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

[Yang, Weijiang]

On 12/5/2023 5:53 PM, Maxim Levitsky wrote:
> On Fri, 2023-12-01 at 14:51 +0800, Yang, Weijiang wrote:
> > On 12/1/2023 1:26 AM, Maxim Levitsky wrote:
> > > On Fri, 2023-11-24 at 00:53 -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);
> > > > +
> > > >    	if (!cpu_feature_enabled(X86_FEATURE_XFD))
> > > >    		fpu_kernel_cfg.max_features &= ~XFEATURE_MASK_USER_DYNAMIC;
> > > >    
> > > I am curious:
> > >
> > > Any reason why my review feedback was not applied even though you did agree
> > > that it is reasonable?
> > My apology! I changed the patch per you feedback but found XFEATURE_CET_USER didn't
> > work before sending out v7 version, after a close look at the existing code:
> >
> >           for (i = 0; i < ARRAY_SIZE(xsave_cpuid_features); i++) {
> >                   unsigned short cid = xsave_cpuid_features[i];
> >
> >                   /* Careful: X86_FEATURE_FPU is 0! */
> >                   if ((i != XFEATURE_FP && !cid) || !boot_cpu_has(cid))
> >                           fpu_kernel_cfg.max_features &= ~BIT_ULL(i);
> >           }
> >
> > With removal of XFEATURE_CET_USER entry from xsave_cpuid_features, actually
> > above check will clear the bit from fpu_kernel_cfg.max_features.
> Are you sure about this? If we remove the XFEATURE_CET_USER from the xsave_cpuid_features,
> then the above loop will not touch it - it loops only over the items in the xsave_cpuid_features
> array.

No,  the code is a bit tricky, the actual array size is XFEATURE_XTILE_DATA( ie, 18) + 1, those xfeature bits not listed in init code leave a blank entry with xsave_cpuid_features[i] == 0, so for the blank elements, the loop hits (i != XFEATURE_FP && !cid) then the relevant xfeature bit for i is cleared in fpu_kernel_cfg.max_features. I had the same illusion at first when I replied your comments in v6, and modified the code as you suggested but found the issue during tests. Please double check it.
> What I suggested was that we remove the XFEATURE_CET_USER from the xsave_cpuid_features
> and instead do this after the above loop.
>
> if (!boot_cpu_has(X86_FEATURE_SHSTK) && !boot_cpu_has(X86_FEATURE_IBT))
>     fpu_kernel_cfg.max_features &= ~BIT_ULL(XFEATURE_CET_USER);
>
> Which is pretty much just a manual iteration of the loop, just instead of checking
> for absence of single feature, it checks that both features are absent.
>
> Best regards,
> 	Maxim Levitsky
>
>
> > So now I need
> > to add it back conditionally.
> > Your sample code is more consistent with existing code in style, but I don't want to
> > hack into the loop and handle XFEATURE_CET_USER specifically.  Just keep the handling
> > and rewording the comments which is also straightforward.
> >
> > > https://lore.kernel.org/lkml/c72dfaac-1622-94cf-a81d-9d7ed81b2f55@xxxxxxxxx/
> > >
> > > Best regard
> > > 	Maxim Levitsky