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Message-ID: <alpine.DEB.2.22.394.2211301710270.4039@ubuntu-linux-20-04-desktop>
Date: Wed, 30 Nov 2022 17:11:50 -0800 (PST)
From: Stefano Stabellini <sstabellini@...nel.org>
To: Roger Pau Monne <roger.pau@...rix.com>
cc: linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
Jiri Slaby <jirislaby@...nel.org>,
Juergen Gross <jgross@...e.com>,
Boris Ostrovsky <boris.ostrovsky@...cle.com>,
Jan Beulich <jbeulich@...e.com>,
Stefano Stabellini <sstabellini@...nel.org>,
Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2] hvc/xen: lock console list traversal
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@...rix.com>
Reviewed-by: Stefano Stabellini <sstabellini@...nel.org>
> ---
> 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
>
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