Re: [PATCH v6 21/24] KVM: arm64: vgic-its: Device table save/restore

Christoffer Dall
2017-05-05T18:12:21Z

On Fri, May 05, 2017 at 06:23:22PM +0200, Auger Eric wrote:
> Hi Christoffer,
>
> On 05/05/2017 14:44, Christoffer Dall wrote:
> > On Thu, May 04, 2017 at 01:44:41PM +0200, Eric Auger wrote:
> >> This patch saves the device table entries into guest RAM.
> >> Both flat table and 2 stage tables are supported. DeviceId
> >> indexing is used.
> >>
> >> For each device listed in the device table, we also save
> >> the translation table using the vgic_its_save/restore_itt
> >> routines. Those functions will be implemented in a subsequent
> >> patch.
> >>
> >> On restore, devices are re-allocated and their itt are
> >> re-built.
> >>
> >> Signed-off-by: Eric Auger <eric.auger@redhat.com>
> >>
> >> ---
> >> v5 -> v6:
> >> - accomodate vgic_its_alloc_device change of proto
> >> - define bit fields for L1 entries
> >> - s/handle_l1_entry/handle_l1_dte
> >> - s/ite_esz/dte_esz in handle_l1_dte
> >> - check BASER valid bit
> >> - s/nb_eventid_bits/num_eventid_bits
> >> - new convention for returned values
> >> - itt functions implemented in subsequent patch
> >>
> >> v4 -> v5:
> >> - sort the device list by deviceid on device table save
> >> - use defines for shifts and masks
> >> - use abi->dte_esz
> >> - clatify entry sizes for L1 and L2 tables
> >>
> >> v3 -> v4:
> >> - use the new proto for its_alloc_device
> >> - compute_next_devid_offset, vgic_its_flush/restore_itt
> >> become static in this patch
> >> - change in the DTE entry format with the introduction of the
> >> valid bit and next field width decrease; ittaddr encoded
> >> on its full range
> >> - fix handle_l1_entry entry handling
> >> - correct vgic_its_table_restore error handling
> >>
> >> v2 -> v3:
> >> - fix itt_addr bitmask in vgic_its_restore_dte
> >> - addition of return 0 in vgic_its_restore_ite moved to
> >> the ITE related patch
> >>
> >> v1 -> v2:
> >> - use 8 byte format for DTE and ITE
> >> - support 2 stage format
> >> - remove kvm parameter
> >> - ITT flush/restore moved in a separate patch
> >> - use deviceid indexing
> >> ---
> >> virt/kvm/arm/vgic/vgic-its.c | 194 +++++++++++++++++++++++++++++++++++++++++--
> >> virt/kvm/arm/vgic/vgic.h | 10 +++
> >> 2 files changed, 199 insertions(+), 5 deletions(-)
> >>
> >> diff --git a/virt/kvm/arm/vgic/vgic-its.c b/virt/kvm/arm/vgic/vgic-its.c
> >> index a3ed52a..c5b388d 100644
> >> --- a/virt/kvm/arm/vgic/vgic-its.c
> >> +++ b/virt/kvm/arm/vgic/vgic-its.c
> >> @@ -23,6 +23,7 @@
> >> #include <linux/interrupt.h>
> >> #include <linux/list.h>
> >> #include <linux/uaccess.h>
> >> +#include <linux/list_sort.h>
> >>
> >> #include <linux/irqchip/arm-gic-v3.h>
> >>
> >> @@ -1701,7 +1702,8 @@ int vgic_its_attr_regs_access(struct kvm_device *dev,
> >> return ret;
> >> }
> >>
> >> -u32 compute_next_devid_offset(struct list_head *h, struct its_device *dev)
> >> +static u32 compute_next_devid_offset(struct list_head *h,
> >> + struct its_device *dev)
> >> {
> >> struct its_device *next;
> >> u32 next_offset;
> >> @@ -1755,8 +1757,8 @@ typedef int (*entry_fn_t)(struct vgic_its *its, u32 id, void *entry,
> >> * Return: < 0 on error, 0 if last element was identified, 1 otherwise
> >> * (the last element may not be found on second level tables)
> >> */
> >> -int scan_its_table(struct vgic_its *its, gpa_t base, int size, int esz,
> >> - int start_id, entry_fn_t fn, void *opaque)
> >> +static int scan_its_table(struct vgic_its *its, gpa_t base, int size, int esz,
> >> + int start_id, entry_fn_t fn, void *opaque)
> >> {
> >> void *entry = kzalloc(esz, GFP_KERNEL);
> >> struct kvm *kvm = its->dev->kvm;
> >> @@ -1791,13 +1793,171 @@ int scan_its_table(struct vgic_its *its, gpa_t base, int size, int esz,
> >> return ret;
> >> }
> >>
> >> +static int vgic_its_save_itt(struct vgic_its *its, struct its_device *device)
> >> +{
> >> + return -ENXIO;
> >> +}
> >> +
