x86/paging: make log-dirty operations preemptible

Both the freeing and the inspection of the bitmap get done in (nested)
loops which - besides having a rather high iteration count in general,
albeit that would be covered by XSA-77 - have the number of non-trivial
iterations they need to perform (indirectly) controllable by both the
guest they are for and any domain controlling the guest (including the
one running qemu for it).

Note that the tying of the continuations to the invoking domain (which
previously [wrongly] used the invoking vCPU instead) implies that the
tools requesting such operations have to make sure they don't issue
multiple similar operations in parallel.

Note further that this breaks supervisor-mode kernel assumptions in
hypercall_create_continuation() (where regs->eip gets rewound to the
current hypercall stub beginning), but otoh
hypercall_cancel_continuation() doesn't work in that mode either.
Perhaps time to rip out all the remains of that feature?

This is part of CVE-2014-5146 / XSA-97.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tim Deegan <tim@xen.org>
Tested-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/domain.c

@@ -1941,7 +1941,9 @@ int domain_relinquish_resources(struct domain *d)
         pci_release_devices(d);
 
         /* Tear down paging-assistance stuff. */
-        paging_teardown(d);
+        ret = paging_teardown(d);
+        if ( ret )
+            return ret;
 
         /* Drop the in-use references to page-table bases. */
         for_each_vcpu ( d, v )

xen/arch/x86/domctl.c

@@ -61,9 +61,11 @@ long arch_do_domctl(
 
     case XEN_DOMCTL_shadow_op:
     {
-        ret = paging_domctl(d,
-                            &domctl->u.shadow_op,
-                            guest_handle_cast(u_domctl, void));
+        ret = paging_domctl(d, &domctl->u.shadow_op,
+                            guest_handle_cast(u_domctl, void), 0);
+        if ( ret == -ERESTART )
+            return hypercall_create_continuation(__HYPERVISOR_arch_1,
+                                                 "h", u_domctl);
         copyback = 1;
     }
     break;

xen/arch/x86/hvm/hvm.c

@@ -4770,7 +4770,8 @@ static hvm_hypercall_t *const hvm_hypercall64_table[NR_hypercalls] = {
     HYPERCALL(hvm_op),
     HYPERCALL(sysctl),
     HYPERCALL(domctl),
-    HYPERCALL(tmem_op)
+    HYPERCALL(tmem_op),
+    [ __HYPERVISOR_arch_1 ] = (hvm_hypercall_t *)paging_domctl_continuation
 };
 
 #define COMPAT_CALL(x)                                        \
@@ -4790,7 +4791,8 @@ static hvm_hypercall_t *const hvm_hypercall32_table[NR_hypercalls] = {
     HYPERCALL(hvm_op),
     HYPERCALL(sysctl),
     HYPERCALL(domctl),
-    HYPERCALL(tmem_op)
+    HYPERCALL(tmem_op),
+    [ __HYPERVISOR_arch_1 ] = (hvm_hypercall_t *)paging_domctl_continuation
 };
 
 /* PVH 32bitfixme. */
@@ -4808,7 +4810,8 @@ static hvm_hypercall_t *const pvh_hypercall64_table[NR_hypercalls] = {
     [ __HYPERVISOR_physdev_op ]      = (hvm_hypercall_t *)hvm_physdev_op,
     HYPERCALL(hvm_op),
     HYPERCALL(sysctl),
-    HYPERCALL(domctl)
+    HYPERCALL(domctl),
+    [ __HYPERVISOR_arch_1 ] = (hvm_hypercall_t *)paging_domctl_continuation
 };
 
 int hvm_do_hypercall(struct cpu_user_regs *regs)

xen/arch/x86/mm/paging.c

@@ -26,6 +26,7 @@
 #include <asm/shadow.h>
 #include <asm/p2m.h>
 #include <asm/hap.h>
+#include <asm/event.h>
 #include <asm/hvm/nestedhvm.h>
 #include <xen/numa.h>
 #include <xsm/xsm.h>
@@ -116,26 +117,46 @@ static void paging_free_log_dirty_page(struct domain *d, mfn_t mfn)
     d->arch.paging.free_page(d, mfn_to_page(mfn));
 }
 
