CVE-2026-63825 in Linux
Summary
by MITRE • 07/19/2026
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.
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Analysis
by VulDB Data Team • 07/19/2026
The vulnerability in question relates to a concurrency issue within the Linux kernel's garbage collection and profiling mechanisms, specifically involving the GNU Compiler Collection's GCOV instrumentation system. This flaw manifests when multiple CPUs execute the inflate_fast() function simultaneously during IPComp (IP Payload Compression) processing, leading to unpredictable memory access patterns and potential system crashes. The root cause stems from how GCC optimizes global branch counters by merging them with loop induction variables in certain contexts, particularly within compression routines where performance optimization is critical.
The technical implementation of this vulnerability involves the compiler's aggressive optimization of GCOV profiling counters through a process that loads the same global counter value multiple times during loop computation. When concurrent execution occurs across different CPU cores, the counter value can change between these individual load operations, creating inconsistent state information for loop bounds calculation. This inconsistency directly translates into out-of-bounds memory writes that can corrupt kernel memory structures and ultimately result in system crashes as evidenced by the page fault error trace showing RIP at inflate_fast+1431.
From a cybersecurity perspective, this vulnerability represents a classic race condition scenario that aligns with CWE-362 (Concurrent Execution using Shared Resource with Improper Synchronization) and can be mapped to ATT&CK technique T1059.008 (Command and Scripting Interpreter: Python). The impact extends beyond simple system instability to potentially enabling privilege escalation or denial of service conditions, particularly in network processing contexts where IPComp is actively utilized. The crash pattern demonstrates memory corruption that could theoretically be exploited to gain unauthorized access to kernel memory spaces.
The mitigation strategy implemented involves modifying the kernel build configuration to use atomic counter updates for GCOV instrumentation, specifically adding -fprofile-update=prefer-atomic to global compiler flags. This approach employs a sophisticated compile-time detection mechanism that tests whether the flag would introduce linking dependencies on libatomic libraries by comparing undefined symbols in generated object files. The solution is architecture-aware, enabling the atomic update preference only on platforms where GCC can inline 64-bit atomic operations without external library dependencies. For platforms where such support is lacking, the flag is silently omitted to maintain backward compatibility while preserving existing behavior.
The implementation details show careful attention to build system placement by moving the CFLAGS_GCOV configuration block to ensure proper flag evaluation during the compilation process. This strategic relocation allows the test to capture arch-specific compiler flags like -m32 and -march= while avoiding interference from GCC plugin flags that might not yet be available in clean builds. The approach demonstrates adherence to kernel development best practices by ensuring atomic operations are only enabled where they can function properly without introducing additional dependencies or breaking existing build processes, thereby maintaining system stability across different hardware platforms while addressing the specific concurrency issue affecting network compression functionality.