Re: [PATCH] powerpc/kvm: don't crash on missing rng, and use darn

[Fabiano Rosas <farosas@linux.ibm.com>]

"Jason A. Donenfeld" <Jason@zx2c4.com> writes:

> On POWER8 systems that don't have ibm,power-rng available, a guest that
> ignores the KVM_CAP_PPC_HWRNG flag and calls H_RANDOM anyway will
> dereference a NULL pointer. And on machines with darn instead of
> ibm,power-rng, H_RANDOM won't work at all.
>
> This patch kills two birds with one stone, by routing H_RANDOM calls to
> ppc_md.get_random_seed, and doing the real mode check inside of it.
>
> Cc: stable@vger.kernel.org # v4.1+
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Fixes: e928e9cb3601 ("KVM: PPC: Book3S HV: Add fast real-mode H_RANDOM implementation.")
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
>
> This patch must be applied ontop of:
> 1) https://github.com/linuxppc/linux/commit/f3eac426657d985b97c92fa5f7ae1d43f04721f3
> 2) https://lore.kernel.org/all/20220622102532.173393-1-Jason@zx2c4.com/
>
>
>  arch/powerpc/include/asm/archrandom.h |  5 ----
>  arch/powerpc/kvm/book3s_hv_builtin.c  |  5 ++--
>  arch/powerpc/platforms/powernv/rng.c  | 33 +++++++--------------------
>  3 files changed, 11 insertions(+), 32 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/archrandom.h b/arch/powerpc/include/asm/archrandom.h
> index 11d4815841ab..3af27bb84a3d 100644
> --- a/arch/powerpc/include/asm/archrandom.h
> +++ b/arch/powerpc/include/asm/archrandom.h
> @@ -38,12 +38,7 @@ static inline bool __must_check arch_get_random_seed_int(unsigned int *v)
>  #endif /* CONFIG_ARCH_RANDOM */
>
>  #ifdef CONFIG_PPC_POWERNV
> -int pnv_hwrng_present(void);
>  int pnv_get_random_long(unsigned long *v);
> -int pnv_get_random_real_mode(unsigned long *v);
> -#else
> -static inline int pnv_hwrng_present(void) { return 0; }
> -static inline int pnv_get_random_real_mode(unsigned long *v) { return 0; }
>  #endif
>
>  #endif /* _ASM_POWERPC_ARCHRANDOM_H */
> diff --git a/arch/powerpc/kvm/book3s_hv_builtin.c b/arch/powerpc/kvm/book3s_hv_builtin.c
> index 799d40c2ab4f..1c6672826db5 100644
> --- a/arch/powerpc/kvm/book3s_hv_builtin.c
> +++ b/arch/powerpc/kvm/book3s_hv_builtin.c
> @@ -176,13 +176,14 @@ EXPORT_SYMBOL_GPL(kvmppc_hcall_impl_hv_realmode);
>
>  int kvmppc_hwrng_present(void)
>  {
> -	return pnv_hwrng_present();
> +	return ppc_md.get_random_seed != NULL;
>  }
>  EXPORT_SYMBOL_GPL(kvmppc_hwrng_present);
>
>  long kvmppc_rm_h_random(struct kvm_vcpu *vcpu)
>  {
> -	if (pnv_get_random_real_mode(&vcpu->arch.regs.gpr[4]))
> +	if (ppc_md.get_random_seed &&
> +	    ppc_md.get_random_seed(&vcpu->arch.regs.gpr[4]))
>  		return H_SUCCESS;

This is the same as arch_get_random_seed_long, perhaps you could use it
instead.

Otherwise, the archrandom.h include is not needed anymore and could be
replaced with #include <asm/machdep.h> for ppc_md.

>
>  	return H_HARDWARE;
> diff --git a/arch/powerpc/platforms/powernv/rng.c b/arch/powerpc/platforms/powernv/rng.c
> index 868bb9777425..c748567cd47e 100644
> --- a/arch/powerpc/platforms/powernv/rng.c
> +++ b/arch/powerpc/platforms/powernv/rng.c
> @@ -29,15 +29,6 @@ struct pnv_rng {
>
>  static DEFINE_PER_CPU(struct pnv_rng *, pnv_rng);
>
> -int pnv_hwrng_present(void)
> -{
> -	struct pnv_rng *rng;
> -
> -	rng = get_cpu_var(pnv_rng);
> -	put_cpu_var(rng);
> -	return rng != NULL;
> -}
> -
>  static unsigned long rng_whiten(struct pnv_rng *rng, unsigned long val)
>  {
>  	unsigned long parity;
> @@ -58,17 +49,6 @@ static unsigned long rng_whiten(struct pnv_rng *rng, unsigned long val)
>  	return val;
>  }
>
> -int pnv_get_random_real_mode(unsigned long *v)
> -{
> -	struct pnv_rng *rng;
> -
> -	rng = raw_cpu_read(pnv_rng);
> -
> -	*v = rng_whiten(rng, __raw_rm_readq(rng->regs_real));
> -
> -	return 1;
> -}
> -
>  static int pnv_get_random_darn(unsigned long *v)
>  {
>  	unsigned long val;
> @@ -105,11 +85,14 @@ int pnv_get_random_long(unsigned long *v)
>  {
>  	struct pnv_rng *rng;
>
> -	rng = get_cpu_var(pnv_rng);
> -
> -	*v = rng_whiten(rng, in_be64(rng->regs));
> -
> -	put_cpu_var(rng);
> +	if (mfmsr() & MSR_DR) {
> +		rng = raw_cpu_read(pnv_rng);
> +		*v = rng_whiten(rng, __raw_rm_readq(rng->regs_real));
> +	} else {
> +		rng = get_cpu_var(pnv_rng);
> +		*v = rng_whiten(rng, in_be64(rng->regs));
> +		put_cpu_var(rng);
> +	}
>
>  	return 1;
>  }