-void paging_free_log_dirty_bitmap(struct domain *d)
+static int paging_free_log_dirty_bitmap(struct domain *d, int rc)
 {
     mfn_t *l4, *l3, *l2;
     int i4, i3, i2;
 
-    if ( !mfn_valid(d->arch.paging.log_dirty.top) )
-        return;
-
     paging_lock(d);
 
-    l4 = map_domain_page(mfn_x(d->arch.paging.log_dirty.top));
+    if ( !mfn_valid(d->arch.paging.log_dirty.top) )
+    {
+        paging_unlock(d);
+        return 0;
+    }
 
-    for ( i4 = 0; i4 < LOGDIRTY_NODE_ENTRIES; i4++ )
+    if ( !d->arch.paging.preempt.dom )
+    {
+        memset(&d->arch.paging.preempt.log_dirty, 0,
+               sizeof(d->arch.paging.preempt.log_dirty));
+        ASSERT(rc <= 0);
+        d->arch.paging.preempt.log_dirty.done = -rc;
+    }
+    else if ( d->arch.paging.preempt.dom != current->domain ||
+              d->arch.paging.preempt.op != XEN_DOMCTL_SHADOW_OP_OFF )
+    {
+        paging_unlock(d);
+        return -EBUSY;
+    }
+
+    l4 = map_domain_page(mfn_x(d->arch.paging.log_dirty.top));
+    i4 = d->arch.paging.preempt.log_dirty.i4;
+    i3 = d->arch.paging.preempt.log_dirty.i3;
+    rc = 0;
+
+    for ( ; i4 < LOGDIRTY_NODE_ENTRIES; i4++, i3 = 0 )
     {
         if ( !mfn_valid(l4[i4]) )
             continue;
 
         l3 = map_domain_page(mfn_x(l4[i4]));
 
-        for ( i3 = 0; i3 < LOGDIRTY_NODE_ENTRIES; i3++ )
+        for ( ; i3 < LOGDIRTY_NODE_ENTRIES; i3++ )
         {
             if ( !mfn_valid(l3[i3]) )
                 continue;
@@ -148,20 +169,54 @@ void paging_free_log_dirty_bitmap(struct domain *d)
 
             unmap_domain_page(l2);
             paging_free_log_dirty_page(d, l3[i3]);
+            l3[i3] = _mfn(INVALID_MFN);
+
+            if ( i3 < LOGDIRTY_NODE_ENTRIES - 1 && hypercall_preempt_check() )
+            {
+                d->arch.paging.preempt.log_dirty.i3 = i3 + 1;
+                d->arch.paging.preempt.log_dirty.i4 = i4;
+                rc = -ERESTART;
+                break;
+            }
         }
 
         unmap_domain_page(l3);
+        if ( rc )
+            break;
         paging_free_log_dirty_page(d, l4[i4]);
+        l4[i4] = _mfn(INVALID_MFN);
+
+        if ( i4 < LOGDIRTY_NODE_ENTRIES - 1 && hypercall_preempt_check() )
+        {
+            d->arch.paging.preempt.log_dirty.i3 = 0;
+            d->arch.paging.preempt.log_dirty.i4 = i4 + 1;
+            rc = -ERESTART;
+            break;
+        }
     }
 
     unmap_domain_page(l4);
-    paging_free_log_dirty_page(d, d->arch.paging.log_dirty.top);
-    d->arch.paging.log_dirty.top = _mfn(INVALID_MFN);
 
-    ASSERT(d->arch.paging.log_dirty.allocs == 0);
-    d->arch.paging.log_dirty.failed_allocs = 0;
+    if ( !rc )
+    {
+        paging_free_log_dirty_page(d, d->arch.paging.log_dirty.top);
+        d->arch.paging.log_dirty.top = _mfn(INVALID_MFN);
+
+        ASSERT(d->arch.paging.log_dirty.allocs == 0);
+        d->arch.paging.log_dirty.failed_allocs = 0;
+
+        rc = -d->arch.paging.preempt.log_dirty.done;
+        d->arch.paging.preempt.dom = NULL;
+    }
+    else
+    {
+        d->arch.paging.preempt.dom = current->domain;
+        d->arch.paging.preempt.op = XEN_DOMCTL_SHADOW_OP_OFF;
+    }
 
     paging_unlock(d);
+
+    return rc;
 }
 
 int paging_log_dirty_enable(struct domain *d, bool_t log_global)
@@ -187,15 +242,25 @@ int paging_log_dirty_enable(struct domain *d, bool_t log_global)
     return ret;
 }
 
