Re: [PATCH v7 31/41] x86/shstk: Introduce routines modifying shstk

[Borislav Petkov]

On Mon, Feb 27, 2023 at 02:29:47PM -0800, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> 
> Shadow stacks are normally written to via CALL/RET or specific CET
				       ^
				       indirectly.

> instructions like RSTORSSP/SAVEPREVSSP. However during some Linux
> operations the kernel will need to write to directly using the ring-0 only

"However, sometimes the kernel will need to..."

> WRUSS instruction.
> 
> A shadow stack restore token marks a restore point of the shadow stack, and
> the address in a token must point directly above the token, which is within
> the same shadow stack. This is distinctively different from other pointers
> on the shadow stack, since those pointers point to executable code area.
> 
> Introduce token setup and verify routines. Also introduce WRUSS, which is
> a kernel-mode instruction but writes directly to user shadow stack.
> 
> In future patches that enable shadow stack to work with signals, the kernel
> will need something to denote the point in the stack where sigreturn may be
> called. This will prevent attackers calling sigreturn at arbitrary places
> in the stack, in order to help prevent SROP attacks.
> 
> To do this, something that can only be written by the kernel needs to be
> placed on the shadow stack. This can be accomplished by setting bit 63 in
> the frame written to the shadow stack. Userspace return addresses can't
> have this bit set as it is in the kernel range. It is also can't be a

s/is //

> valid restore token.

...

> diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
> index de48d1389936..d6cd9344f6c7 100644
> --- a/arch/x86/include/asm/special_insns.h
> +++ b/arch/x86/include/asm/special_insns.h
> @@ -202,6 +202,19 @@ static inline void clwb(volatile void *__p)
>  		: [pax] "a" (p));
>  }
>  
> +#ifdef CONFIG_X86_USER_SHADOW_STACK
> +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;

Nice!

> +}
> +#endif /* CONFIG_X86_USER_SHADOW_STACK */
> +
>  #define nop() asm volatile ("nop")
>  
>  static inline void serialize(void)

...

> +static int put_shstk_data(u64 __user *addr, u64 data)
> +{
> +	if (WARN_ON_ONCE(data & BIT(63)))

Dunno, maybe something like:

/*
 * A comment explaining what that is...
 */
#define SHSTK_SIGRETURN_TOKEN	BIT_ULL(63)

or so?

And use that instead of that magical bit 63.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette