Re: [PATCH v7 02/26] x86/fpu/xstate: Refine CET user xstate bit enabling

[Yang, Weijiang]

On 11/24/2023 5:40 PM, Peter Zijlstra wrote:
> On Fri, Nov 24, 2023 at 12:53:06AM -0500, Yang Weijiang wrote:
> > Remove XFEATURE_CET_USER entry from dependency array as the entry doesn't
> > reflect true dependency between CET features and the user xstate bit.
> > Enable the bit in fpu_kernel_cfg.max_features when either SHSTK or IBT is
> > available.
> >
> > Both user mode shadow stack and indirect branch tracking features depend
> > on XFEATURE_CET_USER bit in XSS to automatically save/restore user mode
> > xstate registers, i.e., IA32_U_CET and IA32_PL3_SSP whenever necessary.
> >
> > Note, the issue, i.e., CPUID only enumerates IBT but no SHSTK is resulted
> > from CET KVM series which synthesizes guest CPUIDs based on userspace
> > settings,in real world the case is rare. In other words, the exitings
> > dependency check is correct when only user mode SHSTK is available.
> >
> > Signed-off-by: Yang Weijiang <weijiang.yang@xxxxxxxxx>
> > Reviewed-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> > Tested-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
> > ---
> >   arch/x86/kernel/fpu/xstate.c | 9 ++++++++-
> >   1 file changed, 8 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
> > index 73f6bc00d178..6e50a4251e2b 100644
> > --- a/arch/x86/kernel/fpu/xstate.c
> > +++ b/arch/x86/kernel/fpu/xstate.c
> > @@ -73,7 +73,6 @@ static unsigned short xsave_cpuid_features[] __initdata = {
> >   	[XFEATURE_PT_UNIMPLEMENTED_SO_FAR]	= X86_FEATURE_INTEL_PT,
> >   	[XFEATURE_PKRU]				= X86_FEATURE_OSPKE,
> >   	[XFEATURE_PASID]			= X86_FEATURE_ENQCMD,
> > -	[XFEATURE_CET_USER]			= X86_FEATURE_SHSTK,
> >   	[XFEATURE_XTILE_CFG]			= X86_FEATURE_AMX_TILE,
> >   	[XFEATURE_XTILE_DATA]			= X86_FEATURE_AMX_TILE,
> >   };
> > @@ -798,6 +797,14 @@ void __init fpu__init_system_xstate(unsigned int legacy_size)
> >   			fpu_kernel_cfg.max_features &= ~BIT_ULL(i);
> >   	}
> >   
> > +	/*
> > +	 * CET user mode xstate bit has been cleared by above sanity check.
> > +	 * Now pick it up if either SHSTK or IBT is available. Either feature
> > +	 * depends on the xstate bit to save/restore user mode states.
> > +	 */
> > +	if (boot_cpu_has(X86_FEATURE_SHSTK) || boot_cpu_has(X86_FEATURE_IBT))
> > +		fpu_kernel_cfg.max_features |= BIT_ULL(XFEATURE_CET_USER);
> So booting a host with "ibt=off" will clear the FEATURE_IBT, this was
> fine before this patch-set, but possibly not with.
>
> That kernel argument really only wants to tell the kernel not to use IBT
> itself, but not inhibit IBT from being used by guests.

Thanks for pointing this out!
If ibt=off actually causes XFEATURE_CET_USER unset in fpu_kernel_cfg.max_features, in KVM part (patch 24), we already have below check to disable SHSTK/IBT in this cases, so looks like it won't bring issues.

+if ((kvm_caps.supported_xss & (XFEATURE_MASK_CET_USER |

+XFEATURE_MASK_CET_KERNEL)) !=

+(XFEATURE_MASK_CET_USER | XFEATURE_MASK_CET_KERNEL)) {

+kvm_cpu_cap_clear(X86_FEATURE_SHSTK);

+kvm_cpu_cap_clear(X86_FEATURE_IBT);

+kvm_caps.supported_xss &= ~XFEATURE_CET_USER;

+kvm_caps.supported_xss &= ~XFEATURE_CET_KERNEL;

+}

+

>