CVE-2026-63825
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 i
CSIRTS triage
- What
- Concurrent access crashes can occur due to improper atomic counter updates in gcov.
- Who is affected
- Deployments using gcov for code coverage analysis.
- Urgency
- Remediation is necessary as the severity is unknown and exploitation is not confirmed.
- Action
- Apply the patch for CVE-2026-63825.
AI-assisted analysis generated from the source advisory — verify against the original.
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Exploitation outlook
- Low exploitation risk0.41% 30-day exploitation probability — currently an unlikely target, but scores change as exploit code circulates. Riskier than 34% of all EPSS-scored CVEs.
Advisory coverage (2)
External references
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