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

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

Posted by Aleksandr Nogikh 1 week, 1 day 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

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

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

Posted by tip-bot2 for Aleksandr Nogikh 3 days, 9 hours ago

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     917e3ad3321e75ca0223d5ccf26ceda116aa51e1
Gitweb:        https://git.kernel.org/tip/917e3ad3321e75ca0223d5ccf26ceda116aa51e1
Author:        Aleksandr Nogikh <nogikh@google.com>
AuthorDate:    Wed, 25 Mar 2026 16:48:24 +01:00
Committer:     Borislav Petkov (AMD) <bp@alien8.de>
CommitterDate: Mon, 30 Mar 2026 14:15:25 +02:00

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
---
 arch/x86/kernel/Makefile | 14 ++++++++++++++
 arch/x86/mm/Makefile     |  2 ++
 2 files changed, 16 insertions(+)

diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile
index e9aeeea..47a32f5 100644
--- 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 5b9908f..3a53648 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