[PATCH v3 2/6] x86/HVM: split restore state checking from state loading

[Jan Beulich <jbeulich@xxxxxxxx>]

..., at least as reasonably feasible without making a check hook
mandatory (in particular strict vs relaxed/zero-extend length checking
can't be done early this way).

Note that only one of the two uses of hvm_load() is accompanied with
hvm_check(). The other directly consumes hvm_save() output, which ought
to be well-formed. This means that while input data related checks don't
need repeating in the "load" function when already done by the "check"
one (albeit assertions to this effect may be desirable), domain state
related checks (e.g. has_xyz(d)) will be required in both places.

Suggested-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Do we really need all the copying involved in use of _hvm_read_entry()
(backing hvm_load_entry()? Zero-extending loads are likely easier to
handle that way, but for strict loads all we gain is a reduced risk of
unaligned accesses (compared to simply pointing into h->data[]).

Would the hvm_sr_handlers[] better use array_access_nospec()?
---
v2: New.

--- a/xen/arch/x86/domctl.c
+++ b/xen/arch/x86/domctl.c
@@ -379,6 +379,10 @@ long arch_do_domctl(
         if ( copy_from_guest(c.data, domctl->u.hvmcontext.buffer, c.size) != 0 
)
             goto sethvmcontext_out;
 
+        ret = hvm_check(d, &c);
+        if ( ret )
+            goto sethvmcontext_out;
+
         domain_pause(d);
         ret = hvm_load(d, &c);
         domain_unpause(d);
--- a/xen/arch/x86/hvm/save.c
+++ b/xen/arch/x86/hvm/save.c
@@ -30,7 +30,8 @@ static void arch_hvm_save(struct domain
     d->arch.hvm.sync_tsc = rdtsc();
 }
 
-static int arch_hvm_load(struct domain *d, const struct hvm_save_header *hdr)
+static int arch_hvm_check(const struct domain *d,
+                          const struct hvm_save_header *hdr)
 {
     uint32_t eax, ebx, ecx, edx;
 
@@ -55,6 +56,11 @@ static int arch_hvm_load(struct domain *
                "(%#"PRIx32") and restored on another (%#"PRIx32").\n",
                d->domain_id, hdr->cpuid, eax);
 
+    return 0;
+}
+
+static void arch_hvm_load(struct domain *d, const struct hvm_save_header *hdr)
+{
     /* Restore guest's preferred TSC frequency. */
     if ( hdr->gtsc_khz )
         d->arch.tsc_khz = hdr->gtsc_khz;
@@ -66,13 +72,12 @@ static int arch_hvm_load(struct domain *
 
     /* VGA state is not saved/restored, so we nobble the cache. */
     d->arch.hvm.stdvga.cache = STDVGA_CACHE_DISABLED;
-
-    return 0;
 }
 
 /* List of handlers for various HVM save and restore types */
 static struct {
     hvm_save_handler save;
+    hvm_check_handler check;
     hvm_load_handler load;
     const char *name;
     size_t size;
@@ -88,6 +93,7 @@ void __init hvm_register_savevm(uint16_t
 {
     ASSERT(typecode <= HVM_SAVE_CODE_MAX);
     ASSERT(hvm_sr_handlers[typecode].save == NULL);
+    ASSERT(hvm_sr_handlers[typecode].check == NULL);
     ASSERT(hvm_sr_handlers[typecode].load == NULL);
     hvm_sr_handlers[typecode].save = save_state;
     hvm_sr_handlers[typecode].load = load_state;
@@ -275,6 +281,78 @@ int hvm_save(struct domain *d, hvm_domai
     return 0;
 }
 
