x86/kexec: Disable KCOV instrumentation after load_segments()

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

Posted by Aleksandr Nogikh 2 weeks, 2 days ago

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
---
 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

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

Posted by Dmitry Vyukov 5 days, 15 hours ago

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
>

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

Posted by Aleksandr Nogikh 1 week, 2 days ago

On Mon, Feb 16, 2026 at 6:37 PM 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
> ---
>  arch/x86/kernel/Makefile | 4 ++++
>  arch/x86/mm/Makefile     | 4 ++++
>  2 files changed, 8 insertions(+)

A gentle ping on this patch.

Should it go through the x86 tree or the mm tree like other kcov patches?

>
> 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
>