Low [CVE-2026-63825] use atomic counter updates to fix concurrent access crashes
This low-severity Red Hat Linux advisory covers CVE-2026-63825.
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Summary
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.
Affected versions
No affected-version range was extracted from the source record. The vendor advisory is authoritative. Check it before change work.
Official advisory · medium-confidence parse· fetched 8 days ago·verify at source
Fixed versions
No fixed release is recorded yet. That does not prove no patch exists. Confirm against the vendor advisory.
Official advisory · medium-confidence parse· fetched 8 days ago·verify at source
Mitigation
The source record does not include mitigation steps. That does not mean no fix exists. Read the vendor advisory below for the authoritative guidance.
Official advisory · medium-confidence parse· fetched 8 days ago·verify at source
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