[PATCH v3] x86/kexec: Disable KCOV instrumentation after load_segments()

[PATCH v3] x86/kexec: Disable KCOV instrumentation after load_segments()

[Aleksandr Nogikh]

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

The real-world context for this problem is enabling crash dump
collection in syzkaller. For this, the tool loads a panic kernel
before fuzzing and then calls makedumpfile after the panic. This
workflow requires both CONFIG_KEXEC and CONFIG_KCOV to be enabled
simultaneously.

Adding safeguards directly to the KCOV fast-path
(__sanitizer_cov_trace_pc()) is also undesirable as it would
introduce an extra performance overhead.

Disabling instrumentation for the individual functions would be too
fragile, so disable KCOV instrumentation for the entire
machine_kexec_64.c and physaddr.c. If coverage-guided fuzzing ever
needs these components in the future, other approaches should be
considered.

The problem is not relevant for 32 bit kernels as CONFIG_KCOV is not
supported there.

Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Aleksandr Nogikh <nogikh@google.com>
Fixes: 0d345996e4cb ("x86/kernel: increase kcov coverage under arch/x86/kernel folder")
Cc: stable@vger.kernel.org
---
v3:
Changed the wording of the commit description and the comments.
Added a Fixes tag.

v2:
https://lore.kernel.org/all/20260317220319.788561-1-nogikh@google.com/

Updated the comments to explain the underlying context.

v1:
https://lore.kernel.org/all/20260216173716.2279847-1-nogikh@google.com/
---
 arch/x86/kernel/Makefile | 11 +++++++++++
 arch/x86/mm/Makefile     |  2 ++
 2 files changed, 13 insertions(+)

diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index e9aeeeafad173..febf6f49207b3 100644
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -43,6 +43,17 @@ KCOV_INSTRUMENT_dumpstack_$(BITS).o			:= n
 KCOV_INSTRUMENT_unwind_orc.o				:= n
 KCOV_INSTRUMENT_unwind_frame.o				:= n
 KCOV_INSTRUMENT_unwind_guess.o				:= n
+# Disable KCOV to prevent crashes during kexec: load_segments() invalidates
+# the GS base, which KCOV relies on for per-CPU data.
+# As KCOV && KEXEC compatibility should be preserved (e.g. syzkaller is
+# using it to collect crash dumps during kernel fuzzing), disabling
+# KCOV for KEXEC kernels is not an option. Selectively disabling KCOV
+# instrumentation for individual affected functions can be fragile, while
+# adding more checks to KCOV would slow it down.
+# As a compromise solution, disable KCOV instrumentation for the whole
+# source code file. If its coverage is ever needed, other approaches
+# should be considered.
+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..3a5364853eab8 100644
--- a/arch/x86/mm/Makefile
+++ b/arch/x86/mm/Makefile
@@ -4,6 +4,8 @@ KCOV_INSTRUMENT_tlb.o			:= n
 KCOV_INSTRUMENT_mem_encrypt.o		:= n
 KCOV_INSTRUMENT_mem_encrypt_amd.o	:= n
 KCOV_INSTRUMENT_pgprot.o		:= n
+# See the "Disable KCOV" comment in arch/x86/kernel/Makefile.
+KCOV_INSTRUMENT_physaddr.o		:= n
 
 KASAN_SANITIZE_mem_encrypt.o		:= n
 KASAN_SANITIZE_mem_encrypt_amd.o	:= n

base-commit: c369299895a591d96745d6492d4888259b004a9e
-- 
2.53.0.1018.g2bb0e51243-goog