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-From 5c4fe018c025740fef4a0a4421e8162db0c3eefd Mon Sep 17 00:00:00 2001
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-From: Eric Blake <eblake@redhat.com>
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-Date: Mon, 8 Jun 2020 13:26:37 -0500
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-Subject: [PATCH] nbd/server: Avoid long error message assertions
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- CVE-2020-10761
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-
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-Ever since commit 36683283 (v2.8), the server code asserts that error
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-strings sent to the client are well-formed per the protocol by not
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-exceeding the maximum string length of 4096. At the time the server
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-first started sending error messages, the assertion could not be
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-triggered, because messages were completely under our control.
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-However, over the years, we have added latent scenarios where a client
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-could trigger the server to attempt an error message that would
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-include the client's information if it passed other checks first:
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-
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-- requesting NBD_OPT_INFO/GO on an export name that is not present
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- (commit 0cfae925 in v2.12 echoes the name)
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-
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-- requesting NBD_OPT_LIST/SET_META_CONTEXT on an export name that is
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- not present (commit e7b1948d in v2.12 echoes the name)
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-
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-At the time, those were still safe because we flagged names larger
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-than 256 bytes with a different message; but that changed in commit
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-93676c88 (v4.2) when we raised the name limit to 4096 to match the NBD
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-string limit. (That commit also failed to change the magic number
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-4096 in nbd_negotiate_send_rep_err to the just-introduced named
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-constant.) So with that commit, long client names appended to server
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-text can now trigger the assertion, and thus be used as a denial of
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-service attack against a server. As a mitigating factor, if the
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-server requires TLS, the client cannot trigger the problematic paths
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-unless it first supplies TLS credentials, and such trusted clients are
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-less likely to try to intentionally crash the server.
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-
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-We may later want to further sanitize the user-supplied strings we
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-place into our error messages, such as scrubbing out control
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-characters, but that is less important to the CVE fix, so it can be a
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-later patch to the new nbd_sanitize_name.
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-
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-Consideration was given to changing the assertion in
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-nbd_negotiate_send_rep_verr to instead merely log a server error and
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-truncate the message, to avoid leaving a latent path that could
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-trigger a future CVE DoS on any new error message. However, this
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-merely complicates the code for something that is already (correctly)
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-flagging coding errors, and now that we are aware of the long message
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-pitfall, we are less likely to introduce such errors in the future,
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-which would make such error handling dead code.
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-
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-Reported-by: Xueqiang Wei <xuwei@redhat.com>
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-CC: qemu-stable@nongnu.org
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-Fixes: https://bugzilla.redhat.com/1843684 CVE-2020-10761
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-Fixes: 93676c88d7
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-Signed-off-by: Eric Blake <eblake@redhat.com>
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-Message-Id: <20200610163741.3745251-2-eblake@redhat.com>
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-Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
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-
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-Upstream-Status: Backport [https://github.com/qemu/qemu/commit/5c4fe018c025740fef4a0a4421e8162db0c3eefd]
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-CVE: CVE-2020-10761
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-Signed-off-by: Chee Yang Lee <chee.yang.lee@intel.com>
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-
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----
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- nbd/server.c | 23 ++++++++++++++++++++---
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- tests/qemu-iotests/143 | 4 ++++
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- tests/qemu-iotests/143.out | 2 ++
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- 3 files changed, 26 insertions(+), 3 deletions(-)
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-
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-diff --git a/nbd/server.c b/nbd/server.c
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-index 02b1ed08014..20754e9ebc3 100644
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---- a/nbd/server.c
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-+++ b/nbd/server.c
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-@@ -217,7 +217,7 @@ nbd_negotiate_send_rep_verr(NBDClient *client, uint32_t type,
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-
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- msg = g_strdup_vprintf(fmt, va);
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- len = strlen(msg);
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-- assert(len < 4096);
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-+ assert(len < NBD_MAX_STRING_SIZE);
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- trace_nbd_negotiate_send_rep_err(msg);
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- ret = nbd_negotiate_send_rep_len(client, type, len, errp);
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- if (ret < 0) {
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-@@ -231,6 +231,19 @@ nbd_negotiate_send_rep_verr(NBDClient *client, uint32_t type,
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- return 0;
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- }
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-
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-+/*
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-+ * Return a malloc'd copy of @name suitable for use in an error reply.
