x86/MCE: Fix race condition in mctelem_reserve

These lines (in mctelem_reserve)

        newhead = oldhead->mcte_next;
        if (cmpxchgptr(freelp, oldhead, newhead) == oldhead) {

are racy. After you read the newhead pointer it can happen that another
flow (thread or recursive invocation) change all the list but set head
with same value. So oldhead is the same as *freelp but you are setting
a new head that could point to whatever element (even already used).

This patch use instead a bit array and atomic bit operations.

Signed-off-by: Frediano Ziglio <frediano.ziglio@citrix.com>
Reviewed-by: Liu Jinsong <jinsong.liu@intel.com>

diff --git a/xen/arch/x86/cpu/mcheck/mctelem.c b/xen/arch/x86/cpu/mcheck/mctelem.c
index 895ce1adf23de8b09f1cf88c12335f993e61c092..ed8e8d20b7cbc300d2961a5a08f9bd4521ae053a 100644 (file)
--- a/xen/arch/x86/cpu/mcheck/mctelem.c
+++ b/xen/arch/x86/cpu/mcheck/mctelem.c
@@ -37,24 +37,19 @@ struct mctelem_ent {
        void *mcte_data;                /* corresponding data payload */
 };
 
-#define        MCTE_F_HOME_URGENT              0x0001U /* free to urgent freelist */
-#define        MCTE_F_HOME_NONURGENT           0x0002U /* free to nonurgent freelist */
-#define        MCTE_F_CLASS_URGENT             0x0004U /* in use - urgent errors */
-#define        MCTE_F_CLASS_NONURGENT          0x0008U /* in use - nonurgent errors */
+#define        MCTE_F_CLASS_URGENT             0x0001U /* in use - urgent errors */
+#define        MCTE_F_CLASS_NONURGENT          0x0002U /* in use - nonurgent errors */
 #define        MCTE_F_STATE_FREE               0x0010U /* on a freelist */
 #define        MCTE_F_STATE_UNCOMMITTED        0x0020U /* reserved; on no list */
 #define        MCTE_F_STATE_COMMITTED          0x0040U /* on a committed list */
 #define        MCTE_F_STATE_PROCESSING         0x0080U /* on a processing list */
 
-#define        MCTE_F_MASK_HOME        (MCTE_F_HOME_URGENT | MCTE_F_HOME_NONURGENT)
 #define        MCTE_F_MASK_CLASS       (MCTE_F_CLASS_URGENT | MCTE_F_CLASS_NONURGENT)
 #define        MCTE_F_MASK_STATE       (MCTE_F_STATE_FREE | \
                                MCTE_F_STATE_UNCOMMITTED | \
                                MCTE_F_STATE_COMMITTED | \
                                MCTE_F_STATE_PROCESSING)
 
-#define        MCTE_HOME(tep) ((tep)->mcte_flags & MCTE_F_MASK_HOME)
-
 #define        MCTE_CLASS(tep) ((tep)->mcte_flags & MCTE_F_MASK_CLASS)
 #define        MCTE_SET_CLASS(tep, new) do { \
     (tep)->mcte_flags &= ~MCTE_F_MASK_CLASS; \
@@ -69,6 +64,8 @@ struct mctelem_ent {
 #define        MC_URGENT_NENT          10
 #define        MC_NONURGENT_NENT       20
 
+#define MC_NENT (MC_URGENT_NENT + MC_NONURGENT_NENT)
+
 #define        MC_NCLASSES             (MC_NONURGENT + 1)
 
 #define        COOKIE2MCTE(c)          ((struct mctelem_ent *)(c))
@@ -77,11 +74,9 @@ struct mctelem_ent {
 static struct mc_telem_ctl {
        /* Linked lists that thread the array members together.
         *
-        * The free lists are singly-linked via mcte_next, and we allocate
-        * from them by atomically unlinking an element from the head.
-        * Consumed entries are returned to the head of the free list.
-        * When an entry is reserved off the free list it is not linked
-        * on any list until it is committed or dismissed.
+        * The free lists is a bit array where bit 1 means free.
+        * This as element number is quite small and is easy to
+        * atomically allocate that way.
         *
         * The committed list grows at the head and we do not maintain a
         * tail pointer; insertions are performed atomically.  The head
@@ -101,7 +96,7 @@ static struct mc_telem_ctl {
         * we can lock it for updates.  The head of the processing list
         * always has the oldest telemetry, and we append (as above)
         * at the tail of the processing list. */
-       struct mctelem_ent *mctc_free[MC_NCLASSES];
+       DECLARE_BITMAP(mctc_free, MC_NENT);
        struct mctelem_ent *mctc_committed[MC_NCLASSES];
        struct mctelem_ent *mctc_processing_head[MC_NCLASSES];
        struct mctelem_ent *mctc_processing_tail[MC_NCLASSES];
@@ -207,14 +202,14 @@ int mctelem_has_deferred(unsigned int cpu)
  */
 static void mctelem_free(struct mctelem_ent *tep)
 {
-       mctelem_class_t target = MCTE_HOME(tep) == MCTE_F_HOME_URGENT ?
-           MC_URGENT : MC_NONURGENT;
-
        BUG_ON(tep->mcte_refcnt != 0);
        BUG_ON(MCTE_STATE(tep) != MCTE_F_STATE_FREE);
 
