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

Yu, Yu-cheng
2021-05-17T20:55:18Z

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