> >> +static int vgic_its_restore_itt(struct vgic_its *its, struct its_device *dev)
> >> +{
> >> + return -ENXIO;
> >> +}
> >> +
> >> +/**
> >> + * vgic_its_save_dte - Save a device table entry at a given GPA
> >> + *
> >> + * @its: ITS handle
> >> + * @dev: ITS device
> >> + * @ptr: GPA
> >> + */
> >> +static int vgic_its_save_dte(struct vgic_its *its, struct its_device *dev,
> >> + gpa_t ptr, int dte_esz)
> >> +{
> >> + struct kvm *kvm = its->dev->kvm;
> >> + u64 val, itt_addr_field;
> >> + u32 next_offset;
> >> +
> >> + itt_addr_field = dev->itt_addr >> 8;
> >> + next_offset = compute_next_devid_offset(&its->device_list, dev);
> >> + val = (1ULL << KVM_ITS_DTE_VALID_SHIFT |
> >> + ((u64)next_offset << KVM_ITS_DTE_NEXT_SHIFT) |
> >> + (itt_addr_field << KVM_ITS_DTE_ITTADDR_SHIFT) |
> >> + (dev->num_eventid_bits - 1));
> >> + val = cpu_to_le64(val);
> >> + return kvm_write_guest(kvm, ptr, &val, dte_esz);
> >> +}
> >> +
> >> +/**
> >> + * vgic_its_restore_dte - restore a device table entry
> >> + *
> >> + * @its: its handle
> >> + * @id: device id the DTE corresponds to
> >> + * @ptr: kernel VA where the 8 byte DTE is located
> >> + * @opaque: unused
> >> + *
> >> + * Return: < 0 on error, 0 if the dte is the last one, id offset to the
> >> + * next dte otherwise
> >> + */
> >> +static int vgic_its_restore_dte(struct vgic_its *its, u32 id,
> >> + void *ptr, void *opaque)
> >> +{
> >> + struct its_device *dev;
> >> + gpa_t itt_addr;
> >> + u8 num_eventid_bits;
> >> + u64 entry = *(u64 *)ptr;
> >> + bool valid;
> >> + u32 offset;
> >> + int ret;
> >> +
> >> + entry = le64_to_cpu(entry);
> >> +
> >> + valid = entry >> KVM_ITS_DTE_VALID_SHIFT;
> >> + num_eventid_bits = (entry & KVM_ITS_DTE_SIZE_MASK) + 1;
> >> + itt_addr = ((entry & KVM_ITS_DTE_ITTADDR_MASK)
> >> + >> KVM_ITS_DTE_ITTADDR_SHIFT) << 8;
> >> +
> >> + if (!valid)
> >> + return 1;
> >> +
> >> + /* dte entry is valid */
> >> + offset = (entry & KVM_ITS_DTE_NEXT_MASK) >> KVM_ITS_DTE_NEXT_SHIFT;
> >> +
> >> + dev = vgic_its_alloc_device(its, id, itt_addr, num_eventid_bits);
> >> + if (IS_ERR(dev))
> >> + return PTR_ERR(dev);
> >> +
> >> + ret = vgic_its_restore_itt(its, dev);
> >> + if (ret)
> >> + return ret;
> >> +
> >> + return offset;
> >> +}
> >> +
> >> +static int vgic_its_device_cmp(void *priv, struct list_head *a,
> >> + struct list_head *b)
> >> +{
> >> + struct its_device *deva = container_of(a, struct its_device, dev_list);
> >> + struct its_device *devb = container_of(b, struct its_device, dev_list);
> >> +
> >> + if (deva->device_id < devb->device_id)
> >> + return -1;
> >> + else
> >> + return 1;
> >> +}
> >> +
> >> /**
> >> * vgic_its_save_device_tables - Save the device table and all ITT
> >> * into guest RAM
> >> + *
> >> + * L1/L2 handling is hidden by vgic_its_check_id() helper which directly
> >> + * returns the GPA of the device entry
> >> */
> >> static int vgic_its_save_device_tables(struct vgic_its *its)
> >> {
> >> - return -ENXIO;
> >> + const struct vgic_its_abi *abi = vgic_its_get_abi(its);
> >> + struct its_device *dev;
> >> + int dte_esz = abi->dte_esz;
> >> + u64 baser;
> >> +
> >> + baser = its->baser_device_table;
> >> +
> >> + list_sort(NULL, &its->device_list, vgic_its_device_cmp);
> >
> > this list is protected by the ITS mutex but you seem to be only holding
> > the KVM mutex here, so don't we have a potential exploit here?
>
> Updates to the device, ite list are done when running commands. As we
> hold the KVM mutex, commands cannot run. Then there is
> vgic_its_destroy() which happens on kvm_put_kvm when all users have
> released their reference. So to me holding the kvm lock looks sufficient.
>

But we don't hold the KVM mutex when running commands, we run the its
mutex? What am I missing?

Even worse, the vgic_its_trigger_msi also only takes the its->its_lock
mutex (or rather its caller does) and that surely can run while we are
saving the tables can it not?

Thanks,
-Christoffer