diff options
| author | Aleksandr Nogikh <nogikh@google.com> | 2026-03-25 16:48:24 +0100 |
|---|---|---|
| committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2026-04-11 14:29:43 +0200 |
| commit | de05c66fab8847237a9ca216934e56d3ee837f08 (patch) | |
| tree | 0bc6c297850169658ec0ba38544b592410fa472e /arch | |
| parent | fb1064fcf848529b221f32e254e399aa9751ab37 (diff) | |
x86/kexec: Disable KCOV instrumentation after load_segments()
commit 917e3ad3321e75ca0223d5ccf26ceda116aa51e1 upstream.
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
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Diffstat (limited to 'arch')
| -rw-r--r-- | arch/x86/kernel/Makefile | 14 | ||||
| -rw-r--r-- | arch/x86/mm/Makefile | 2 |
2 files changed, 16 insertions, 0 deletions
diff --git a/arch/x86/kernel/Makefile b/arch/x86/kernel/Makefile index bc184dd38d99..558b96d53e00 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 5b9908f13dcf..3a5364853eab 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 |