[PATCH v2 1/2] powerpc: Avoid signed to unsigned conversion in set_thread_tidr()

[Michael Ellerman]

Vaibhav Jain <vaibhav at linux.vnet.ibm.com> writes:

> There is an unsafe signed to unsigned conversion in set_thread_tidr()
> that may cause an error value to be assigned to SPRN_TIDR register and
> used as thread-id.
>
> The issue happens as assign_thread_tidr() returns an int and
> thread.tidr is an unsigned-long. So a negative error code returned
> from assign_thread_tidr() will fail the error check and gets assigned
> as tidr as a large positive value.
>
> To fix this the patch assigns the return value of assign_thread_tidr()
> to a temporary int and assigns it to thread.tidr iff its '> 0'.
>
> The patch shouldn't impact the calling convention of set_thread_tidr()

But it does? I'm really confused.

Please fix and resend.

cheers

>  int set_thread_tidr(struct task_struct *t)
>  {
> +	int rc;
> +
>  	if (!cpu_has_feature(CPU_FTR_ARCH_300))
>  		return -EINVAL;
                ^
                -ve error

>  
>  	if (t != current)
>  		return -EINVAL;
                ^
                -ve error

>  
> -	t->thread.tidr = assign_thread_tidr();
> -	if (t->thread.tidr < 0)
> -		return t->thread.tidr;
                ^
                -ve error

Yes I know we never hit that case because of the bug, that is what this
patch should be fixing, and only that.

> -
> -	mtspr(SPRN_TIDR, t->thread.tidr);
> +	rc = assign_thread_tidr();
> +	if (rc > 0) {
> +		t->thread.tidr = rc;
> +		mtspr(SPRN_TIDR, t->thread.tidr);
> +	}
>  
> -	return 0;

Success == 0

> +	return rc;

Success == +ve TIDR

>  }

cheers