sd-bus: skip sending formatted UIDs via SASL

The dbus external authentication takes as optional argument the UID the
sender wants to authenticate as. This uid is purely optional. The
AF_UNIX socket already conveys the same information through the
auxiliary socket data, so we really don't have to provide that
information.

Unfortunately, there is no way to send empty arguments, since they are
interpreted as "missing argument", which has a different meaning. The
SASL negotiation thus changes from:

    AUTH EXTERNAL <uid>
    NEGOTIATE_UNIX_FD                   (optional)
    BEGIN

to:

    AUTH EXTERNAL
    DATA
    NEGOTIATE_UNIX_FD                   (optional)
    BEGIN

And thus the replies we expect as a client change from:

    OK <server-id>
    AGREE_UNIX_FD                       (optional)

to:

    DATA
    OK <server-id>
    AGREE_UNIX_FD                       (optional)

Since the old sd-bus server implementation used the wrong reply for
"AUTH" requests that do not carry the arguments inlined, we decided to
make sd-bus clients accept this as well. Hence, sd-bus now allows
"OK <server-id>\r\n" replies instead of "DATA\r\n" replies.

Signed-off-by: David Rheinsberg <david.rheinsberg@gmail.com>
(cherry picked from commit 1ed4723d38cd0d1423c8fe650f90fa86007ddf55)

Resolves: #1838081

1 files changed, 64 insertions, 42 deletions

diff --git a/src/libsystemd/sd-bus/bus-socket.c b/src/libsystemd/sd-bus/bus-socket.c
index e505d43c6b..8813dd5efd 100644
--- a/src/libsystemd/sd-bus/bus-socket.c
+++ b/src/libsystemd/sd-bus/bus-socket.c
@@ -162,17 +162,25 @@ static int bus_socket_write_auth(sd_bus *b) {
 }
 
 static int bus_socket_auth_verify_client(sd_bus *b) {
-        char *e, *f, *start;
+        char *d, *e, *f, *start;
         sd_id128_t peer;
         unsigned i;
         int r;
 
         assert(b);
 
-        /* We expect two response lines: "OK" and possibly
-         * "AGREE_UNIX_FD" */
+        /*
+         * We expect three response lines:
+         *   "DATA\r\n"
+         *   "OK <server-id>\r\n"
+         *   "AGREE_UNIX_FD\r\n"        (optional)
+         */
 
-        e = memmem_safe(b->rbuffer, b->rbuffer_size, "\r\n", 2);
+        d = memmem_safe(b->rbuffer, b->rbuffer_size, "\r\n", 2);
+        if (!d)
+                return 0;
+
+        e = memmem(d + 2, b->rbuffer_size - (d - (char*) b->rbuffer) - 2, "\r\n", 2);
         if (!e)
                 return 0;
 
@@ -187,13 +195,30 @@ static int bus_socket_auth_verify_client(sd_bus *b) {
                 start = e + 2;
         }
 
-        /* Nice! We got all the lines we need. First check the OK
-         * line */
+        /* Nice! We got all the lines we need. First check the DATA line. */
+
+        if (d - (char*) b->rbuffer == 4) {
+                if (memcmp(b->rbuffer, "DATA", 4))
+                        return -EPERM;
+        } else if (d - (char*) b->rbuffer == 3 + 32) {
+                /*
+                 * Old versions of the server-side implementation of `sd-bus` replied with "OK <id>" to
+                 * "AUTH" requests from a client, even if the "AUTH" line did not contain inlined
+                 * arguments. Therefore, we also accept "OK <id>" here, even though it is technically the
+                 * wrong reply. We ignore the "<id>" parameter, though, since it has no real value.
+                 */
+                if (memcmp(b->rbuffer, "OK ", 3))
+                        return -EPERM;
+        } else {
+                return -EPERM;
+        }
+
+        /* Now check the OK line. */
 
-        if (e - (char*) b->rbuffer != 3 + 32)
+        if (e - d != 2 + 3 + 32)
                 return -EPERM;
 
