From 5c4fe018c025740fef4a0a4421e8162db0c3eefd Mon Sep 17 00:00:00 2001 From: Eric Blake Date: Mon, 8 Jun 2020 13:26:37 -0500 Subject: [PATCH] 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 CC: qemu-stable@nongnu.org Fixes: https://bugzilla.redhat.com/1843684 CVE-2020-10761 Fixes: 93676c88d7 Signed-off-by: Eric Blake Message-Id: <20200610163741.3745251-2-eblake@redhat.com> Reviewed-by: Vladimir Sementsov-Ogievskiy Upstream-Status: Backport [https://github.com/qemu/qemu/commit/5c4fe018c025740fef4a0a4421e8162db0c3eefd] CVE: CVE-2020-10761 Signed-off-by: Chee Yang Lee --- 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 02b1ed08014..20754e9ebc3 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 f649b361950..d2349903b1b 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 1f4001c6013..fc9c0a761fa 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"}}