-int paging_log_dirty_disable(struct domain *d)
+static int paging_log_dirty_disable(struct domain *d, bool_t resuming)
 {
-    int ret;
+    int ret = 1;
+
+    if ( !resuming )
+    {
+        domain_pause(d);
+        /* Safe because the domain is paused. */
+        if ( paging_mode_log_dirty(d) )
+        {
+            ret = d->arch.paging.log_dirty.disable_log_dirty(d);
+            ASSERT(ret <= 0);
+        }
+    }
+
+    ret = paging_free_log_dirty_bitmap(d, ret);
+    if ( ret == -ERESTART )
+        return ret;
 
-    domain_pause(d);
-    /* Safe because the domain is paused. */
-    ret = d->arch.paging.log_dirty.disable_log_dirty(d);
-    if ( !paging_mode_log_dirty(d) )
-        paging_free_log_dirty_bitmap(d);
     domain_unpause(d);
 
     return ret;
@@ -335,7 +400,9 @@ int paging_mfn_is_dirty(struct domain *d, mfn_t gmfn)
 
 /* Read a domain's log-dirty bitmap and stats.  If the operation is a CLEAN,
  * clear the bitmap and stats as well. */
-int paging_log_dirty_op(struct domain *d, struct xen_domctl_shadow_op *sc)
+static int paging_log_dirty_op(struct domain *d,
+                               struct xen_domctl_shadow_op *sc,
+                               bool_t resuming)
 {
     int rv = 0, clean = 0, peek = 1;
     unsigned long pages = 0;
@@ -343,9 +410,22 @@ int paging_log_dirty_op(struct domain *d, struct xen_domctl_shadow_op *sc)
     unsigned long *l1 = NULL;
     int i4, i3, i2;
 
-    domain_pause(d);
+    if ( !resuming )
+        domain_pause(d);
     paging_lock(d);
 
+    if ( !d->arch.paging.preempt.dom )
+        memset(&d->arch.paging.preempt.log_dirty, 0,
+               sizeof(d->arch.paging.preempt.log_dirty));
+    else if ( d->arch.paging.preempt.dom != current->domain ||
+              d->arch.paging.preempt.op != sc->op )
+    {
+        paging_unlock(d);
+        ASSERT(!resuming);
+        domain_unpause(d);
+        return -EBUSY;
+    }
+
     clean = (sc->op == XEN_DOMCTL_SHADOW_OP_CLEAN);
 
     PAGING_DEBUG(LOGDIRTY, "log-dirty %s: dom %u faults=%u dirty=%u\n",
@@ -357,12 +437,6 @@ int paging_log_dirty_op(struct domain *d, struct xen_domctl_shadow_op *sc)
     sc->stats.fault_count = d->arch.paging.log_dirty.fault_count;
     sc->stats.dirty_count = d->arch.paging.log_dirty.dirty_count;
 
-    if ( clean )
-    {
-        d->arch.paging.log_dirty.fault_count = 0;
-        d->arch.paging.log_dirty.dirty_count = 0;
-    }
-
     if ( guest_handle_is_null(sc->dirty_bitmap) )
         /* caller may have wanted just to clean the state or access stats. */
         peek = 0;
@@ -374,17 +448,15 @@ int paging_log_dirty_op(struct domain *d, struct xen_domctl_shadow_op *sc)
         goto out;
     }
 
-    pages = 0;
     l4 = paging_map_log_dirty_bitmap(d);
+    i4 = d->arch.paging.preempt.log_dirty.i4;
+    i3 = d->arch.paging.preempt.log_dirty.i3;
+    pages = d->arch.paging.preempt.log_dirty.done;
 
