tools/xenstored: Prevent a buffer overflow in dump_state_node_perms()

ASAN reported one issue when Live Updating Xenstored:

=================================================================
==873==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7ffc194f53e0 at pc 0x555c6b323292 bp 0x7ffc194f5340 sp 0x7ffc194f5338
WRITE of size 1 at 0x7ffc194f53e0 thread T0
    #0 0x555c6b323291 in dump_state_node_perms xen/tools/xenstore/xenstored_core.c:2468
    #1 0x555c6b32746e in dump_state_special_node xen/tools/xenstore/xenstored_domain.c:1257
    #2 0x555c6b32a702 in dump_state_special_nodes xen/tools/xenstore/xenstored_domain.c:1273
    xen-project#3 0x555c6b32ddb3 in lu_dump_state xen/tools/xenstore/xenstored_control.c:521
    xen-project#4 0x555c6b32e380 in do_lu_start xen/tools/xenstore/xenstored_control.c:660
    xen-project#5 0x555c6b31b461 in call_delayed xen/tools/xenstore/xenstored_core.c:278
    xen-project#6 0x555c6b32275e in main xen/tools/xenstore/xenstored_core.c:2357
    xen-project#7 0x7f95eecf3d09 in __libc_start_main ../csu/libc-start.c:308
    xen-project#8 0x555c6b3197e9 in _start (/usr/local/sbin/xenstored+0xc7e9)

Address 0x7ffc194f53e0 is located in stack of thread T0 at offset 80 in frame
    #0 0x555c6b32713e in dump_state_special_node xen/tools/xenstore/xenstored_domain.c:1232

  This frame has 2 object(s):
    [32, 40) 'head' (line 1233)
    [64, 80) 'sn' (line 1234) <== Memory access at offset 80 overflows this variable

This is happening because the callers are passing a pointer to a variable
allocated on the stack. However, the field perms is a dynamic array, so
Xenstored will end up to read outside of the variable.

Rework the code so the permissions are written one by one in the fd.

Fixes: ed6eebf ("tools/xenstore: dump the xenstore state for live update")
Signed-off-by: Julien Grall <[email protected]>
Reviewed-by: Juergen Gross <[email protected]>
Reviewed-by: Luca Fancellu <[email protected]>

tools/xenstore/xenstored_core.c

@@ -2447,34 +2447,36 @@ const char *dump_state_buffered_data(FILE *fp, const struct connection *c,
 	return NULL;
 }
 
-const char *dump_state_node_perms(FILE *fp, struct xs_state_node *sn,
-				  const struct xs_permissions *perms,
+const char *dump_state_node_perms(FILE *fp, const struct xs_permissions *perms,
 				  unsigned int n_perms)
 {
 	unsigned int p;
 
 	for (p = 0; p < n_perms; p++) {
+		struct xs_state_node_perm sp;
+
 		switch ((int)perms[p].perms & ~XS_PERM_IGNORE) {
 		case XS_PERM_READ:
-			sn->perms[p].access = XS_STATE_NODE_PERM_READ;
+			sp.access = XS_STATE_NODE_PERM_READ;
 			break;
 		case XS_PERM_WRITE:
-			sn->perms[p].access = XS_STATE_NODE_PERM_WRITE;
+			sp.access = XS_STATE_NODE_PERM_WRITE;
 			break;
 		case XS_PERM_READ | XS_PERM_WRITE:
-			sn->perms[p].access = XS_STATE_NODE_PERM_BOTH;
+			sp.access = XS_STATE_NODE_PERM_BOTH;
 			break;
 		default:
-			sn->perms[p].access = XS_STATE_NODE_PERM_NONE;
+			sp.access = XS_STATE_NODE_PERM_NONE;
 			break;
 		}
-		sn->perms[p].flags = (perms[p].perms & XS_PERM_IGNORE)
+		sp.flags = (perms[p].perms & XS_PERM_IGNORE)
 				     ? XS_STATE_NODE_PERM_IGNORE : 0;
-		sn->perms[p].domid = perms[p].id;
-	}
+		sp.domid = perms[p].id;
 
-	if (fwrite(sn->perms, sizeof(*sn->perms), n_perms, fp) != n_perms)
-		return "Dump node permissions error";
+		if (fwrite(&sp, sizeof(sp), 1, fp) != 1)
+			return "Dump node permission error";
+
+	}
 
 	return NULL;
 }
@@ -2519,7 +2521,7 @@ static const char *dump_state_node_tree(FILE *fp, char *path)
 	if (fwrite(&sn, sizeof(sn), 1, fp) != 1)
 		return "Dump node state error";
 
-	ret = dump_state_node_perms(fp, &sn, hdr->perms, hdr->num_perms);
+	ret = dump_state_node_perms(fp, hdr->perms, hdr->num_perms);
 	if (ret)
 		return ret;
 

tools/xenstore/xenstored_core.h

@@ -271,8 +271,7 @@ const char *dump_state_buffered_data(FILE *fp, const struct connection *c,
 				     const struct connection *conn,
 				     struct xs_state_connection *sc);
 const char *dump_state_nodes(FILE *fp, const void *ctx);
-const char *dump_state_node_perms(FILE *fp, struct xs_state_node *sn,
-				  const struct xs_permissions *perms,
+const char *dump_state_node_perms(FILE *fp, const struct xs_permissions *perms,
 				  unsigned int n_perms);
 
 void read_state_global(const void *ctx, const void *state);

tools/xenstore/xenstored_domain.c

@@ -1254,7 +1254,7 @@ static const char *dump_state_special_node(FILE *fp, const char *name,
 	if (fwrite(&sn, sizeof(sn), 1, fp) != 1)
 		return "Dump special node error";
 
-	ret = dump_state_node_perms(fp, &sn, perms->p, perms->num);
+	ret = dump_state_node_perms(fp, perms->p, perms->num);
 	if (ret)
 		return ret;