linux-kernel - Re: RIP: 0010:param_get_bool.cold+0x0/0x2 - LTP read_all_sys

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Message-ID: <YkXv0NoBjLBYBzX8@google.com>
Date:   Thu, 31 Mar 2022 18:15:44 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Jan Stancek <jstancek@...hat.com>
Cc:     Bruno Goncalves <bgoncalv@...hat.com>, kvm <kvm@...r.kernel.org>,
        "Bonzini, Paolo" <pbonzini@...hat.com>,
        lkml <linux-kernel@...r.kernel.org>,
        CKI Project <cki-project@...hat.com>,
        Li Wang <liwang@...hat.com>
Subject: Re: 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.