[Skiboot] [PATCH] flash: Handle nullptr dereference of system_flash

[Mahesh J Salgaonkar]

On 2025-03-07 11:01:02 Fri, Aditya Gupta wrote:
> With QEMU with NO support for MPIPL, 'p9_sbe_terminate' returns early
> at:
> 
>     /* Return if MPIPL is not supported */
>     if (!is_mpipl_enabled())
>     	return;
> 
> But with MPIPL supported in QEMU, 'p9_sbe_terminate' continues further and
> calls 'flash_unregister' which causes a Machine Check due to nullptr
> dereference of 'system_flash':
> 
[...]
> Previously above machine check was never hit as QEMU platform didn't
> had MPIPL, and hence the caller 'p9_sbe_terminate' used to return early.
> 
> Add null check to ignore the unregister request if system_flash is not set.
> 
> Signed-off-by: Aditya Gupta <adityag at linux.ibm.com>
> 
> ---
> Initial QEMU MPIPL support was posted to [1]. It has not been merged
> yet.
> 
> [1]: https://lore.kernel.org/qemu-devel/20250217071934.86131-1-adityag@linux.ibm.com/
> 
> Changelog
> ==========
> rfc -> v1:
>   + reword warning message to say system flash not registered
> ---
> ---
>  core/flash.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/core/flash.c b/core/flash.c
> index a14bfc68fd1a..6d9aeb1cbfda 100644
> --- a/core/flash.c
> +++ b/core/flash.c
> @@ -88,7 +88,16 @@ void flash_release(void)
>  
>  bool flash_unregister(void)
>  {
> -	struct blocklevel_device *bl = system_flash->bl;
> +	struct blocklevel_device *bl;
> +
> +	if (!system_flash) {
> +		prlog(PR_WARNING, "System Flash is not registered, ignoring"\
> +				" unregister request\n");
> +
> +		return true;
> +	}
> +
> +	bl = system_flash->bl;
>  
>  	if (bl->exit)
>  		return bl->exit(bl);
> -- 

Looks good to me.

Reviewed-by: Mahesh Salgaonkar <mahesh at linux.ibm.com>

Thanks,
-Mahesh.