[xen staging] x86/shadow: properly handle get_page() failing

[patchbot@xxxxxxx]

commit 3629759626ac7201a670a8a2d4d4a536e7443575
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Fri Jul 29 08:48:26 2022 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Fri Jul 29 08:48:26 2022 +0200

    x86/shadow: properly handle get_page() failing
    
    We should not blindly (in a release build) insert the new entry in the
    hash if a reference to the guest page cannot be obtained, or else an
    excess reference would be put when removing the hash entry again. Crash
    the domain in that case instead. The sole caller doesn't further care
    about the state of the guest page: All it does is return the
    corresponding shadow page (which was obtained successfully before) to
    its caller.
    
    To compensate we further need to adjust hash removal: Since the shadow
    page already has had its backlink set, domain cleanup code would try to
    destroy the shadow, and hence still cause a put_page() without
    corresponding get_page(). Leverage that the failed get_page() leads to
    no hash insertion, making shadow_hash_delete() no longer assume it will
    find the requested entry. Instead return back whether the entry was
    found. This way delete_shadow_status() can avoid calling put_page() in
    the problem scenario.
    
    For the other caller of shadow_hash_delete() simply reinstate the
    otherwise dropped assertion at the call site.
    
    While touching the conditionals in {set,delete}_shadow_status() anyway,
    also switch around their two pre-existing parts, to have the cheap one
    first (frequently allowing to avoid evaluation of the expensive - due to
    evaluate_nospec() - one altogether).
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/arch/x86/mm/shadow/common.c  | 10 ++++++----
 xen/arch/x86/mm/shadow/multi.c   |  8 +++++++-
 xen/arch/x86/mm/shadow/private.h | 19 ++++++++++---------
 3 files changed, 23 insertions(+), 14 deletions(-)

diff --git a/xen/arch/x86/mm/shadow/common.c b/xen/arch/x86/mm/shadow/common.c
index c37c3bb077..0fd00a2f96 100644
--- a/xen/arch/x86/mm/shadow/common.c
+++ b/xen/arch/x86/mm/shadow/common.c
@@ -1581,7 +1581,7 @@ void shadow_hash_insert(struct domain *d, unsigned long 
n, unsigned int t,
     sh_hash_audit_bucket(d, key);
 }
 
-void shadow_hash_delete(struct domain *d, unsigned long n, unsigned int t,
+bool shadow_hash_delete(struct domain *d, unsigned long n, unsigned int t,
                         mfn_t smfn)
 /* Excise the mapping (n,t)->smfn from the hash table */
 {
@@ -1606,10 +1606,8 @@ void shadow_hash_delete(struct domain *d, unsigned long 
n, unsigned int t,
     {
         /* Need to search for the one we want */
         x = d->arch.paging.shadow.hash_table[key];
-        while ( 1 )
+        while ( x )
         {
-            ASSERT(x); /* We can't have hit the end, since our target is
-                        * still in the chain somehwere... */
             if ( next_shadow(x) == sp )
             {
                 x->next_shadow = sp->next_shadow;
@@ -1617,10 +1615,14 @@ void shadow_hash_delete(struct domain *d, unsigned long 
n, unsigned int t,
             }
             x = next_shadow(x);
         }
+        if ( !x )
+            return false;
     }
     set_next_shadow(sp, NULL);
 
