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SubjectRe: RIP: 0010:param_get_bool.cold+0x0/0x2 - LTP read_all_sys - 5.17.0
On Wed, Mar 30, 2022, Jan Stancek wrote:
> +CC kvm
>
> Issue seems to be that nx_huge_pages is not initialized (-1) and
> attempted to be used as boolean when reading
> /sys/module/kvm/parameters/nx_huge_pages

Ugh, CONFIG_UBSAN_BOOL=y complains about a bool not being 0 or 1. What a pain.

> CONFIG_KVM=Y, CONFIG_UBSAN=y, but kvm_mmu_module_init() doesn't
> appear to run, since kvm detects no HW support:
> # dmesg |grep kvm
> [ 0.000000] kvm-clock: Using msrs 4b564d01 and 4b564d00
> [ 0.000003] kvm-clock: using sched offset of 1155425753112 cycles
> [ 0.000007] clocksource: kvm-clock: mask: 0xffffffffffffffff
> max_cycles: 0x1cd42e4dffb, max_idle_ns: 881590591483 ns
> [ 0.045066] kvm-guest: PV spinlocks enabled
> [ 0.705370] clocksource: Switched to clocksource kvm-clock
> [ 0.913593] kvm: no hardware support for 'kvm_intel'
> [ 0.915574] kvm: no hardware support for 'kvm_amd'
> [ 2.284925] systemd[1]: Detected virtualization kvm.
> [ 4.158909] Stack Depot allocating hash table with kvmalloc
> [ 8.120446] systemd[1]: Detected virtualization kvm.
>
> Initializing 'nx_huge_pages' to 0 (in out branch) or write to
> /sys/module/kvm/parameters/nx_huge_pages before read makes it go away
> too:
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 02cf0a7e1d14..b3b8b9a22e20 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8921,6 +8921,7 @@ int kvm_arch_init(void *opaque)
> out_free_x86_emulator_cache:
> kmem_cache_destroy(x86_emulator_cache);
> out:
> + nx_huge_pages = 0;

This won't help, because nx_huge_pages is deliberately left as -1 if the vendor
module isn't loaded, in which case kvm_arch_init() won't be reached. This would
also incorrectly disable the mitigation.

We could fix it by adding a proper accessor, but that's rather silly because KVM
doesn't actually need to wait until a vendor module is loaded to finalize its
value (-1 means "auto"). kvm.ko doesn't have its own module_init() hook on x86,
but that's easily solved and I think less gross than having Schrödinger's param.

I'll test and send a patch.

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