[PATCH v3] powerpc: Avoid signed to unsigned conversion in set_thread_tidr()

[Sukadev Bhattiprolu]

Vaibhav Jain [vaibhav at linux.vnet.ibm.com] wrote:
> 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.

Thanks for fixing this. I have a comment below
> 
> 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()
> i.e all -ve return-values are error codes and a return value of '0'
> indicates success.
> 
> Fixes: ec233ede4c86("powerpc: Add support for setting SPRN_TIDR")
> Signed-off-by: Vaibhav Jain <vaibhav at linux.vnet.ibm.com>
> 
> ---
> Changelog:
> 
> v3  ->  Updated the patch to not impact the calling convention [Mpe, Christophe]
> 
> v2  ->	* Update the patch description to document the calling
> 	convention of set_thread_tidr(). [Mpe]
> 	* Fix a tidr allocation leak.
> ---
>  arch/powerpc/kernel/process.c | 17 ++++++++++-------
>  1 file changed, 10 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
> index bfdd783e3916..9fb69211a3d4 100644
> --- a/arch/powerpc/kernel/process.c
> +++ b/arch/powerpc/kernel/process.c
> @@ -1569,19 +1569,22 @@ void arch_release_task_struct(struct task_struct *t)
>   */
>  int set_thread_tidr(struct task_struct *t)
>  {
> +	int rc;
> +
>  	if (!cpu_has_feature(CPU_FTR_ARCH_300))
>  		return -EINVAL;
> 
>  	if (t != current)
>  		return -EINVAL;
> 
> -	t->thread.tidr = assign_thread_tidr();
> -	if (t->thread.tidr < 0)
> -		return t->thread.tidr;
> -
> -	mtspr(SPRN_TIDR, t->thread.tidr);
> -
> -	return 0;
> +	rc = assign_thread_tidr();
> +	if (rc > 0) {
> +		t->thread.tidr = rc;
> +		mtspr(SPRN_TIDR, t->thread.tidr);
> +		return 0;
> +	} else {
> +		return rc;
> +	}

We can eliminate the 'else' and be consistent with existing code, if
we check for error (i.e rc < 0) and return rc. assign_thread_tidr() will
not return 0, but even if it did, setting the register and thread.tidr
to 0 should not be a problem.

Sukadev