Re: [PATCH v2] hvc/xen: lock console list traversal

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 30 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.
> 
> Adjust current lock takers to use the _irq{save,restore} helpers,
> since the context in which vtermno_to_xencons() is called can have
> interrupts disabled.  Use the _irq{save,restore} set of helpers to
> switch the current callers to disable interrupts in the locked region.
> I haven't checked if existing users could instead use the _irq
> variant, as I think it's safer to use _irq{save,restore} upfront.
> 
> 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>

Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>


> ---
> Changes since v1:
>  - Switch current lock users to disable interrupts in the locked
>    region.
> ---
>  drivers/tty/hvc/hvc_xen.c | 46 ++++++++++++++++++++++++---------------
>  1 file changed, 29 insertions(+), 17 deletions(-)
> 
> diff --git a/drivers/tty/hvc/hvc_xen.c b/drivers/tty/hvc/hvc_xen.c
> index e63c1761a361..d9d023275328 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 (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;
>  }
> @@ -234,7 +239,7 @@ static int xen_hvm_console_init(void)
>  {
>       int r;
>       uint64_t v = 0;
> -     unsigned long gfn;
> +     unsigned long gfn, flags;
>       struct xencons_info *info;
>  
>       if (!xen_hvm_domain())
> @@ -270,9 +275,9 @@ static int xen_hvm_console_init(void)
>               goto err;
>       info->vtermno = HVC_COOKIE;
>  
> -     spin_lock(&xencons_lock);
> +     spin_lock_irqsave(&xencons_lock, flags);
>       list_add_tail(&info->list, &xenconsoles);
> -     spin_unlock(&xencons_lock);
> +     spin_unlock_irqrestore(&xencons_lock, flags);
>  
>       return 0;
>  err:
> @@ -296,6 +301,7 @@ static int xencons_info_pv_init(struct xencons_info 
> *info, int vtermno)
>  static int xen_pv_console_init(void)
>  {
>       struct xencons_info *info;
> +     unsigned long flags;
>  
>       if (!xen_pv_domain())
>               return -ENODEV;
> @@ -312,9 +318,9 @@ static int xen_pv_console_init(void)
>               /* already configured */
>               return 0;
>       }
> -     spin_lock(&xencons_lock);
> +     spin_lock_irqsave(&xencons_lock, flags);
>       xencons_info_pv_init(info, HVC_COOKIE);
> -     spin_unlock(&xencons_lock);
> +     spin_unlock_irqrestore(&xencons_lock, flags);
>  
>       return 0;
>  }
> @@ -322,6 +328,7 @@ static int xen_pv_console_init(void)
>  static int xen_initial_domain_console_init(void)
>  {
>       struct xencons_info *info;
> +     unsigned long flags;
>  
>       if (!xen_initial_domain())
>               return -ENODEV;
> @@ -337,9 +344,9 @@ static int xen_initial_domain_console_init(void)
>       info->irq = bind_virq_to_irq(VIRQ_CONSOLE, 0, false);
>       info->vtermno = HVC_COOKIE;
>  
> -     spin_lock(&xencons_lock);
> +     spin_lock_irqsave(&xencons_lock, flags);
>       list_add_tail(&info->list, &xenconsoles);
> -     spin_unlock(&xencons_lock);
> +     spin_unlock_irqrestore(&xencons_lock, flags);
>  
>       return 0;
>  }
> @@ -394,10 +401,12 @@ static void xencons_free(struct xencons_info *info)
>  
>  static int xen_console_remove(struct xencons_info *info)
>  {
> +     unsigned long flags;
> +
>       xencons_disconnect_backend(info);
> -     spin_lock(&xencons_lock);
> +     spin_lock_irqsave(&xencons_lock, flags);
>       list_del(&info->list);
> -     spin_unlock(&xencons_lock);
> +     spin_unlock_irqrestore(&xencons_lock, flags);
>       if (info->xbdev != NULL)
>               xencons_free(info);
>       else {
> @@ -478,6 +487,7 @@ static int xencons_probe(struct xenbus_device *dev,
>  {
>       int ret, devid;
>       struct xencons_info *info;
> +     unsigned long flags;
>  
>       devid = dev->nodename[strlen(dev->nodename) - 1] - '0';
>       if (devid == 0)
> @@ -497,9 +507,9 @@ static int xencons_probe(struct xenbus_device *dev,
>       ret = xencons_connect_backend(dev, info);
>       if (ret < 0)
>               goto error;
> -     spin_lock(&xencons_lock);
> +     spin_lock_irqsave(&xencons_lock, flags);
>       list_add_tail(&info->list, &xenconsoles);
> -     spin_unlock(&xencons_lock);
> +     spin_unlock_irqrestore(&xencons_lock, flags);
>  
>       return 0;
>  
> @@ -599,10 +609,12 @@ static int __init xen_hvc_init(void)
>  
>       info->hvc = hvc_alloc(HVC_COOKIE, info->irq, ops, 256);
>       if (IS_ERR(info->hvc)) {
> +             unsigned long flags;
> +
>               r = PTR_ERR(info->hvc);
> -             spin_lock(&xencons_lock);
> +             spin_lock_irqsave(&xencons_lock, flags);
>               list_del(&info->list);
> -             spin_unlock(&xencons_lock);
> +             spin_unlock_irqrestore(&xencons_lock, flags);
>               if (info->irq)
>                       unbind_from_irqhandler(info->irq, NULL);
>               kfree(info);
> -- 
> 2.37.3
>