[qemu-xen stable-4.14] nbd/server: Avoid long error message assertions CVE-2020-10761

[patchbot@xxxxxxx]

commit d48973dc26f9f852eb32b89ae33db717d192df5c
Author:     Eric Blake <eblake@xxxxxxxxxx>
AuthorDate: Mon Jun 8 13:26:37 2020 -0500
Commit:     Michael Roth <mdroth@xxxxxxxxxxxxxxxxxx>
CommitDate: Mon Aug 24 18:56:03 2020 -0500

    nbd/server: Avoid long error message assertions CVE-2020-10761
    
    Ever since commit 36683283 (v2.8), the server code asserts that error
    strings sent to the client are well-formed per the protocol by not
    exceeding the maximum string length of 4096.  At the time the server
    first started sending error messages, the assertion could not be
    triggered, because messages were completely under our control.
    However, over the years, we have added latent scenarios where a client
    could trigger the server to attempt an error message that would
    include the client's information if it passed other checks first:
    
    - requesting NBD_OPT_INFO/GO on an export name that is not present
      (commit 0cfae925 in v2.12 echoes the name)
    
    - requesting NBD_OPT_LIST/SET_META_CONTEXT on an export name that is
      not present (commit e7b1948d in v2.12 echoes the name)
    
    At the time, those were still safe because we flagged names larger
    than 256 bytes with a different message; but that changed in commit
    93676c88 (v4.2) when we raised the name limit to 4096 to match the NBD
    string limit.  (That commit also failed to change the magic number
    4096 in nbd_negotiate_send_rep_err to the just-introduced named
    constant.)  So with that commit, long client names appended to server
    text can now trigger the assertion, and thus be used as a denial of
    service attack against a server.  As a mitigating factor, if the
    server requires TLS, the client cannot trigger the problematic paths
    unless it first supplies TLS credentials, and such trusted clients are
    less likely to try to intentionally crash the server.
    
    We may later want to further sanitize the user-supplied strings we
    place into our error messages, such as scrubbing out control
    characters, but that is less important to the CVE fix, so it can be a
    later patch to the new nbd_sanitize_name.
    
    Consideration was given to changing the assertion in
    nbd_negotiate_send_rep_verr to instead merely log a server error and
    truncate the message, to avoid leaving a latent path that could
    trigger a future CVE DoS on any new error message.  However, this
    merely complicates the code for something that is already (correctly)
    flagging coding errors, and now that we are aware of the long message
    pitfall, we are less likely to introduce such errors in the future,
    which would make such error handling dead code.
    
    Reported-by: Xueqiang Wei <xuwei@xxxxxxxxxx>
    CC: qemu-stable@xxxxxxxxxx
    Fixes: https://bugzilla.redhat.com/1843684 CVE-2020-10761
    Fixes: 93676c88d7
    Signed-off-by: Eric Blake <eblake@xxxxxxxxxx>
    Message-Id: <20200610163741.3745251-2-eblake@xxxxxxxxxx>
    Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@xxxxxxxxxxxxx>
    (cherry picked from commit 5c4fe018c025740fef4a0a4421e8162db0c3eefd)
    Signed-off-by: Michael Roth <mdroth@xxxxxxxxxxxxxxxxxx>
---
 nbd/server.c               | 23 ++++++++++++++++++++---
 tests/qemu-iotests/143     |  4 ++++
 tests/qemu-iotests/143.out |  2 ++
 3 files changed, 26 insertions(+), 3 deletions(-)

diff --git a/nbd/server.c b/nbd/server.c
index 02b1ed0801..20754e9ebc 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -217,7 +217,7 @@ nbd_negotiate_send_rep_verr(NBDClient *client, uint32_t 
type,
 
     msg = g_strdup_vprintf(fmt, va);
     len = strlen(msg);
-    assert(len < 4096);
+    assert(len < NBD_MAX_STRING_SIZE);
     trace_nbd_negotiate_send_rep_err(msg);
     ret = nbd_negotiate_send_rep_len(client, type, len, errp);
     if (ret < 0) {
@@ -231,6 +231,19 @@ nbd_negotiate_send_rep_verr(NBDClient *client, uint32_t 
type,
     return 0;
 }
 
