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Jan Beulich
02cbeeb620
The way the lock is currently being used in get_maptrack_handle(), it protects only the maptrack limit: The function acts on current's list only, so races on list accesses are impossible even without the lock. Otoh list access races are possible between __get_maptrack_handle() and put_maptrack_handle(), due to the invocation of the former for other than current from steal_maptrack_handle(). Introduce a per-vCPU lock for list accesses to become race free again. This lock will be uncontended except when it becomes necessary to take the steal path, i.e. in the common case there should be no meaningful performance impact. When in get_maptrack_handle adds a stolen entry to a fresh, empty, freelist, we think that there is probably no concurrency. However, this is not a fast path and adding the locking there makes the code clearly correct. Also, while we are here: the stolen maptrack_entry's tail pointer was not properly set. Set it. This is CVE-2017-12136 / XSA-228. Reported-by: Ian Jackson <ian.jackson@eu.citrix.com> Signed-off-by: Jan Beulich <jbeulich@suse.com> Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
384 lines
14 KiB
Plaintext
384 lines
14 KiB
Plaintext
********************************************************************************
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A Rough Introduction to Using Grant Tables
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Christopher Clark, March, 2005.
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Grant tables are a mechanism for sharing and transferring frames between
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domains, without requiring the participating domains to be privileged.
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The first mode of use allows domA to grant domB access to a specific frame,
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whilst retaining ownership. The block front driver uses this to grant memory
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access to the block back driver, so that it may read or write as requested.
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1. domA creates a grant access reference, and transmits the ref id to domB.
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2. domB uses the reference to map the granted frame.
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3. domB performs the memory access.
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4. domB unmaps the granted frame.
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5. domA removes its grant.
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The second mode allows domA to accept a transfer of ownership of a frame from
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domB. The net front and back driver will use this for packet tx/rx. This
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mechanism is still being implemented, though the xen<->guest interface design
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is complete.
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1. domA creates an accept transfer grant reference, and transmits it to domB.
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2. domB uses the ref to hand over a frame it owns.
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3. domA accepts the transfer
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4. domA clears the used reference.
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********************************************************************************
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Data structures
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~~~~~~~~~~~~~~~
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The following data structures are used by Xen and the guests to implement
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grant tables:
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1. Shared grant entries
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2. Active grant entries
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3. Map tracking
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These are not the users primary interface to grant tables, but are discussed
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because an understanding of how they work may be useful. Each of these is a
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finite resource.
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Shared grant entries
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~~~~~~~~~~~~~~~~~~~~
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A set of pages are shared between Xen and a guest, holding the shared grant
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entries. The guest writes into these entries to create grant references. The
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index of the entry is transmitted to the remote domain: this is the
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reference used to activate an entry. Xen will write into a shared entry to
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indicate to a guest that its grant is in use.
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sha->domid : remote domain being granted rights
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sha->frame : machine frame being granted
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sha->flags : allow access, allow transfer, remote is reading/writing, etc.
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Active grant entries
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~~~~~~~~~~~~~~~~~~~~
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Xen maintains a set of private frames per domain, holding the active grant
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entries for safety, and to reference count mappings.
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act->domid : remote domain being granted rights
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act->frame : machine frame being granted
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act->pin : used to hold reference counts
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act->lock : spinlock used to serialize access to active entry state
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Map tracking
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~~~~~~~~~~~~
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Every time a frame is mapped, a map track entry is stored in the metadata of
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the mapping domain. The index of this entry is returned from the map call,
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and is used to unmap the frame. Map track entries are also searched whenever a
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page table entry containing a foreign frame number is overwritten: the first
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matching map track entry is then removed, as if unmap had been invoked.
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These are not used by the transfer mechanism.
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map->domid : owner of the mapped frame
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map->ref : grant reference
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map->flags : ro/rw, mapped for host or device access
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********************************************************************************
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Locking
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~~~~~~~
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Xen uses several locks to serialize access to the internal grant table state.
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grant_table->lock : rwlock used to prevent readers from accessing
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inconsistent grant table state such as current
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version, partially initialized active table pages,
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etc.
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grant_table->maptrack_lock : spinlock used to protect the maptrack limit
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v->maptrack_freelist_lock : spinlock used to protect the maptrack free list
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active_grant_entry->lock : spinlock used to serialize modifications to
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active entries
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The primary lock for the grant table is a read/write spinlock. All
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functions that access members of struct grant_table must acquire a
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read lock around critical sections. Any modification to the members
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of struct grant_table (e.g., nr_status_frames, nr_grant_frames,
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active frames, etc.) must only be made if the write lock is
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held. These elements are read-mostly, and read critical sections can
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be large, which makes a rwlock a good choice.
