Re: [OPTIONAL/RFC v2 39/39] x86: Add alt shadow stack support

[Andy Lutomirski]

On 9/29/22 15:29, Rick Edgecombe wrote:
> To handle stack overflows, applications can register a separate signal alt
> stack to use for the stack to handle signals. To handle shadow stack
> overflows the kernel can similarly provide the ability to have an alt
> shadow stack.


The overall SHSTK mechanism has a concept of a shadow stack that is 
valid and not in use and a shadow stack that is in use.  This is used, 
for example, by RSTORSSP.  I would like to imagine that this serves a 
real purpose (presumably preventing two different threads from using the 
same shadow stack and thus corrupting each others' state).

So maybe altshstk should use exactly the same mechanism.  Either signal 
delivery should do the atomic very-and-mark-busy routine or registering 
the stack as an altstack should do it.

I think your patch has this maybe 1/3 implemented, but I don't see any 
atomics, and you seem to have removed (?) the code that actually 
modifies the token on the stack.

>   
> +static bool on_alt_shstk(unsigned long ssp)
> +{
> +	unsigned long alt_ss_start = current->thread.sas_shstk_sp;
> +	unsigned long alt_ss_end = alt_ss_start + current->thread.sas_shstk_size;
> +
> +	return ssp >= alt_ss_start && ssp < alt_ss_end;
> +}

We're forcing AUTODISARM behavior (right?), so I don't think this is 
needed at all.  User code is never "on the alt stack".  It's either "on 
the alt stack but the alt stack is disarmed, so it's not on the alt 
stack" or it's just straight up not on the alt stack.