+/*
+ * Return a malloc'd copy of @name suitable for use in an error reply.
+ */
+static char *
+nbd_sanitize_name(const char *name)
+{
+    if (strnlen(name, 80) < 80) {
+        return g_strdup(name);
+    }
+    /* XXX Should we also try to sanitize any control characters? */
+    return g_strdup_printf("%.80s...", name);
+}
+
 /* Send an error reply.
  * Return -errno on error, 0 on success. */
 static int GCC_FMT_ATTR(4, 5)
@@ -595,9 +608,11 @@ static int nbd_negotiate_handle_info(NBDClient *client, 
Error **errp)
 
     exp = nbd_export_find(name);
     if (!exp) {
+        g_autofree char *sane_name = nbd_sanitize_name(name);
+
         return nbd_negotiate_send_rep_err(client, NBD_REP_ERR_UNKNOWN,
                                           errp, "export '%s' not present",
-                                          name);
+                                          sane_name);
     }
 
     /* Don't bother sending NBD_INFO_NAME unless client requested it */
@@ -995,8 +1010,10 @@ static int nbd_negotiate_meta_queries(NBDClient *client,
 
     meta->exp = nbd_export_find(export_name);
     if (meta->exp == NULL) {
+        g_autofree char *sane_name = nbd_sanitize_name(export_name);
+
         return nbd_opt_drop(client, NBD_REP_ERR_UNKNOWN, errp,
-                            "export '%s' not present", export_name);
+                            "export '%s' not present", sane_name);
     }
 
     ret = nbd_opt_read(client, &nb_queries, sizeof(nb_queries), errp);
diff --git a/tests/qemu-iotests/143 b/tests/qemu-iotests/143
index f649b36195..d2349903b1 100755
--- a/tests/qemu-iotests/143
+++ b/tests/qemu-iotests/143
@@ -58,6 +58,10 @@ _send_qemu_cmd $QEMU_HANDLE \
 $QEMU_IO_PROG -f raw -c quit \
     "nbd+unix:///no_such_export?socket=$SOCK_DIR/nbd" 2>&1 \
     | _filter_qemu_io | _filter_nbd
+# Likewise, with longest possible name permitted in NBD protocol
+$QEMU_IO_PROG -f raw -c quit \
+    "nbd+unix:///$(printf %4096d 1 | tr ' ' a)?socket=$SOCK_DIR/nbd" 2>&1 \
+    | _filter_qemu_io | _filter_nbd | sed 's/aaaa*aa/aa--aa/'
 
 _send_qemu_cmd $QEMU_HANDLE \
     "{ 'execute': 'quit' }" \
diff --git a/tests/qemu-iotests/143.out b/tests/qemu-iotests/143.out
index 1f4001c601..fc9c0a761f 100644
--- a/tests/qemu-iotests/143.out
+++ b/tests/qemu-iotests/143.out
@@ -5,6 +5,8 @@ QA output created by 143
 {"return": {}}
 qemu-io: can't open device nbd+unix:///no_such_export?socket=SOCK_DIR/nbd: 
Requested export not available
 server reported: export 'no_such_export' not present
+qemu-io: can't open device nbd+unix:///aa--aa1?socket=SOCK_DIR/nbd: Requested 
export not available
+server reported: export 'aa--aa...' not present
 { 'execute': 'quit' }
 {"return": {}}
 {"timestamp": {"seconds":  TIMESTAMP, "microseconds":  TIMESTAMP}, "event": 
"SHUTDOWN", "data": {"guest": false, "reason": "host-qmp-quit"}}
--
generated by git-patchbot for /home/xen/git/qemu-xen.git#stable-4.14