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

   _deflatedecompress

   cryptocompdecompress

   ipcompdecompress [xfrmipcomp]

   ipcompinput [xfrmipcomp]

   xfrm_input

At the crash point, the compiler generated three loads from the same

global GCOV counter (_gcov0.inflatefast+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 _atomicfetchadd8 on i386 or _aarch64ldadd8_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.