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

Borislav Petkov
2021-05-17T07:45:45Z

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?

> +
> +static inline int write_user_shstk_64(u64 __user *addr, u64 val)
> +{
> + asm_volatile_goto("1: wrussq %[val], (%[addr])\n"
> + _ASM_EXTABLE(1b, %l[fail])
> + :: [addr] "r" (addr), [val] "r" (val)
> + :: fail);
> + return 0;
> +fail:
> + return -EFAULT;
> +}
> +#endif /* CONFIG_X86_SHADOW_STACK */
> +
> #define nop() asm volatile ("nop")
>
> static inline void serialize(void)
> 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.

> +{
> + 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?

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.

> +
> + 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?

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);

> +
> + if (write_user_shstk_64((u64 __user *)addr, (u64)ssp))
> + return -EFAULT;
> +
> + *token_addr = addr;

<---- newline here.

> + return 0;
> +}
> +
> +/*
> + * Create a restore token on shadow stack, and then push the user-mode
> + * function return address.
> + */
> +int shstk_setup_rstor_token(bool ia32, unsigned long ret_addr,
> + unsigned long *token_addr, unsigned long *new_ssp)
> +{
> + struct cet_status *cet = &current->thread.cet;
> + unsigned long ssp = 0;
> + int err = 0;

What are those cleared to 0 for?

> +
> + if (cet->shstk_size) {

Flip logic to save an indentation level:

if (!cet->shstk_size)
return err;

if (!ret_addr)
...

> + if (!ret_addr)
> + return -EINVAL;
> +
> + ssp = _get_user_shstk_addr();

Needs to test retval for 0 here and return error if so.

> + err = _create_rstor_token(ia32, ssp, token_addr);
> + if (err)
> + return err;
> +
> + if (ia32) {
> + *new_ssp = *token_addr - sizeof(u32);
> + err = write_user_shstk_32((u32 __user *)*new_ssp, (u32)ret_addr);
> + } else {
> + *new_ssp = *token_addr - sizeof(u64);
> + err = write_user_shstk_64((u64 __user *)*new_ssp, (u64)ret_addr);

In both cases, you should write *new_ssp only when write_user_shstk_*
functions have succeeded.

> + }
> + }
> +
> + return err;
> +}
> +
> +/*
> + * Verify token_addr point to a valid token, and then set *new_ssp

points

> + * according to the token.
> + */
> +int shstk_check_rstor_token(bool ia32, unsigned long token_addr, unsigned long *new_ssp)
> +{
> + unsigned long token;
> +
> + *new_ssp = 0;

Same as above.

> +
> + if (!IS_ALIGNED(token_addr, 8))
> + return -EINVAL;
> +
> + if (get_user(token, (unsigned long __user *)token_addr))
> + return -EFAULT;
> +
> + /* Is 64-bit mode flag correct? */
> + if (!ia32 && !IS_TOKEN_64(token))
> + return -EINVAL;
> + else if (ia32 && !IS_TOKEN_32(token))
> + return -EINVAL;

That test can be done using the XOR function - i.e., you want to return
an error value when the two things are different.

In order to make this more readable, you call ia32 "proc32" to be clear
what that variable denotes - a 32-bit process. Then, you do

bool shstk32 = !(token & BIT(0));

if (proc32 ^ shstk32)
return -EINVAL;

Voila.

> + token &= ~TOKEN_MODE_MASK;
> +
> + /*
> + * Restore address properly aligned?
> + */
> + if ((!ia32 && !IS_ALIGNED(token, 8)) || !IS_ALIGNED(token, 4))

Flip logic as above.

--
Regards/Gruss,
Boris.