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

Borislav Petkov
2023-03-09T16:56:58Z

On Mon, Feb 27, 2023 at 02:29:47PM -0800, Rick Edgecombe wrote:
> From: Yu-cheng Yu <yu-cheng.yu@intel.com>
>
> 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.