-    for ( i4 = 0;
-          (pages < sc->pages) && (i4 < LOGDIRTY_NODE_ENTRIES);
-          i4++ )
+    for ( ; (pages < sc->pages) && (i4 < LOGDIRTY_NODE_ENTRIES); i4++, i3 = 0 )
     {
         l3 = (l4 && mfn_valid(l4[i4])) ? map_domain_page(mfn_x(l4[i4])) : NULL;
-        for ( i3 = 0;
-              (pages < sc->pages) && (i3 < LOGDIRTY_NODE_ENTRIES);
-              i3++ )
+        for ( ; (pages < sc->pages) && (i3 < LOGDIRTY_NODE_ENTRIES); i3++ )
         {
             l2 = ((l3 && mfn_valid(l3[i3])) ?
                   map_domain_page(mfn_x(l3[i3])) : NULL);
@@ -419,18 +491,58 @@ int paging_log_dirty_op(struct domain *d, struct xen_domctl_shadow_op *sc)
             }
             if ( l2 )
                 unmap_domain_page(l2);
+
+            if ( i3 < LOGDIRTY_NODE_ENTRIES - 1 && hypercall_preempt_check() )
+            {
+                d->arch.paging.preempt.log_dirty.i4 = i4;
+                d->arch.paging.preempt.log_dirty.i3 = i3 + 1;
+                rv = -ERESTART;
+                break;
+            }
         }
         if ( l3 )
             unmap_domain_page(l3);
+
+        if ( !rv && i4 < LOGDIRTY_NODE_ENTRIES - 1 &&
+             hypercall_preempt_check() )
+        {
+            d->arch.paging.preempt.log_dirty.i4 = i4 + 1;
+            d->arch.paging.preempt.log_dirty.i3 = 0;
+            rv = -ERESTART;
+        }
+        if ( rv )
+            break;
     }
     if ( l4 )
         unmap_domain_page(l4);
 
-    if ( pages < sc->pages )
-        sc->pages = pages;
+    if ( !rv )
+    {
+        d->arch.paging.preempt.dom = NULL;
+        if ( clean )
+        {
+            d->arch.paging.log_dirty.fault_count = 0;
+            d->arch.paging.log_dirty.dirty_count = 0;
+        }
+    }
+    else
+    {
+        d->arch.paging.preempt.dom = current->domain;
+        d->arch.paging.preempt.op = sc->op;
+        d->arch.paging.preempt.log_dirty.done = pages;
+    }
 
     paging_unlock(d);
 
+    if ( rv )
+    {
+        /* Never leave the domain paused on real errors. */
+        ASSERT(rv == -ERESTART);
+        return rv;
+    }
+
+    if ( pages < sc->pages )
+        sc->pages = pages;
     if ( clean )
     {
         /* We need to further call clean_dirty_bitmap() functions of specific
@@ -441,6 +553,7 @@ int paging_log_dirty_op(struct domain *d, struct xen_domctl_shadow_op *sc)
     return rv;
 
  out:
+    d->arch.paging.preempt.dom = NULL;
     paging_unlock(d);
     domain_unpause(d);
 
@@ -504,12 +617,6 @@ void paging_log_dirty_init(struct domain *d,
     d->arch.paging.log_dirty.clean_dirty_bitmap = clean_dirty_bitmap;
 }
 
-/* This function fress log dirty bitmap resources. */
-static void paging_log_dirty_teardown(struct domain*d)
-{
-    paging_free_log_dirty_bitmap(d);
-}
-
 /************************************************/
 /*           CODE FOR PAGING SUPPORT            */
 /************************************************/
@@ -551,7 +658,7 @@ void paging_vcpu_init(struct vcpu *v)
 
 
 int paging_domctl(struct domain *d, xen_domctl_shadow_op_t *sc,
-                  XEN_GUEST_HANDLE_PARAM(void) u_domctl)
+                  XEN_GUEST_HANDLE_PARAM(void) u_domctl, bool_t resuming)
 {
     int rc;
 
@@ -575,6 +682,20 @@ int paging_domctl(struct domain *d, xen_domctl_shadow_op_t *sc,
         return -EINVAL;
     }
 
