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

[Maxim Levitsky]

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.

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 regards,
> > 	Maxim Levitsky
> >