        tep->mcte_prev = NULL;
-       mctelem_xchg_head(&mctctl.mctc_free[target], &tep->mcte_next, tep);
+       tep->mcte_next = NULL;
+
+       /* set free in array */
+       set_bit(tep - mctctl.mctc_elems, mctctl.mctc_free);
 }
 
 /* Increment the reference count of an entry that is not linked on to
@@ -274,34 +269,25 @@ void mctelem_init(int reqdatasz)
        }
 
        if ((mctctl.mctc_elems = xmalloc_array(struct mctelem_ent,
-           MC_URGENT_NENT + MC_NONURGENT_NENT)) == NULL ||
-           (datarr = xmalloc_bytes((MC_URGENT_NENT + MC_NONURGENT_NENT) *
-           datasz)) == NULL) {
+           MC_NENT)) == NULL ||
+           (datarr = xmalloc_bytes(MC_NENT * datasz)) == NULL) {
                if (mctctl.mctc_elems)
                        xfree(mctctl.mctc_elems);
                printk("Allocations for MCA telemetry failed\n");
                return;
        }
 
-       for (i = 0; i < MC_URGENT_NENT + MC_NONURGENT_NENT; i++) {
-               struct mctelem_ent *tep, **tepp;
+       for (i = 0; i < MC_NENT; i++) {
+               struct mctelem_ent *tep;
 
                tep = mctctl.mctc_elems + i;
                tep->mcte_flags = MCTE_F_STATE_FREE;
                tep->mcte_refcnt = 0;
                tep->mcte_data = datarr + i * datasz;
 
-               if (i < MC_URGENT_NENT) {
-                       tepp = &mctctl.mctc_free[MC_URGENT];
-                       tep->mcte_flags |= MCTE_F_HOME_URGENT;
-               } else {
-                       tepp = &mctctl.mctc_free[MC_NONURGENT];
-                       tep->mcte_flags |= MCTE_F_HOME_NONURGENT;
-               }
-
-               tep->mcte_next = *tepp;
+               __set_bit(i, mctctl.mctc_free);
+               tep->mcte_next = NULL;
                tep->mcte_prev = NULL;
-               *tepp = tep;
        }
 }
 
@@ -310,32 +296,25 @@ static int mctelem_drop_count;
 
 /* Reserve a telemetry entry, or return NULL if none available.
  * If we return an entry then the caller must subsequently call exactly one of
- * mctelem_unreserve or mctelem_commit for that entry.
+ * mctelem_dismiss or mctelem_commit for that entry.
  */
 mctelem_cookie_t mctelem_reserve(mctelem_class_t which)
 {
-       struct mctelem_ent **freelp;
-       struct mctelem_ent *oldhead, *newhead;
-       mctelem_class_t target = (which == MC_URGENT) ?
-           MC_URGENT : MC_NONURGENT;
+       unsigned bit;
+       unsigned start_bit = (which == MC_URGENT) ? 0 : MC_URGENT_NENT;
 
-       freelp = &mctctl.mctc_free[target];
        for (;;) {
-               if ((oldhead = *freelp) == NULL) {
-                       if (which == MC_URGENT && target == MC_URGENT) {
-                               /* raid the non-urgent freelist */
-                               target = MC_NONURGENT;
-                               freelp = &mctctl.mctc_free[target];
-                               continue;
-                       } else {
-                               mctelem_drop_count++;
-                               return (NULL);
-                       }
+               bit = find_next_bit(mctctl.mctc_free, MC_NENT, start_bit);
+
+               if (bit >= MC_NENT) {
+                       mctelem_drop_count++;
+                       return (NULL);
                }
 
-               newhead = oldhead->mcte_next;
-               if (cmpxchgptr(freelp, oldhead, newhead) == oldhead) {
-                       struct mctelem_ent *tep = oldhead;
+               /* try to allocate, atomically clear free bit */
+               if (test_and_clear_bit(bit, mctctl.mctc_free)) {
+                       /* return element we got */
+                       struct mctelem_ent *tep = mctctl.mctc_elems + bit;
 
                        mctelem_hold(tep);
                        MCTE_TRANSITION_STATE(tep, FREE, UNCOMMITTED);