+    if ( resuming
+         ? (d->arch.paging.preempt.dom != current->domain ||
+            d->arch.paging.preempt.op != sc->op)
+         : (d->arch.paging.preempt.dom &&
+            sc->op != XEN_DOMCTL_SHADOW_OP_GET_ALLOCATION) )
+    {
+        printk(XENLOG_G_DEBUG
+               "%pv: Paging op %#x on Dom%u with unfinished prior op %#x by Dom%u\n",
+               current, sc->op, d->domain_id, d->arch.paging.preempt.op,
+               d->arch.paging.preempt.dom
+               ? d->arch.paging.preempt.dom->domain_id : DOMID_INVALID);
+        return -EBUSY;
+    }
+
     rc = xsm_shadow_control(XSM_HOOK, d, sc->op);
     if ( rc )
         return rc;
@@ -597,14 +718,13 @@ int paging_domctl(struct domain *d, xen_domctl_shadow_op_t *sc,
         return paging_log_dirty_enable(d, 1);
 
     case XEN_DOMCTL_SHADOW_OP_OFF:
-        if ( paging_mode_log_dirty(d) )
-            if ( (rc = paging_log_dirty_disable(d)) != 0 )
-                return rc;
+        if ( (rc = paging_log_dirty_disable(d, resuming)) != 0 )
+            return rc;
         break;
 
     case XEN_DOMCTL_SHADOW_OP_CLEAN:
     case XEN_DOMCTL_SHADOW_OP_PEEK:
-        return paging_log_dirty_op(d, sc);
+        return paging_log_dirty_op(d, sc, resuming);
     }
 
     /* Here, dispatch domctl to the appropriate paging code */
@@ -614,19 +734,67 @@ int paging_domctl(struct domain *d, xen_domctl_shadow_op_t *sc,
         return shadow_domctl(d, sc, u_domctl);
 }
 
-/* Call when destroying a domain */
-void paging_teardown(struct domain *d)
+long paging_domctl_continuation(XEN_GUEST_HANDLE_PARAM(xen_domctl_t) u_domctl)
 {
+    struct xen_domctl op;
+    struct domain *d;
+    int ret;
+
+    if ( copy_from_guest(&op, u_domctl, 1) )
+        return -EFAULT;
+
+    if ( op.interface_version != XEN_DOMCTL_INTERFACE_VERSION ||
+         op.cmd != XEN_DOMCTL_shadow_op )
+        return -EBADRQC;
+
+    d = rcu_lock_domain_by_id(op.domain);
+    if ( d == NULL )
+        return -ESRCH;
+
+    ret = xsm_domctl(XSM_OTHER, d, op.cmd);
+    if ( !ret )
+    {
+        if ( domctl_lock_acquire() )
+        {
+            ret = paging_domctl(d, &op.u.shadow_op,
+                                guest_handle_cast(u_domctl, void), 1);
+
+            domctl_lock_release();
+        }
+        else
+            ret = -ERESTART;
+    }
+
+    rcu_unlock_domain(d);
+
+    if ( ret == -ERESTART )
+        ret = hypercall_create_continuation(__HYPERVISOR_arch_1,
+                                            "h", u_domctl);
+    else if ( __copy_field_to_guest(u_domctl, &op, u.shadow_op) )
+        ret = -EFAULT;
+
+    return ret;
+}
+
+/* Call when destroying a domain */
+int paging_teardown(struct domain *d)
+{
+    int rc;
+
     if ( hap_enabled(d) )
         hap_teardown(d);
     else
         shadow_teardown(d);
 
     /* clean up log dirty resources. */
-    paging_log_dirty_teardown(d);
+    rc = paging_free_log_dirty_bitmap(d, 0);
+    if ( rc == -ERESTART )
+        return rc;
 
     /* Move populate-on-demand cache back to domain_list for destruction */
     p2m_pod_empty_cache(d);
+
+    return rc;
 }
 
