Re: [PATCH] irqchip/gic-v4: Don't allow a VMOVP on a dying VPE

[Marc Zyngier <maz@kernel.org>]

Hi Zenghui,

On Tue, 22 Oct 2024 08:45:17 +0100,
Zenghui Yu <yuzenghui@huawei.com> wrote:
> 
> Hi Marc,
> 
> On 2024/10/3 4:49, Marc Zyngier wrote:
> > Kunkun Jiang reports that there is a small window of opportunity for
> > userspace to force a change of affinity for a VPE while the VPE has
> > already been unmapped, but the corresponding doorbell interrupt still
> > visible in /proc/irq/.
> > 
> > Plug the race by checking the value of vmapp_count, which tracks whether
> > the VPE is mapped ot not, and returning an error in this case.
> > 
> > This involves making vmapp_count common to both GICv4.1 and its v4.0
> > ancestor.
> > 
> > Reported-by: Kunkun Jiang <jiangkunkun@huawei.com>
> > Signed-off-by: Marc Zyngier <maz@kernel.org>
> > Link: https://lore.kernel.org/r/c182ece6-2ba0-ce4f-3404-dba7a3ab6c52@huawei.com
> > ---
> >  drivers/irqchip/irq-gic-v3-its.c   | 18 ++++++++++++------
> >  include/linux/irqchip/arm-gic-v4.h |  4 +++-
> >  2 files changed, 15 insertions(+), 7 deletions(-)
> > 
> > diff --git a/drivers/irqchip/irq-gic-v3-its.c b/drivers/irqchip/irq-gic-v3-its.c
> > index fdec478ba5e7..ab597e74ba08 100644
> > --- a/drivers/irqchip/irq-gic-v3-its.c
> > +++ b/drivers/irqchip/irq-gic-v3-its.c
> > @@ -797,8 +797,8 @@ static struct its_vpe *its_build_vmapp_cmd(struct its_node *its,
> >  	its_encode_valid(cmd, desc->its_vmapp_cmd.valid);
> >  
> >  	if (!desc->its_vmapp_cmd.valid) {
> > +		alloc = !atomic_dec_return(&desc->its_vmapp_cmd.vpe->vmapp_count);
> >  		if (is_v4_1(its)) {
> > -			alloc = !atomic_dec_return(&desc->its_vmapp_cmd.vpe->vmapp_count);
> >  			its_encode_alloc(cmd, alloc);
> >  			/*
> >  			 * Unmapping a VPE is self-synchronizing on GICv4.1,
> > @@ -817,13 +817,13 @@ static struct its_vpe *its_build_vmapp_cmd(struct its_node *its,
> >  	its_encode_vpt_addr(cmd, vpt_addr);
> >  	its_encode_vpt_size(cmd, LPI_NRBITS - 1);
> >  
> > +	alloc = !atomic_fetch_inc(&desc->its_vmapp_cmd.vpe->vmapp_count);
> > +
> >  	if (!is_v4_1(its))
> >  		goto out;
> >  
> >  	vconf_addr = virt_to_phys(page_address(desc->its_vmapp_cmd.vpe->its_vm->vprop_page));
> >  
> > -	alloc = !atomic_fetch_inc(&desc->its_vmapp_cmd.vpe->vmapp_count);
> > -
> >  	its_encode_alloc(cmd, alloc);
> >  
> >  	/*
> > @@ -3806,6 +3806,13 @@ static int its_vpe_set_affinity(struct irq_data *d,
> >  	struct cpumask *table_mask;
> >  	unsigned long flags;
> >  
> > +	/*
> > +	 * Check if we're racing against a VPE being destroyed, for
> > +	 * which we don't want to allow a VMOVP.
> > +	 */
> > +	if (!atomic_read(&vpe->vmapp_count))
> > +		return -EINVAL;
> 
> We lazily map the vPE so that vmapp_count is likely to be 0 on GICv4.0
> implementations with the ITSList feature. Seems that that implementation
> is not affected by the reported race and we don't need to check
> vmapp_count for that.

Indeed, the ITSList guards the sending of VMOVP in that case, and we
avoid the original issue in that case. However, this still translates
in the doorbell being moved for no reason (see its_vpe_db_proxy_move).

How about something like this?

Thanks,

	M.

diff --git a/drivers/irqchip/irq-gic-v3-its.c b/drivers/irqchip/irq-gic-v3-its.c
index ab597e74ba08..ac8ed56f1e48 100644
--- a/drivers/irqchip/irq-gic-v3-its.c
+++ b/drivers/irqchip/irq-gic-v3-its.c
@@ -3810,8 +3810,17 @@ static int its_vpe_set_affinity(struct irq_data *d,
 	 * Check if we're racing against a VPE being destroyed, for
 	 * which we don't want to allow a VMOVP.
 	 */
-	if (!atomic_read(&vpe->vmapp_count))
-		return -EINVAL;
+	if (!atomic_read(&vpe->vmapp_count)) {
+		if (gic_requires_eager_mapping())
+			return -EINVAL;
+
+		/*
+		 * If we lazily map the VPEs, this isn't an error, and
+		 * we exit cleanly.
+		 */
+		irq_data_update_effective_affinity(d, cpumask_of(cpu));
+		return IRQ_SET_MASK_OK_DONE;
+	}
 
 	/*
 	 * Changing affinity is mega expensive, so let's be as lazy as

-- 
Without deviation from the norm, progress is not possible.