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-+ */
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-+static char *
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-+nbd_sanitize_name(const char *name)
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-+{
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-+ if (strnlen(name, 80) < 80) {
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-+ return g_strdup(name);
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-+ }
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-+ /* XXX Should we also try to sanitize any control characters? */
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-+ return g_strdup_printf("%.80s...", name);
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-+}
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-+
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- /* Send an error reply.
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- * Return -errno on error, 0 on success. */
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- static int GCC_FMT_ATTR(4, 5)
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-@@ -595,9 +608,11 @@ static int nbd_negotiate_handle_info(NBDClient *client, Error **errp)
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-
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- exp = nbd_export_find(name);
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- if (!exp) {
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-+ g_autofree char *sane_name = nbd_sanitize_name(name);
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-+
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- return nbd_negotiate_send_rep_err(client, NBD_REP_ERR_UNKNOWN,
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- errp, "export '%s' not present",
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-- name);
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-+ sane_name);
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- }
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-
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- /* Don't bother sending NBD_INFO_NAME unless client requested it */
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-@@ -995,8 +1010,10 @@ static int nbd_negotiate_meta_queries(NBDClient *client,
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-
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- meta->exp = nbd_export_find(export_name);
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- if (meta->exp == NULL) {
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-+ g_autofree char *sane_name = nbd_sanitize_name(export_name);
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-+
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- return nbd_opt_drop(client, NBD_REP_ERR_UNKNOWN, errp,
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-- "export '%s' not present", export_name);
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-+ "export '%s' not present", sane_name);
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- }
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-
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- ret = nbd_opt_read(client, &nb_queries, sizeof(nb_queries), errp);
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-diff --git a/tests/qemu-iotests/143 b/tests/qemu-iotests/143
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-index f649b361950..d2349903b1b 100755
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---- a/tests/qemu-iotests/143
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-+++ b/tests/qemu-iotests/143
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-@@ -58,6 +58,10 @@ _send_qemu_cmd $QEMU_HANDLE \
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- $QEMU_IO_PROG -f raw -c quit \
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- "nbd+unix:///no_such_export?socket=$SOCK_DIR/nbd" 2>&1 \
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- | _filter_qemu_io | _filter_nbd
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-+# Likewise, with longest possible name permitted in NBD protocol
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-+$QEMU_IO_PROG -f raw -c quit \
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-+ "nbd+unix:///$(printf %4096d 1 | tr ' ' a)?socket=$SOCK_DIR/nbd" 2>&1 \
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-+ | _filter_qemu_io | _filter_nbd | sed 's/aaaa*aa/aa--aa/'
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-
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- _send_qemu_cmd $QEMU_HANDLE \
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- "{ 'execute': 'quit' }" \
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-diff --git a/tests/qemu-iotests/143.out b/tests/qemu-iotests/143.out
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-index 1f4001c6013..fc9c0a761fa 100644
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---- a/tests/qemu-iotests/143.out
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-+++ b/tests/qemu-iotests/143.out
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-@@ -5,6 +5,8 @@ QA output created by 143
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- {"return": {}}
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- qemu-io: can't open device nbd+unix:///no_such_export?socket=SOCK_DIR/nbd: Requested export not available
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- server reported: export 'no_such_export' not present
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-+qemu-io: can't open device nbd+unix:///aa--aa1?socket=SOCK_DIR/nbd: Requested export not available
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-+server reported: export 'aa--aa...' not present
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- { 'execute': 'quit' }
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- {"return": {}}
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- {"timestamp": {"seconds": TIMESTAMP, "microseconds": TIMESTAMP}, "event": "SHUTDOWN", "data": {"guest": false, "reason": "host-qmp-quit"}}
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