Re: [PATCH] x86/kexec: Disable KCOV instrumentation after load_segments()

[Aleksandr Nogikh <nogikh@google.com>]

+Cc linux-mm

On Fri, Feb 27, 2026 at 2:26 PM Dmitry Vyukov <dvyukov@google.com> wrote:
>
> On Mon, 16 Feb 2026 at 18:37, Aleksandr Nogikh <nogikh@google.com> wrote:
> >
> > The load_segments() function changes segment registers, invalidating
> > GS base (which KCOV relies on for per-cpu data). When CONFIG_KCOV is
> > enabled, any subsequent instrumented C code call (e.g.
> > native_gdt_invalidate()) begins crashing the kernel in an
> > endless loop.
> >
> > To reproduce the problem, it's sufficient to do kexec on a
> > KCOV-instrumented kernel:
> > $ kexec -l /boot/otherKernel
> > $ kexec -e
> >
> > (additional problems arise when the kernel is booting into a crash
> > kernel)
> >
> > Disabling instrumentation for the individual functions would be too
> > fragile, so let's fix the bug by disabling KCOV instrumentation for
> > the whole machine_kexec_64.c and physaddr.c.
> >
> > The problem is not relevant for 32 bit kernels as CONFIG_KCOV is not
> > supported there.
> >
> > Signed-off-by: Aleksandr Nogikh <nogikh@google.com>
> > Cc: stable@vger.kernel.org
>
> Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
>
> > ---
> >  arch/x86/kernel/Makefile | 4 ++++
> >  arch/x86/mm/Makefile     | 4 ++++
> >  2 files changed, 8 insertions(+)
> >
> > diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
> > index e9aeeeafad173..5703fa6027866 100644
> > --- a/arch/x86/kernel/Makefile
> > +++ b/arch/x86/kernel/Makefile
> > @@ -43,6 +43,10 @@ KCOV_INSTRUMENT_dumpstack_$(BITS).o                  := n
> >  KCOV_INSTRUMENT_unwind_orc.o                           := n
> >  KCOV_INSTRUMENT_unwind_frame.o                         := n
> >  KCOV_INSTRUMENT_unwind_guess.o                         := n
> > +# When a kexec kernel is loaded, calling load_segments() breaks all
> > +# subsequent KCOV instrumentation until new kernel takes control.
> > +# Keep KCOV instrumentation disabled to prevent kernel crashes.
> > +KCOV_INSTRUMENT_machine_kexec_64.o                     := n
> >
> >  CFLAGS_head32.o := -fno-stack-protector
> >  CFLAGS_head64.o := -fno-stack-protector
> > diff --git a/arch/x86/mm/Makefile b/arch/x86/mm/Makefile
> > index 5b9908f13dcfd..a678a38a40266 100644
> > --- a/arch/x86/mm/Makefile
> > +++ b/arch/x86/mm/Makefile
> > @@ -4,6 +4,10 @@ KCOV_INSTRUMENT_tlb.o                  := n
> >  KCOV_INSTRUMENT_mem_encrypt.o          := n
> >  KCOV_INSTRUMENT_mem_encrypt_amd.o      := n
> >  KCOV_INSTRUMENT_pgprot.o               := n
> > +# When a kexec kernel is loaded, calling load_segments() breaks all
> > +# subsequent KCOV instrumentation until new kernel takes control.
> > +# Keep KCOV instrumentation disabled to prevent kernel crashes.
> > +KCOV_INSTRUMENT_physaddr.o             := n
> >
> >  KASAN_SANITIZE_mem_encrypt.o           := n
> >  KASAN_SANITIZE_mem_encrypt_amd.o       := n
> > --
> > 2.53.0.273.g2a3d683680-goog
> >