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The maptrack free list is protected by its own spinlock. The maptrack
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lock may be locked while holding the grant table lock.
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The maptrack_freelist_lock is an innermost lock. It may be locked
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while holding other locks, but no other locks may be acquired within
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it.
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Active entries are obtained by calling active_entry_acquire(gt, ref).
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This function returns a pointer to the active entry after locking its
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spinlock. The caller must hold the grant table read lock before
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calling active_entry_acquire(). This is because the grant table can
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be dynamically extended via gnttab_grow_table() while a domain is
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running and must be fully initialized. Once all access to the active
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entry is complete, release the lock by calling active_entry_release(act).
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Summary of rules for locking:
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active_entry_acquire() and active_entry_release() can only be
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called when holding the relevant grant table's read lock. I.e.:
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read_lock(>->lock);
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act = active_entry_acquire(gt, ref);
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...
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active_entry_release(act);
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read_unlock(>->lock);
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Active entries cannot be acquired while holding the maptrack lock.
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Multiple active entries can be acquired while holding the grant table
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_write_ lock.
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Maptrack entries are protected by the corresponding active entry
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lock. As an exception, new maptrack entries may be populated without
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holding the lock, provided the flags field is written last. This
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requires any maptrack entry user validates the flags field as
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non-zero first.
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********************************************************************************
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Granting a foreign domain access to frames
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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domA [frame]--> domB
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domA: #include <asm-xen/gnttab.h>
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grant_ref_t gref[BATCH_SIZE];
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for ( i = 0; i < BATCH_SIZE; i++ )
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gref[i] = gnttab_grant_foreign_access( domBid, mfn, (readonly ? 1 : 0) );
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.. gref is then somehow transmitted to domB for use.
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Mapping foreign frames
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~~~~~~~~~~~~~~~~~~~~~~
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domB: #include <asm-xen/hypervisor.h>
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unsigned long mmap_vstart;
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gnttab_op_t aop[BATCH_SIZE];
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grant_ref_t mapped_handle[BATCH_SIZE];
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if ( (mmap_vstart = allocate_empty_lowmem_region(BATCH_SIZE)) == 0 )
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BUG();
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for ( i = 0; i < BATCH_SIZE; i++ )
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{
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aop[i].u.map_grant_ref.host_virt_addr =
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mmap_vstart + (i * PAGE_SIZE);
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aop[i].u.map_grant_ref.dom = domAid;
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aop[i].u.map_grant_ref.ref = gref[i];
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aop[i].u.map_grant_ref.flags = ( GNTMAP_host_map | GNTMAP_readonly );
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}
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if ( unlikely(HYPERVISOR_grant_table_op(
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GNTTABOP_map_grant_ref, aop, BATCH_SIZE)))
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BUG();
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for ( i = 0; i < BATCH_SIZE; i++ )
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{
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if ( unlikely(aop[i].u.map_grant_ref.handle < 0) )
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{
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tidyup_all(aop, i);
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goto panic;
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}
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phys_to_machine_mapping[__pa(mmap_vstart + (i * PAGE_SIZE))>>PAGE_SHIFT] =
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FOREIGN_FRAME(aop[i].u.map_grant_ref.dev_bus_addr);
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mapped_handle[i] = aop[i].u.map_grant_ref.handle;
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}
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Unmapping foreign frames
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~~~~~~~~~~~~~~~~~~~~~~~~
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domB:
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for ( i = 0; i < BATCH_SIZE; i++ )
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{
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aop[i].u.unmap_grant_ref.host_virt_addr = mmap_vstart + (i * PAGE_SIZE);
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aop[i].u.unmap_grant_ref.dev_bus_addr = 0;
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aop[i].u.unmap_grant_ref.handle = mapped_handle[i];
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}
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if ( unlikely(HYPERVISOR_grant_table_op(
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GNTTABOP_unmap_grant_ref, aop, BATCH_SIZE)))
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BUG();
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Ending foreign access
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~~~~~~~~~~~~~~~~~~~~~
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Note that this only prevents further mappings; it does _not_ revoke access.
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Should _only_ be used when the remote domain has unmapped the frame.
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gnttab_query_foreign_access( gref ) will indicate the state of any mapping.
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domA:
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if ( gnttab_query_foreign_access( gref[i] ) == 0 )
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gnttab_end_foreign_access( gref[i], readonly );
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TODO: readonly yet to be implemented.