     sh_hash_audit_bucket(d, key);
+
+    return true;
 }
 
 typedef int (*hash_vcpu_callback_t)(struct vcpu *v, mfn_t smfn, mfn_t 
other_mfn);
diff --git a/xen/arch/x86/mm/shadow/multi.c b/xen/arch/x86/mm/shadow/multi.c
index b0b1c31ee0..869d7baed0 100644
--- a/xen/arch/x86/mm/shadow/multi.c
+++ b/xen/arch/x86/mm/shadow/multi.c
@@ -132,7 +132,13 @@ delete_fl1_shadow_status(struct domain *d, gfn_t gfn, 
mfn_t smfn)
     SHADOW_PRINTK("gfn=%"SH_PRI_gfn", type=%08x, smfn=%"PRI_mfn"\n",
                    gfn_x(gfn), SH_type_fl1_shadow, mfn_x(smfn));
     ASSERT(mfn_to_page(smfn)->u.sh.head);
-    shadow_hash_delete(d, gfn_x(gfn), SH_type_fl1_shadow, smfn);
+    if ( !shadow_hash_delete(d, gfn_x(gfn), SH_type_fl1_shadow, smfn) )
+    {
+        printk(XENLOG_G_ERR
+               "%pd: %"PRI_gfn":FL1 hash entry not found for %"PRI_mfn"\n",
+               d, gfn_x(gfn), mfn_x(smfn));
+        domain_crash(d);
+    }
 }
 
 
diff --git a/xen/arch/x86/mm/shadow/private.h b/xen/arch/x86/mm/shadow/private.h
index 3dc024e30f..3a74f45362 100644
--- a/xen/arch/x86/mm/shadow/private.h
+++ b/xen/arch/x86/mm/shadow/private.h
@@ -375,7 +375,7 @@ shadow_size(unsigned int shadow_type)
 mfn_t shadow_hash_lookup(struct domain *d, unsigned long n, unsigned int t);
 void  shadow_hash_insert(struct domain *d,
                          unsigned long n, unsigned int t, mfn_t smfn);
-void  shadow_hash_delete(struct domain *d,
+bool  shadow_hash_delete(struct domain *d,
                          unsigned long n, unsigned int t, mfn_t smfn);
 
 /* shadow promotion */
@@ -773,18 +773,19 @@ static inline void
 set_shadow_status(struct domain *d, mfn_t gmfn, u32 shadow_type, mfn_t smfn)
 /* Put a shadow into the hash table */
 {
-    int res;
-
     SHADOW_PRINTK("d%d gmfn=%lx, type=%08x, smfn=%lx\n",
                   d->domain_id, mfn_x(gmfn), shadow_type, mfn_x(smfn));
 
     ASSERT(mfn_to_page(smfn)->u.sh.head);
 
     /* 32-bit PV guests don't own their l4 pages so can't get_page them */
-    if ( !is_pv_32bit_domain(d) || shadow_type != SH_type_l4_64_shadow )
+    if ( (shadow_type != SH_type_l4_64_shadow || !is_pv_32bit_domain(d)) &&
+         !get_page(mfn_to_page(gmfn), d) )
     {
-        res = get_page(mfn_to_page(gmfn), d);
-        ASSERT(res == 1);
+        printk(XENLOG_G_ERR "%pd: cannot get page for MFN %" PRI_mfn "\n",
+               d, mfn_x(gmfn));
+        domain_crash(d);
+        return;
     }
 
     shadow_hash_insert(d, mfn_x(gmfn), shadow_type, smfn);
@@ -797,9 +798,9 @@ delete_shadow_status(struct domain *d, mfn_t gmfn, u32 
shadow_type, mfn_t smfn)
     SHADOW_PRINTK("d%d gmfn=%"PRI_mfn", type=%08x, smfn=%"PRI_mfn"\n",
                   d->domain_id, mfn_x(gmfn), shadow_type, mfn_x(smfn));
     ASSERT(mfn_to_page(smfn)->u.sh.head);
-    shadow_hash_delete(d, mfn_x(gmfn), shadow_type, smfn);
-    /* 32-bit PV guests don't own their l4 pages; see set_shadow_status */
-    if ( !is_pv_32bit_domain(d) || shadow_type != SH_type_l4_64_shadow )
+    if ( shadow_hash_delete(d, mfn_x(gmfn), shadow_type, smfn) &&
+         /* 32-bit PV guests don't own their l4 pages; see set_shadow_status */
+         (shadow_type != SH_type_l4_64_shadow || !is_pv_32bit_domain(d)) )
         put_page(mfn_to_page(gmfn));
 }
 
--
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