linux-kernel - Re: [PATCH v2 30/38] x86/paravirt: Don't use a PV sched

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite for Android: free password hash cracker in your pocket

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <okuuhll3ymxlvno46dlimlpnkhg5vcxm2jiaew7uce4f35sps3@xaommgjd447m>
Date: Thu, 27 Feb 2025 15:17:26 +0200
From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	Paolo Bonzini <pbonzini@...hat.com>, Juergen Gross <jgross@...e.com>, 
	"K. Y. Srinivasan" <kys@...rosoft.com>, Haiyang Zhang <haiyangz@...rosoft.com>, 
	Wei Liu <wei.liu@...nel.org>, Dexuan Cui <decui@...rosoft.com>, 
	Ajay Kaher <ajay.kaher@...adcom.com>, Jan Kiszka <jan.kiszka@...mens.com>, 
	Andy Lutomirski <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>, 
	Daniel Lezcano <daniel.lezcano@...aro.org>, John Stultz <jstultz@...gle.com>, linux-kernel@...r.kernel.org, 
	linux-coco@...ts.linux.dev, kvm@...r.kernel.org, virtualization@...ts.linux.dev, 
	linux-hyperv@...r.kernel.org, xen-devel@...ts.xenproject.org, 
	Tom Lendacky <thomas.lendacky@....com>, Nikunj A Dadhania <nikunj@....com>
Subject: Re: [PATCH v2 30/38] x86/paravirt: Don't use a PV sched_clock in
 CoCo guests with trusted TSC

On Wed, Feb 26, 2025 at 06:18:46PM -0800, Sean Christopherson wrote:
> Silently ignore attempts to switch to a paravirt sched_clock when running
> as a CoCo guest with trusted TSC.  In hand-wavy theory, a misbehaving
> hypervisor could attack the guest by manipulating the PV clock to affect
> guest scheduling in some weird and/or predictable way.  More importantly,
> reading TSC on such platforms is faster than any PV clock, and sched_clock
> is all about speed.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kernel/paravirt.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
> index a3a1359cfc26..c538c608d9fb 100644
> --- a/arch/x86/kernel/paravirt.c
> +++ b/arch/x86/kernel/paravirt.c
> @@ -89,6 +89,15 @@ DEFINE_STATIC_CALL(pv_sched_clock, native_sched_clock);
>  int __init __paravirt_set_sched_clock(u64 (*func)(void), bool stable,
>  				      void (*save)(void), void (*restore)(void))
>  {
> +	/*
> +	 * Don't replace TSC with a PV clock when running as a CoCo guest and
> +	 * the TSC is secure/trusted; PV clocks are emulated by the hypervisor,
> +	 * which isn't in the guest's TCB.
> +	 */
> +	if (cc_platform_has(CC_ATTR_GUEST_SNP_SECURE_TSC) ||
> +	    boot_cpu_has(X86_FEATURE_TDX_GUEST))
> +		return -EPERM;
> +

Looks like a call for generic CC_ATTR_GUEST_SECURE_TSC that would be true
for TDX and SEV with CC_ATTR_GUEST_SNP_SECURE_TSC.

>  	if (!stable)
>  		clear_sched_clock_stable();
>  
> -- 
> 2.48.1.711.g2feabab25a-goog
> 

-- 
  Kiryl Shutsemau / Kirill A. Shutemov