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********************************************************************************
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Transferring ownership of a frame to another domain
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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[ XXX: Transfer mechanism is alpha-calibre code, untested, use at own risk XXX ]
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[ XXX: show use of batch operations below, rather than single frame XXX ]
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[ XXX: linux internal interface could/should be wrapped to be tidier XXX ]
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Prepare to accept a frame from a foreign domain
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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domA:
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if ( (p = alloc_page(GFP_HIGHUSER)) == NULL )
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{
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printk("Cannot alloc a frame to surrender\n");
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break;
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}
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pfn = p - mem_map;
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mfn = phys_to_machine_mapping[pfn];
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if ( !PageHighMem(p) )
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{
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v = phys_to_virt(pfn << PAGE_SHIFT);
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scrub_pages(v, 1);
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queue_l1_entry_update(get_ptep((unsigned long)v), 0);
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}
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/* Ensure that ballooned highmem pages don't have cached mappings. */
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kmap_flush_unused();
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/* Flush updates through and flush the TLB. */
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xen_tlb_flush();
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phys_to_machine_mapping[pfn] = INVALID_P2M_ENTRY;
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if ( HYPERVISOR_dom_mem_op(
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MEMOP_decrease_reservation, &mfn, 1, 0) != 1 )
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{
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printk("MEMOP_decrease_reservation failed\n");
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/* er... ok. free the page then */
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__free_page(p);
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break;
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}
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accepting_pfn = pfn;
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ref = gnttab_grant_foreign_transfer( (domid_t) args.arg[0], pfn );
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printk("Accepting dom %lu frame at ref (%d)\n", args.arg[0], ref);
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Transfer a frame to a foreign domain
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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domB:
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mmu_update_t update;
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domid_t domid;
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grant_ref_t gref;
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unsigned long pfn, mfn, *v;
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struct page *transfer_page = 0;
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/* alloc a page and grant access.
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* alloc page returns a page struct. */
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if ( (transfer_page = alloc_page(GFP_HIGHUSER)) == NULL )
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return -ENOMEM;
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pfn = transfer_page - mem_map;
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mfn = phys_to_machine_mapping[pfn];
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/* need to remove all references to this page */
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if ( !PageHighMem(transfer_page) )
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{
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v = phys_to_virt(pfn << PAGE_SHIFT);
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scrub_pages(v, 1);
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sprintf((char *)v, "This page (%lx) was transferred.\n", mfn);
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queue_l1_entry_update(get_ptep((unsigned long)v), 0);
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}
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#ifdef CONFIG_XEN_SCRUB_PAGES
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else
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{
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v = kmap(transfer_page);
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scrub_pages(v, 1);
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sprintf((char *)v, "This page (%lx) was transferred.\n", mfn);
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kunmap(transfer_page);
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}
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#endif
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/* Delete any cached kmappings */
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kmap_flush_unused();
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/* Flush updates through and flush the TLB */
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xen_tlb_flush();
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/* invalidate in P2M */
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phys_to_machine_mapping[pfn] = INVALID_P2M_ENTRY;
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domid = (domid_t)args.arg[0];
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gref = (grant_ref_t)args.arg[1];
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update.ptr = MMU_EXTENDED_COMMAND;
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update.ptr |= ((gref & 0x00FF) << 2);
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update.ptr |= mfn << PAGE_SHIFT;
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update.val = MMUEXT_TRANSFER_PAGE;
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update.val |= (domid << 16);
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update.val |= (gref & 0xFF00);
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ret = HYPERVISOR_mmu_update(&update, 1, NULL);
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Map a transferred frame
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~~~~~~~~~~~~~~~~~~~~~~~
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TODO:
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Clear the used transfer reference
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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TODO:
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********************************************************************************
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Using a private reserve of grant references
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Where it is known in advance how many grant references are required, and
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failure to allocate them on demand would cause difficulty, a batch can be
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allocated and held in a private reserve.
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To reserve a private batch:
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/* housekeeping data - treat as opaque: */
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grant_ref_t gref_head, gref_terminal;
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if ( 0 > gnttab_alloc_grant_references( number_to_reserve,
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&gref_head, &gref_terminal ))
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return -ENOSPC;
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To release a batch back to the shared pool:
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gnttab_free_grant_references( number_reserved, gref_head );
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To claim a reserved reference:
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ref = gnttab_claim_grant_reference( &gref_head, gref_terminal );
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To release a claimed reference back to the reserve pool:
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gnttab_release_grant_reference( &gref_head, gref );
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To use a claimed reference to grant access, use these alternative functions
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that take an additional parameter of the grant reference to use:
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gnttab_grant_foreign_access_ref
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gnttab_grant_foreign_transfer_ref
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