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

[Julien Grall <julien@xxxxxxx>]

From: Julien Grall <jgrall@xxxxxxxxxx>

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
    #3 0x555c6b32ddb3 in lu_dump_state 
xen/tools/xenstore/xenstored_control.c:521
    #4 0x555c6b32e380 in do_lu_start xen/tools/xenstore/xenstored_control.c:660
    #5 0x555c6b31b461 in call_delayed xen/tools/xenstore/xenstored_core.c:278
    #6 0x555c6b32275e in main xen/tools/xenstore/xenstored_core.c:2357
    #7 0x7f95eecf3d09 in __libc_start_main ../csu/libc-start.c:308
    #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: ed6eebf17d2c ("tools/xenstore: dump the xenstore state for live update")
Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
---
 tools/xenstore/xenstored_core.c   | 26 ++++++++++++++------------
 tools/xenstore/xenstored_core.h   |  3 +--
 tools/xenstore/xenstored_domain.c |  2 +-
 3 files changed, 16 insertions(+), 15 deletions(-)

diff --git a/tools/xenstore/xenstored_core.c b/tools/xenstore/xenstored_core.c
index d54a6042a9f7..f68da12b5b23 100644
--- a/tools/xenstore/xenstored_core.c
+++ b/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;
 
diff --git a/tools/xenstore/xenstored_core.h b/tools/xenstore/xenstored_core.h
index 1cdbc3dcb5f7..b50ea3f57d5a 100644
--- a/tools/xenstore/xenstored_core.h
+++ b/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);
diff --git a/tools/xenstore/xenstored_domain.c 
b/tools/xenstore/xenstored_domain.c
index 3d4d0649a243..580ed454a3f5 100644
--- a/tools/xenstore/xenstored_domain.c
+++ b/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;
 
-- 
2.17.1