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[xen master] tools/xenstore: limit max number of nodes accessed in a transaction



commit 268369d8e322d227a74a899009c5748d7b0ea142
Author:     Juergen Gross <jgross@xxxxxxxx>
AuthorDate: Tue Sep 13 07:35:09 2022 +0200
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Tue Nov 1 13:05:44 2022 +0000

    tools/xenstore: limit max number of nodes accessed in a transaction
    
    Today a guest is free to access as many nodes in a single transaction
    as it wants. This can lead to unbounded memory consumption in Xenstore
    as there is the need to keep track of all nodes having been accessed
    during a transaction.
    
    In oxenstored the number of requests in a transaction is being limited
    via a quota maxrequests (default is 1024). As multiple accesses of a
    node are not problematic in C Xenstore, limit the number of accessed
    nodes.
    
    In order to let read_node() detect a quota error in case too many nodes
    are being accessed, check the return value of access_node() and return
    NULL in case an error has been seen. Introduce __must_check and add it
    to the access_node() prototype.
    
    This is part of XSA-326 / CVE-2022-42314.
    
    Suggested-by: Julien Grall <julien@xxxxxxx>
    Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
    Reviewed-by: Julien Grall <jgrall@xxxxxxxxxx>
---
 tools/include/xen-tools/libs.h         |  4 +++
 tools/xenstore/xenstored_core.c        | 50 ++++++++++++++++++++++++----------
 tools/xenstore/xenstored_core.h        |  1 +
 tools/xenstore/xenstored_transaction.c |  9 ++++++
 tools/xenstore/xenstored_transaction.h |  4 +--
 5 files changed, 52 insertions(+), 16 deletions(-)

diff --git a/tools/include/xen-tools/libs.h b/tools/include/xen-tools/libs.h
index a16e0c3807..bafc90e2f6 100644
--- a/tools/include/xen-tools/libs.h
+++ b/tools/include/xen-tools/libs.h
@@ -63,4 +63,8 @@
 #define ROUNDUP(_x,_w) (((unsigned long)(_x)+(1UL<<(_w))-1) & ~((1UL<<(_w))-1))
 #endif
 
+#ifndef __must_check
+#define __must_check __attribute__((__warn_unused_result__))
+#endif
+
 #endif /* __XEN_TOOLS_LIBS__ */
diff --git a/tools/xenstore/xenstored_core.c b/tools/xenstore/xenstored_core.c
index 0a684450bc..d4fd005f59 100644
--- a/tools/xenstore/xenstored_core.c
+++ b/tools/xenstore/xenstored_core.c
@@ -106,6 +106,7 @@ int quota_nb_watch_per_domain = 128;
 int quota_max_entry_size = 2048; /* 2K */
 int quota_max_transaction = 10;
 int quota_nb_perms_per_node = 5;
+int quota_trans_nodes = 1024;
 int quota_max_path_len = XENSTORE_REL_PATH_MAX;
 int quota_req_outstanding = 20;
 
@@ -591,6 +592,7 @@ struct node *read_node(struct connection *conn, const void 
*ctx,
        TDB_DATA key, data;
        struct xs_tdb_record_hdr *hdr;
        struct node *node;
+       int err;
 
        node = talloc(ctx, struct node);
        if (!node) {
@@ -612,14 +614,13 @@ struct node *read_node(struct connection *conn, const 
void *ctx,
        if (data.dptr == NULL) {
                if (tdb_error(tdb_ctx) == TDB_ERR_NOEXIST) {
                        node->generation = NO_GENERATION;
-                       access_node(conn, node, NODE_ACCESS_READ, NULL);
-                       errno = ENOENT;
+                       err = access_node(conn, node, NODE_ACCESS_READ, NULL);
+                       errno = err ? : ENOENT;
                } else {
                        log("TDB error on read: %s", tdb_errorstr(tdb_ctx));
                        errno = EIO;
                }
-               talloc_free(node);
-               return NULL;
+               goto error;
        }
 
        node->parent = NULL;
@@ -634,19 +635,36 @@ struct node *read_node(struct connection *conn, const 
void *ctx,
 
        /* Permissions are struct xs_permissions. */
        node->perms.p = hdr->perms;
-       if (domain_adjust_node_perms(conn, node)) {
-               talloc_free(node);
-               return NULL;
-       }
+       if (domain_adjust_node_perms(conn, node))
+               goto error;
 
        /* Data is binary blob (usually ascii, no nul). */
        node->data = node->perms.p + hdr->num_perms;
        /* Children is strings, nul separated. */
        node->children = node->data + node->datalen;
 
-       access_node(conn, node, NODE_ACCESS_READ, NULL);
+       if (access_node(conn, node, NODE_ACCESS_READ, NULL))
+               goto error;
 
        return node;
+
+ error:
+       err = errno;
+       talloc_free(node);
+       errno = err;
+       return NULL;
+}
+
+static bool read_node_can_propagate_errno(void)
+{
+       /*
+        * 2 error cases for read_node() can always be propagated up:
+        * ENOMEM, because this has nothing to do with the node being in the
+        * data base or not, but is caused by a general lack of memory.
+        * ENOSPC, because this is related to hitting quota limits which need
+        * to be respected.
+        */
+       return errno == ENOMEM || errno == ENOSPC;
 }
 
 int write_node_raw(struct connection *conn, TDB_DATA *key, struct node *node,
@@ -763,7 +781,7 @@ static int ask_parents(struct connection *conn, const void 
*ctx,
                node = read_node(conn, ctx, name);
                if (node)
                        break;
-               if (errno == ENOMEM)
+               if (read_node_can_propagate_errno())
                        return errno;
        } while (!streq(name, "/"));
 