+int hvm_check(const struct domain *d, hvm_domain_context_t *h)
+{
+    const struct hvm_save_header *hdr;
+    int rc;
+
+    if ( d->is_dying )
+        return -EINVAL;
+
+    /* Get at the save header, which must be first. */
+    hdr = hvm_get_entry(HEADER, h);
+    if ( !hdr )
+        return -ENODATA;
+
+    rc = arch_hvm_check(d, hdr);
+    if ( rc )
+        return rc;
+
+    for ( ; ; )
+    {
+        const struct hvm_save_descriptor *desc;
+        hvm_check_handler handler;
+
+        if ( h->size - h->cur < sizeof(*desc) )
+        {
+            /* Run out of data */
+            printk(XENLOG_G_ERR
+                   "HVM restore %pd: save did not end with a null entry\n",
+                   d);
+            return -ENODATA;
+        }
+
+        /* Read the typecode of the next entry and check for the end-marker. */
+        desc = (const void *)&h->data[h->cur];
+        if ( desc->typecode == HVM_SAVE_CODE(END) )
+        {
+            /* Reset cursor for hvm_load(). */
+            h->cur = 0;
+            return 0;
+        }
+
+        /* Find the handler for this entry. */
+        if ( desc->typecode >= ARRAY_SIZE(hvm_sr_handlers) ||
+             !hvm_sr_handlers[desc->typecode].name ||
+             !hvm_sr_handlers[desc->typecode].load )
+        {
+            printk(XENLOG_G_ERR "HVM restore %pd: unknown entry typecode %u\n",
+                   d, desc->typecode);
+            return -EINVAL;
+        }
+
+        /* Check the entry. */
+        handler = hvm_sr_handlers[desc->typecode].check;
+        if ( !handler )
+        {
+            if ( desc->length > h->size - h->cur - sizeof(*desc) )
+                return -ENODATA;
+            h->cur += sizeof(*desc) + desc->length;
+        }
+        else if ( (rc = handler(d, h)) )
+        {
+            printk(XENLOG_G_ERR
+                   "HVM restore %pd: failed to check %s:%u rc %d\n",
+                   d, hvm_sr_handlers[desc->typecode].name, desc->instance, 
rc);
+            return rc;
+        }
+
+        process_pending_softirqs();
+    }
+
+    /* Not reached */
+}
+
 int hvm_load(struct domain *d, hvm_domain_context_t *h)
 {
     const struct hvm_save_header *hdr;
@@ -291,9 +369,8 @@ int hvm_load(struct domain *d, hvm_domai
     if ( !hdr )
         return -ENODATA;
 
-    rc = arch_hvm_load(d, hdr);
-    if ( rc )
-        return rc;
+    ASSERT(!arch_hvm_check(d, hdr));
+    arch_hvm_load(d, hdr);
 
     /* Down all the vcpus: we only re-enable the ones that had state saved. */
     for_each_vcpu(d, v)
@@ -304,10 +381,7 @@ int hvm_load(struct domain *d, hvm_domai
     {
         if ( h->size - h->cur < sizeof(struct hvm_save_descriptor) )
         {
-            /* Run out of data */
-            printk(XENLOG_G_ERR
-                   "HVM%d restore: save did not end with a null entry\n",
-                   d->domain_id);
+            ASSERT_UNREACHABLE();
             return -ENODATA;
         }
 
@@ -320,8 +394,7 @@ int hvm_load(struct domain *d, hvm_domai
         if ( (desc->typecode > HVM_SAVE_CODE_MAX) ||
              ((handler = hvm_sr_handlers[desc->typecode].load) == NULL) )
         {
-            printk(XENLOG_G_ERR "HVM%d restore: unknown entry typecode %u\n",
-                   d->domain_id, desc->typecode);
+            ASSERT_UNREACHABLE();
             return -EINVAL;
         }
 
--- a/xen/arch/x86/include/asm/hvm/save.h
+++ b/xen/arch/x86/include/asm/hvm/save.h
@@ -103,6 +103,8 @@ static inline unsigned int hvm_load_inst
  * restoring.  Both return non-zero on error. */
 typedef int (*hvm_save_handler) (struct vcpu *v,
                                  hvm_domain_context_t *h);
+typedef int (*hvm_check_handler)(const struct domain *d,
+                                 hvm_domain_context_t *h);
 typedef int (*hvm_load_handler) (struct domain *d,
                                  hvm_domain_context_t *h);
 
@@ -140,6 +142,7 @@ size_t hvm_save_size(struct domain *d);
 int hvm_save(struct domain *d, hvm_domain_context_t *h);
 int hvm_save_one(struct domain *d, unsigned int typecode, unsigned int 
instance,
                  XEN_GUEST_HANDLE_64(uint8) handle, uint64_t *bufsz);
+int hvm_check(const struct domain *d, hvm_domain_context_t *h);
 int hvm_load(struct domain *d, hvm_domain_context_t *h);
 
 #endif /* __XEN_HVM_SAVE_H__ */