In the Linux kernel, the following vulnerability has been resolved:
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. ]