Re: [PATCH v8 15/27] ARM: vITS: handle CLEAR command

[Julien Grall <julien.grall@arm.com>]

Hi Andre,

On 12/04/17 01:44, Andre Przywara wrote:
> This introduces the ITS command handler for the CLEAR command, which
> clears the pending state of an LPI.
> This removes a not-yet injected, but already queued IRQ from a VCPU.
> As read_itte() is now eventually used, we add the static keyword.
>
> Signed-off-by: Andre Przywara <andre.przywara@arm.com>
> ---
>  xen/arch/arm/vgic-v3-its.c | 52 ++++++++++++++++++++++++++++++++++++++++++++--
>  1 file changed, 50 insertions(+), 2 deletions(-)
>
> diff --git a/xen/arch/arm/vgic-v3-its.c b/xen/arch/arm/vgic-v3-its.c
> index 632ab84..e24ab60 100644
> --- a/xen/arch/arm/vgic-v3-its.c
> +++ b/xen/arch/arm/vgic-v3-its.c
> @@ -227,8 +227,8 @@ static bool read_itte_locked(struct virt_its *its, uint32_t devid,
>   * This function takes care of the locking by taking the its_lock itself, so
>   * a caller shall not hold this. Before returning, the lock is dropped again.
>   */
> -bool read_itte(struct virt_its *its, uint32_t devid, uint32_t evid,
> -               struct vcpu **vcpu_ptr, uint32_t *vlpi_ptr)
> +static bool read_itte(struct virt_its *its, uint32_t devid, uint32_t evid,
> +                      struct vcpu **vcpu_ptr, uint32_t *vlpi_ptr)
>  {
>      bool ret;
>
> @@ -297,6 +297,51 @@ static uint64_t its_cmd_mask_field(uint64_t *its_cmd, unsigned int word,
>  #define its_cmd_get_validbit(cmd)       its_cmd_mask_field(cmd, 2, 63,  1)
>  #define its_cmd_get_ittaddr(cmd)        (its_cmd_mask_field(cmd, 2, 8, 44) << 8)
>
> +/*
> + * CLEAR removes the pending state from an LPI. */
> +static int its_handle_clear(struct virt_its *its, uint64_t *cmdptr)
> +{
> +    uint32_t devid = its_cmd_get_deviceid(cmdptr);
> +    uint32_t eventid = its_cmd_get_id(cmdptr);
> +    struct pending_irq *p;
> +    struct vcpu *vcpu;
> +    uint32_t vlpi;
> +    unsigned long flags;
> +
> +    /* Translate the DevID/EvID pair into a vCPU/vLPI pair. */
> +    if ( !read_itte(its, devid, eventid, &vcpu, &vlpi) )

So the vCPU ID is retrieved from guest memory, therefore you cannot 
trust the value for taking a lock.

Indeed, a malicious guest could rewrite the value with another vCPU ID 
and you would take the wrong vCPU vGIC lock.

What is the plan to fix that?

> +        return -1;
> +
> +    spin_lock_irqsave(&vcpu->arch.vgic.lock, flags);

I don't think this lock will protect correctly the pending_irq because 
if you do a MOVI before hand you will use the new vCPU ID and not the 
old one (see my explanation on patch #2).

> +
> +    p = its->d->arch.vgic.handler->lpi_to_pending(its->d, vlpi);
> +    if ( !p )

Please detail what means NULL here.

> +    {
> +        spin_unlock_irqrestore(&vcpu->arch.vgic.lock, flags);
> +        return -1;
> +    }
> +
> +    /*
> +     * If the LPI is already visible on the guest, it is too late to
> +     * clear the pending state. However this is a benign race that can
> +     * happen on real hardware, too: If the LPI has already been forwarded
> +     * to a CPU interface, a CLEAR request reaching the redistributor has
> +     * no effect on that LPI anymore. Since LPIs are edge triggered and
> +     * have no active state, we don't need to care about this here.
> +     */
> +    if ( !test_bit(GIC_IRQ_GUEST_VISIBLE, &p->status) )
> +    {
> +        /* Remove a pending, but not yet injected guest IRQ. */
> +        clear_bit(GIC_IRQ_GUEST_QUEUED, &p->status);
> +        list_del_init(&p->inflight);
> +        list_del_init(&p->lr_queue);

I am not sure to understand why you open-code gic_remove_from_queues 
rather than reworking it.

> +    }
> +
> +    spin_unlock_irqrestore(&vcpu->arch.vgic.lock, flags);
> +
> +    return 0;
> +}
> +
>  #define ITS_CMD_BUFFER_SIZE(baser)      ((((baser) & 0xff) + 1) << 12)
>  #define ITS_CMD_OFFSET(reg)             ((reg) & GENMASK(19, 5))
>
> @@ -326,6 +371,9 @@ static int vgic_its_handle_cmds(struct domain *d, struct virt_its *its)
>
>          switch ( its_cmd_get_command(command) )
>          {
> +        case GITS_CMD_CLEAR:
> +            ret = its_handle_clear(its, command);
> +            break;
>          case GITS_CMD_SYNC:
>              /* We handle ITS commands synchronously, so we ignore SYNC. */
>              break;
>

Cheers,

-- 
Julien Grall

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