Commit 917e3ad3 authored by Aleksandr Nogikh's avatar Aleksandr Nogikh Committed by Borislav Petkov (AMD)
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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: 0d345996 ("x86/kernel: increase kcov coverage under arch/x86/kernel folder")
Signed-off-by: default avatarAleksandr Nogikh <nogikh@google.com>
Signed-off-by: default avatarBorislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: default avatarDmitry Vyukov <dvyukov@google.com>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/20260325154825.551191-1-nogikh@google.com
parent 7aaa8047

arch/x86/kernel/Makefile

+14 −0
Original line number Diff line number Diff line
@@ -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

arch/x86/mm/Makefile

+2 −0
Original line number Diff line number Diff line
@@ -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