@@ -825,7 +843,7 @@ static struct node *get_node(struct connection *conn,
                }
        }
        /* Clean up errno if they weren't supposed to know. */
-       if (!node && errno != ENOMEM)
+       if (!node && !read_node_can_propagate_errno())
                errno = errno_from_parents(conn, ctx, name, errno, perm);
        return node;
 }
@@ -1231,7 +1249,7 @@ static struct node *construct_node(struct connection 
*conn, const void *ctx,
 
        /* If parent doesn't exist, create it. */
        parent = read_node(conn, parentname, parentname);
-       if (!parent)
+       if (!parent && errno == ENOENT)
                parent = construct_node(conn, ctx, parentname);
        if (!parent)
                return NULL;
@@ -1505,7 +1523,7 @@ static int _rm(struct connection *conn, const void *ctx, 
struct node *node,
 
        parent = read_node(conn, ctx, parentname);
        if (!parent)
-               return (errno == ENOMEM) ? ENOMEM : EINVAL;
+               return read_node_can_propagate_errno() ? errno : EINVAL;
        node->parent = parent;
 
        return delete_node(conn, ctx, parent, node, false);
@@ -1535,7 +1553,7 @@ static int do_rm(struct connection *conn, struct 
buffered_data *in)
                                return 0;
                        }
                        /* Restore errno, just in case. */
-                       if (errno != ENOMEM)
+                       if (!read_node_can_propagate_errno())
                                errno = ENOENT;
                }
                return errno;
@@ -2368,6 +2386,8 @@ static void usage(void)
 "  -M, --path-max <chars>  limit the allowed Xenstore node path length,\n"
 "  -Q, --quota <what>=<nb> set the quota <what> to the value <nb>, allowed\n"
 "                          quotas are:\n"
+"                          transaction-nodes: number of accessed node per\n"
+"                                             transaction\n"
 "                          outstanding: number of outstanding requests\n"
 "  -w, --timeout <what>=<seconds>   set the timeout in seconds for <what>,\n"
 "                          allowed timeout candidates are:\n"
@@ -2452,6 +2472,8 @@ static void set_quota(const char *arg)
        val = get_optval_int(eq + 1);
        if (what_matches(arg, "outstanding"))
                quota_req_outstanding = val;
+       else if (what_matches(arg, "transaction-nodes"))
+               quota_trans_nodes = val;
        else
                barf("unknown quota \"%s\"\n", arg);
 }
diff --git a/tools/xenstore/xenstored_core.h b/tools/xenstore/xenstored_core.h
index aa0dedde64..9c572a3c6e 100644
--- a/tools/xenstore/xenstored_core.h
+++ b/tools/xenstore/xenstored_core.h
@@ -266,6 +266,7 @@ extern int dom0_event;
 extern int priv_domid;
 extern int quota_nb_entry_per_domain;
 extern int quota_req_outstanding;
+extern int quota_trans_nodes;
 
 extern unsigned int timeout_watch_event_msec;
 
diff --git a/tools/xenstore/xenstored_transaction.c 
b/tools/xenstore/xenstored_transaction.c
index 86caf6c398..7bd41eb475 100644
--- a/tools/xenstore/xenstored_transaction.c
+++ b/tools/xenstore/xenstored_transaction.c
@@ -156,6 +156,9 @@ struct transaction
        /* Connection-local identifier for this transaction. */
        uint32_t id;
 
+       /* Node counter. */
+       unsigned int nodes;
+
        /* Generation when transaction started. */
        uint64_t generation;
 
@@ -260,6 +263,11 @@ int access_node(struct connection *conn, struct node *node,
 
        i = find_accessed_node(trans, node->name);
        if (!i) {
+               if (trans->nodes >= quota_trans_nodes &&
+                   domain_is_unprivileged(conn)) {
+                       ret = ENOSPC;
+                       goto err;
+               }
                i = talloc_zero(trans, struct accessed_node);
                if (!i)
                        goto nomem;
@@ -297,6 +305,7 @@ int access_node(struct connection *conn, struct node *node,
                                i->ta_node = true;
                        }
                }
+               trans->nodes++;
                list_add_tail(&i->list, &trans->accessed);
        }
 
diff --git a/tools/xenstore/xenstored_transaction.h 
b/tools/xenstore/xenstored_transaction.h
index 0093cac807..e3cbd6b230 100644
--- a/tools/xenstore/xenstored_transaction.h
+++ b/tools/xenstore/xenstored_transaction.h
@@ -39,8 +39,8 @@ void transaction_entry_inc(struct transaction *trans, 
unsigned int domid);
 void transaction_entry_dec(struct transaction *trans, unsigned int domid);
 
 /* This node was accessed. */
-int access_node(struct connection *conn, struct node *node,
-                enum node_access_type type, TDB_DATA *key);
+int __must_check access_node(struct connection *conn, struct node *node,
+                             enum node_access_type type, TDB_DATA *key);
 
 /* Queue watches for a modified node. */
 void queue_watches(struct connection *conn, const char *name, bool 
watch_exact);
--
generated by git-patchbot for /home/xen/git/xen.git#master



 


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