x86/kexec: Disable KCOV instrumentation after load_segments()

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.

  [ bp: Space out comment for better readability. ]

Fixes: 0d345996e4cb ("x86/kernel: increase kcov coverage under arch/x86/kernel folder")
Signed-off-by: Aleksandr Nogikh <nogikh@google.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260325154825.551191-1-nogikh@google.com

diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index e9aeeeafad173a3b79abfad21bf064f6b79fd3e4..47a32f5839300438045bdd4a33dc92c0ffb6afbc 100644 (file)
--- a/arch/x86/kernel/Makefile
+++ b/arch/x86/kernel/Makefile
@@ -44,6 +44,20 @@ 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 and 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
 CFLAGS_irq.o := -I $(src)/../include/asm/trace

diff --git a/arch/x86/mm/Makefile b/arch/x86/mm/Makefile
index 5b9908f13dcfd092897f3778ee56ea4d45bdb868..3a5364853eab876856c9132f44b886c8b1607f4c 100644 (file)
--- 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