x86/shadow: account for log-dirty mode when pre-allocating

Pre-allocation is intended to ensure that in the course of constructing
or updating shadows there won't be any risk of just made shadows or
shadows being acted upon can disappear under our feet. The amount of
pages pre-allocated then, however, needs to account for all possible
subsequent allocations. While the use in sh_page_fault() accounts for
all shadows which may need making, so far it didn't account for
allocations coming from log-dirty tracking (which piggybacks onto the
P2M allocation functions).

Since shadow_prealloc() takes a count of shadows (or other data
structures) rather than a count of pages, putting the adjustment at the
call site of this function won't work very well: We simply can't express
the correct count that way in all cases. Instead take care of this in
the function itself, by "snooping" for L1 type requests. (While not
applicable right now, future new request sites of L1 tables would then
also be covered right away.)

It is relevant to note here that pre-allocations like the one done from
shadow_alloc_p2m_page() are benign when they fall in the "scope" of an
earlier pre-alloc which already included that count: The inner call will
simply find enough pages available then; it'll bail right away.

This is CVE-2022-42332 / XSA-427.

Signed-off-by: Jan Beulich <[email protected]>
Reviewed-by: Tim Deegan <[email protected]>

xen/arch/x86/include/asm/paging.h

@@ -187,6 +187,10 @@ bool paging_mfn_is_dirty(const struct domain *d, mfn_t gmfn);
 #define L4_LOGDIRTY_IDX(pfn) ((pfn_x(pfn) >> (PAGE_SHIFT + 3 + PAGETABLE_ORDER * 2)) & \
                               (LOGDIRTY_NODE_ENTRIES-1))
 
+#define paging_logdirty_levels() \
+    (DIV_ROUND_UP(PADDR_BITS - PAGE_SHIFT - (PAGE_SHIFT + 3), \
+                  PAGE_SHIFT - ilog2(sizeof(mfn_t))) + 1)
+
 #ifdef CONFIG_HVM
 /* VRAM dirty tracking support */
 struct sh_dirty_vram {

xen/arch/x86/mm/paging.c

@@ -282,6 +282,7 @@ void paging_mark_pfn_dirty(struct domain *d, pfn_t pfn)
     if ( unlikely(!VALID_M2P(pfn_x(pfn))) )
         return;
 
+    BUILD_BUG_ON(paging_logdirty_levels() != 4);
     i1 = L1_LOGDIRTY_IDX(pfn);
     i2 = L2_LOGDIRTY_IDX(pfn);
     i3 = L3_LOGDIRTY_IDX(pfn);

xen/arch/x86/mm/shadow/common.c

@@ -1012,7 +1012,17 @@ bool shadow_prealloc(struct domain *d, unsigned int type, unsigned int count)
     if ( unlikely(d->is_dying) )
        return false;
 
-    ret = _shadow_prealloc(d, shadow_size(type) * count);
+    count *= shadow_size(type);
+    /*
+     * Log-dirty handling may result in allocations when populating its
+     * tracking structures.  Tie this to the caller requesting space for L1
+     * shadows.
+     */
+    if ( paging_mode_log_dirty(d) &&
+         ((SHF_L1_ANY | SHF_FL1_ANY) & (1u << type)) )
+        count += paging_logdirty_levels();
+
+    ret = _shadow_prealloc(d, count);
     if ( !ret && (!d->is_shutting_down || d->shutdown_code != SHUTDOWN_crash) )
         /*
          * Failing to allocate memory required for shadow usage can only result in