 /* Call once all of the references to the domain have gone away */

xen/arch/x86/x86_64/compat/entry.S

@@ -421,6 +421,7 @@ ENTRY(compat_hypercall_table)
         .quad compat_ni_hypercall
         .endr
         .quad do_mca                    /* 48 */
+        .quad paging_domctl_continuation
         .rept NR_hypercalls-((.-compat_hypercall_table)/8)
         .quad compat_ni_hypercall
         .endr
@@ -469,6 +470,7 @@ ENTRY(compat_hypercall_args_table)
         .byte 0 /* compat_ni_hypercall      */
         .endr
         .byte 1 /* do_mca                   */
+        .byte 1 /* paging_domctl_continuation      */
         .rept NR_hypercalls-(.-compat_hypercall_args_table)
         .byte 0 /* compat_ni_hypercall      */
         .endr

xen/arch/x86/x86_64/entry.S

@@ -776,6 +776,7 @@ ENTRY(hypercall_table)
         .quad do_ni_hypercall
         .endr
         .quad do_mca                /* 48 */
+        .quad paging_domctl_continuation
         .rept NR_hypercalls-((.-hypercall_table)/8)
         .quad do_ni_hypercall
         .endr
@@ -824,6 +825,7 @@ ENTRY(hypercall_args_table)
         .byte 0 /* do_ni_hypercall      */
         .endr
         .byte 1 /* do_mca               */  /* 48 */
+        .byte 1 /* paging_domctl_continuation */
         .rept NR_hypercalls-(.-hypercall_args_table)
         .byte 0 /* do_ni_hypercall      */
         .endr

xen/common/domain.c

@@ -619,7 +619,6 @@ int domain_kill(struct domain *d)
         {
             if ( rc == -ERESTART )
                 rc = -EAGAIN;
-            BUG_ON(rc != -EAGAIN);
             break;
         }
         if ( sched_move_domain(d, cpupool0) )

xen/include/asm-x86/domain.h

@@ -187,6 +187,20 @@ struct paging_domain {
     struct hap_domain       hap;
     /* log dirty support */
     struct log_dirty_domain log_dirty;
+
+    /* preemption handling */
+    struct {
+        const struct domain *dom;
+        unsigned int op;
+        union {
+            struct {
+                unsigned long done:PADDR_BITS - PAGE_SHIFT;
+                unsigned long i4:PAGETABLE_ORDER;
+                unsigned long i3:PAGETABLE_ORDER;
+            } log_dirty;
+        };
+    } preempt;
+
     /* alloc/free pages from the pool for paging-assistance structures
      * (used by p2m and log-dirty code for their tries) */
     struct page_info * (*alloc_page)(struct domain *d);

xen/include/asm-x86/paging.h

@@ -132,9 +132,6 @@ struct paging_mode {
 /*****************************************************************************
  * Log dirty code */
 
-/* free log dirty bitmap resource */
-void paging_free_log_dirty_bitmap(struct domain *d);
-
 /* get the dirty bitmap for a specific range of pfns */
 void paging_log_dirty_range(struct domain *d,
                             unsigned long begin_pfn,
@@ -144,9 +141,6 @@ void paging_log_dirty_range(struct domain *d,
 /* enable log dirty */
 int paging_log_dirty_enable(struct domain *d, bool_t log_global);
 
-/* disable log dirty */
-int paging_log_dirty_disable(struct domain *d);
-
 /* log dirty initialization */
 void paging_log_dirty_init(struct domain *d,
                            int  (*enable_log_dirty)(struct domain *d,
@@ -203,10 +197,13 @@ int paging_domain_init(struct domain *d, unsigned int domcr_flags);
  * and disable ephemeral shadow modes (test mode and log-dirty mode) and
  * manipulate the log-dirty bitmap. */
 int paging_domctl(struct domain *d, xen_domctl_shadow_op_t *sc,
-                  XEN_GUEST_HANDLE_PARAM(void) u_domctl);
+                  XEN_GUEST_HANDLE_PARAM(void) u_domctl, bool_t resuming);
+
+/* Helper hypercall for dealing with continuations. */
+long paging_domctl_continuation(XEN_GUEST_HANDLE_PARAM(xen_domctl_t));
 
 /* Call when destroying a domain */
-void paging_teardown(struct domain *d);
+int paging_teardown(struct domain *d);
 
 /* Call once all of the references to the domain have gone away */
 void paging_final_teardown(struct domain *d);