Re: [PATCH v26 24/30] x86/cet/shstk: Introduce shadow stack token setup/verify routines

[Yu, Yu-cheng]

On 5/17/2021 12:45 AM, Borislav Petkov wrote:
> On Tue, Apr 27, 2021 at 01:43:09PM -0700, Yu-cheng Yu wrote:
> > +static inline int write_user_shstk_32(u32 __user *addr, u32 val)
> > +{
> > +	WARN_ONCE(1, "%s used but not supported.\n", __func__);
> > +	return -EFAULT;
> > +}
> > +#endif
>
> What is that supposed to catch? Any concrete (mis-)use cases?

If 32-bit apps are not supported, there should be no need of 32-bit 
shadow stack write, otherwise there is a bug.

[...]

> > diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
> > index d387df84b7f1..48a0c87414ef 100644
> > --- a/arch/x86/kernel/shstk.c
> > +++ b/arch/x86/kernel/shstk.c
> > @@ -20,6 +20,7 @@
> >   #include <asm/fpu/xstate.h>
> >   #include <asm/fpu/types.h>
> >   #include <asm/cet.h>
> > +#include <asm/special_insns.h>
> >   
> >   static void start_update_msrs(void)
> >   {
> > @@ -176,3 +177,128 @@ void shstk_disable(void)
> >   
> >   	shstk_free(current);
> >   }
> > +
> > +static unsigned long _get_user_shstk_addr(void)
>
> What's the "_" prefix in the name supposed to denote?
>
> Ditto for the other functions with "_" prefix you're adding.

These are static functions.  I thought that would make the static scope 
clear.  I can remove "_".

> > +{
> > +	struct fpu *fpu = &current->thread.fpu;
> > +	unsigned long ssp = 0;
> > +
> > +	fpregs_lock();
> > +
> > +	if (fpregs_state_valid(fpu, smp_processor_id())) {
> > +		rdmsrl(MSR_IA32_PL3_SSP, ssp);
> > +	} else {
> > +		struct cet_user_state *p;
> > +
> > +		p = get_xsave_addr(&fpu->state.xsave, XFEATURE_CET_USER);
> > +		if (p)
> > +			ssp = p->user_ssp;
> > +	}
> > +
> > +	fpregs_unlock();
>
> <---- newline here.
>
> > +	return ssp;
> > +}
> > +
> > +#define TOKEN_MODE_MASK	3UL
> > +#define TOKEN_MODE_64	1UL
> > +#define IS_TOKEN_64(token) (((token) & TOKEN_MODE_MASK) == TOKEN_MODE_64)
> > +#define IS_TOKEN_32(token) (((token) & TOKEN_MODE_MASK) == 0)
>
> Why do you have to look at the second, busy bit, too in order to
> determine the mode?

If the busy bit is set, it is only for SAVEPREVSSP, and invalid as a 
normal restore token.

> Also, you don't need most of those defines - see below.
>
> > +/*
> > + * Create a restore token on the shadow stack.  A token is always 8-byte
> > + * and aligned to 8.
> > + */
> > +static int _create_rstor_token(bool ia32, unsigned long ssp,
> > +			       unsigned long *token_addr)
> > +{
> > +	unsigned long addr;
> > +
> > +	*token_addr = 0;
>
> What for? Callers should check this function's retval and then interpret
> the validity of token_addr and it should not unconditionally write into
> it.

Ok.

> > +
> > +	if ((!ia32 && !IS_ALIGNED(ssp, 8)) || !IS_ALIGNED(ssp, 4))
>
> Flip this logic:
>
> 	if ((ia32 && !IS_ALIGNED(ssp, 4)) || !IS_ALIGNED(ssp, 8))
>
> > +		return -EINVAL;
> > +
> > +	addr = ALIGN_DOWN(ssp, 8) - 8;
>
> Yah, so this is weird. Why does the restore token need to be at -8
> instead on the shadow stack address itself?

With the lower two bits masked out, the restore token must point 
directly above itself.

> Looking at
>
> Figure 18-2. RSTORSSP to Switch to New Shadow Stack
> Figure 18-3. SAVEPREVSSP to Save a Restore Point
>
> in the SDM, it looks like unnecessarily more complex than it should be.
> But maybe there's some magic I'm missing.
>
> > +
> > +	/* Is the token for 64-bit? */
> > +	if (!ia32)
> > +		ssp |= TOKEN_MODE_64;
>
> 		    |= BIT(0);

Ok, then, we don't use #define's.  I will put in comments about what it 
is doing, and fix the rest.

Thanks,
Yu-cheng