superrob1500
/
mirror-linux
mirror of https://github.com/torvalds/linux
1
0
Fork 0
Code Issues Packages Projects Releases Wiki Activity

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: 0d345996e4 ("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
master
Aleksandr Nogikh 2026-03-25 16:48:24 +01:00 committed by Borislav Petkov (AMD)
parent 7aaa8047ea
commit 917e3ad332
2 changed files with 16 additions and 0 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

14
arch/x86/kernel/Makefile
View File

@ -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

2
arch/x86/mm/Makefile
View File

@ -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