Re: [PATCH] hvc/xen: lock console list traversal

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 29 Nov 2022, Roger Pau Monne wrote:
> The currently lockless access to the xen console list in
> vtermno_to_xencons() is incorrect, as additions and removals from the
> list can happen anytime, and as such the traversal of the list to get
> the private console data for a given termno needs to happen with the
> lock held.  Note users that modify the list already do so with the
> lock taken.
> 
> While there switch from using list_for_each_entry_safe to
> list_for_each_entry: the current entry cursor won't be removed as
> part of the code in the loop body, so using the _safe variant is
> pointless.
> 
> Fixes: 02e19f9c7cac ('hvc_xen: implement multiconsole support')
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> ---
>  drivers/tty/hvc/hvc_xen.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/tty/hvc/hvc_xen.c b/drivers/tty/hvc/hvc_xen.c
> index d65741983837..117dc48f980e 100644
> --- a/drivers/tty/hvc/hvc_xen.c
> +++ b/drivers/tty/hvc/hvc_xen.c
> @@ -53,17 +53,22 @@ static DEFINE_SPINLOCK(xencons_lock);
>  
>  static struct xencons_info *vtermno_to_xencons(int vtermno)
>  {
> -     struct xencons_info *entry, *n, *ret = NULL;
> +     struct xencons_info *entry, *ret = NULL;
> +     unsigned long flags;
>  
> -     if (list_empty(&xenconsoles))
> -                     return NULL;
> +     spin_lock_irqsave(&xencons_lock, flags);

If xencons_lock requires irqsave then we need to change all the
xencons_lock spinlocks to call irqsave, including the ones in
xen_hvm_console_init if they can happen at runtime.


> +     if (list_empty(&xenconsoles)) {
> +             spin_unlock_irqrestore(&xencons_lock, flags);
> +             return NULL;
> +     }
>  
> -     list_for_each_entry_safe(entry, n, &xenconsoles, list) {
> +     list_for_each_entry(entry, &xenconsoles, list) {
>               if (entry->vtermno == vtermno) {
>                       ret  = entry;
>                       break;
>               }
>       }
> +     spin_unlock_irqrestore(&xencons_lock, flags);
>  
>       return ret;
>  }
> -- 
> 2.37.3
>