inherently cross a VM, container, or IOMMU boundary.
- C:HConfidentiality impact, high
- The bug produces a large out-of-bounds kernel write (documented 3.4 MB past a 65 KB buffer), which is arbitrary memory corruption that can be leveraged for kernel memory disclosure, not merely a bounded crash.
- I:HIntegrity impact, high
- The corrupted write targets kernel heap memory far outside the decompression buffer, providing a memory-corruption primitive suitable for control-flow hijacking and arbitrary kernel code execution.
- A:HAvailability impact, high
- The reported failure is a kernel page fault in inflate_fast() during IPComp processing, and the underlying out-of-bounds write can reliably cause kernel oops/panic or hang.