-        if (memcmp(b->rbuffer, "OK ", 3))
+        if (memcmp(d + 2, "OK ", 3))
                 return -EPERM;
 
         b->auth = b->anonymous_auth ? BUS_AUTH_ANONYMOUS : BUS_AUTH_EXTERNAL;
@@ -201,8 +226,8 @@ static int bus_socket_auth_verify_client(sd_bus *b) {
         for (i = 0; i < 32; i += 2) {
                 int x, y;
 
-                x = unhexchar(((char*) b->rbuffer)[3 + i]);
-                y = unhexchar(((char*) b->rbuffer)[3 + i + 1]);
+                x = unhexchar(d[2 + 3 + i]);
+                y = unhexchar(d[2 + 3 + i + 1]);
 
                 if (x < 0 || y < 0)
                         return -EINVAL;
@@ -216,7 +241,7 @@ static int bus_socket_auth_verify_client(sd_bus *b) {
 
         b->server_id = peer;
 
-        /* And possibly check the second line, too */
+        /* And possibly check the third line, too */
 
         if (f)
                 b->can_fds =
@@ -616,42 +641,39 @@ static void bus_get_peercred(sd_bus *b) {
 }
 
 static int bus_socket_start_auth_client(sd_bus *b) {
-        size_t l;
-        const char *auth_suffix, *auth_prefix;
+        static const char sasl_auth_anonymous[] = {
+                /*
+                 * We use an arbitrary trace-string for the ANONYMOUS authentication. It can be used by the
+                 * message broker to aid debugging of clients. We fully anonymize the connection and use a
+                 * static default.
+                 */
+                "\0AUTH ANONYMOUS\r\n"
+                /* HEX a n o n y m o u s */
+                "DATA 616e6f6e796d6f7573\r\n"
+        };
+        static const char sasl_auth_external[] = {
+                "\0AUTH EXTERNAL\r\n"
+                "DATA\r\n"
+        };
+        static const char sasl_negotiate_unix_fd[] = {
+                "NEGOTIATE_UNIX_FD\r\n"
+        };
+        static const char sasl_begin[] = {
+                "BEGIN\r\n"
+        };
+        size_t i = 0;
 
         assert(b);
 
-        if (b->anonymous_auth) {
-                auth_prefix = "\0AUTH ANONYMOUS ";
-
-                /* For ANONYMOUS auth we send some arbitrary "trace" string */
-                l = 9;
-                b->auth_buffer = hexmem("anonymous", l);
-        } else {
-                char text[DECIMAL_STR_MAX(uid_t) + 1];
-
-                auth_prefix = "\0AUTH EXTERNAL ";
-
-                xsprintf(text, UID_FMT, geteuid());
-
-                l = strlen(text);
-                b->auth_buffer = hexmem(text, l);
-        }
-
-        if (!b->auth_buffer)
-                return -ENOMEM;
+        if (b->anonymous_auth)
+                b->auth_iovec[i++] = IOVEC_MAKE((char*) sasl_auth_anonymous, sizeof(sasl_auth_anonymous) - 1);
+        else
+                b->auth_iovec[i++] = IOVEC_MAKE((char*) sasl_auth_external, sizeof(sasl_auth_external) - 1);
 
         if (b->accept_fd)
-                auth_suffix = "\r\nNEGOTIATE_UNIX_FD\r\nBEGIN\r\n";
-        else
-                auth_suffix = "\r\nBEGIN\r\n";
-
-        b->auth_iovec[0].iov_base = (void*) auth_prefix;
-        b->auth_iovec[0].iov_len = 1 + strlen(auth_prefix + 1);
-        b->auth_iovec[1].iov_base = (void*) b->auth_buffer;
-        b->auth_iovec[1].iov_len = l * 2;
-        b->auth_iovec[2].iov_base = (void*) auth_suffix;
-        b->auth_iovec[2].iov_len = strlen(auth_suffix);
+                b->auth_iovec[i++] = IOVEC_MAKE_STRING(sasl_negotiate_unix_fd);
+
+        b->auth_iovec[i++] = IOVEC_MAKE_STRING(sasl_begin);
 
         return bus_socket_write_auth(b);
 }