journald: do not store the iovec entry for process commandline on stack

This fixes a crash where we would read the commandline, whose length is under
control of the sending program, and then crash when trying to create a stack
allocation for it.

CVE-2018-16864
https://bugzilla.redhat.com/show_bug.cgi?id=1653855

The message actually doesn't get written to disk, because
journal_file_append_entry() returns -E2BIG.

4 files changed, 38 insertions, 30 deletions

diff --git a/src/basic/io-util.c b/src/basic/io-util.c
index 1f64cc933..575398fbe 100644
--- a/src/basic/io-util.c
+++ b/src/basic/io-util.c
@@ -8,6 +8,7 @@
 #include <unistd.h>
 
 #include "io-util.h"
+#include "string-util.h"
 #include "time-util.h"
 
 int flush_fd(int fd) {
@@ -252,3 +253,12 @@ ssize_t sparse_write(int fd, const void *p, size_t sz, size_t run_length) {
 
         return q - (const uint8_t*) p;
 }
+
+char* set_iovec_string_field(struct iovec *iovec, size_t *n_iovec, const char *field, const char *value) {
+        char *x;
+
+        x = strappend(field, value);
+        if (x)
+                iovec[(*n_iovec)++] = IOVEC_MAKE_STRING(x);
+        return x;
+}
diff --git a/src/basic/io-util.h b/src/basic/io-util.h
index ed189b582..792a64ad5 100644
--- a/src/basic/io-util.h
+++ b/src/basic/io-util.h
@@ -71,3 +71,5 @@ static inline bool FILE_SIZE_VALID_OR_INFINITY(uint64_t l) {
 #define IOVEC_MAKE(base, len) (struct iovec) IOVEC_INIT(base, len)
 #define IOVEC_INIT_STRING(string) IOVEC_INIT((char*) string, strlen(string))
 #define IOVEC_MAKE_STRING(string) (struct iovec) IOVEC_INIT_STRING(string)
+
+char* set_iovec_string_field(struct iovec *iovec, size_t *n_iovec, const char *field, const char *value);
diff --git a/src/coredump/coredump.c b/src/coredump/coredump.c
index 20c1fb024..db2cf64dd 100644
--- a/src/coredump/coredump.c
+++ b/src/coredump/coredump.c
@@ -1063,19 +1063,10 @@ static int send_iovec(const struct iovec iovec[], size_t n_iovec, int input_fd)
         return 0;
 }
 
-static char* set_iovec_field(struct iovec *iovec, size_t *n_iovec, const char *field, const char *value) {
-        char *x;
-
-        x = strappend(field, value);
-        if (x)
-                iovec[(*n_iovec)++] = IOVEC_MAKE_STRING(x);
-        return x;
-}
-
 static char* set_iovec_field_free(struct iovec *iovec, size_t *n_iovec, const char *field, char *value) {
         char *x;
 
-        x = set_iovec_field(iovec, n_iovec, field, value);
+        x = set_iovec_string_field(iovec, n_iovec, field, value);
         free(value);
         return x;
 }
@@ -1125,36 +1116,36 @@ static int gather_pid_metadata(
                         disable_coredumps();
                 }
 
-                set_iovec_field(iovec, n_iovec, "COREDUMP_UNIT=", context[CONTEXT_UNIT]);
+                set_iovec_string_field(iovec, n_iovec, "COREDUMP_UNIT=", context[CONTEXT_UNIT]);
         }
 
         if (cg_pid_get_user_unit(pid, &t) >= 0)
                 set_iovec_field_free(iovec, n_iovec, "COREDUMP_USER_UNIT=", t);
 
         /* The next few are mandatory */
-        if (!set_iovec_field(iovec, n_iovec, "COREDUMP_PID=", context[CONTEXT_PID]))
+        if (!set_iovec_string_field(iovec, n_iovec, "COREDUMP_PID=", context[CONTEXT_PID]))
                 return log_oom();
 
-        if (!set_iovec_field(iovec, n_iovec, "COREDUMP_UID=", context[CONTEXT_UID]))
+        if (!set_iovec_string_field(iovec, n_iovec, "COREDUMP_UID=", context[CONTEXT_UID]))
                 return log_oom();
 
-        if (!set_iovec_field(iovec, n_iovec, "COREDUMP_GID=", context[CONTEXT_GID]))
+        if (!set_iovec_string_field(iovec, n_iovec, "COREDUMP_GID=", context[CONTEXT_GID]))
                 return log_oom();
 
-        if (!set_iovec_field(iovec, n_iovec, "COREDUMP_SIGNAL=", context[CONTEXT_SIGNAL]))
+        if (!set_iovec_string_field(iovec, n_iovec, "COREDUMP_SIGNAL=", context[CONTEXT_SIGNAL]))
                 return log_oom();
 
-        if (!set_iovec_field(iovec, n_iovec, "COREDUMP_RLIMIT=", context[CONTEXT_RLIMIT]))
+        if (!set_iovec_string_field(iovec, n_iovec, "COREDUMP_RLIMIT=", context[CONTEXT_RLIMIT]))
                 return log_oom();
 
-        if (!set_iovec_field(iovec, n_iovec, "COREDUMP_HOSTNAME=", context[CONTEXT_HOSTNAME]))
+        if (!set_iovec_string_field(iovec, n_iovec, "COREDUMP_HOSTNAME=", context[CONTEXT_HOSTNAME]))
                 return log_oom();
 
-        if (!set_iovec_field(iovec, n_iovec, "COREDUMP_COMM=", context[CONTEXT_COMM]))
+        if (!set_iovec_string_field(iovec, n_iovec, "COREDUMP_COMM=", context[CONTEXT_COMM]))
                 return log_oom();
 
         if (context[CONTEXT_EXE] &&
-            !set_iovec_field(iovec, n_iovec, "COREDUMP_EXE=", context[CONTEXT_EXE]))
+            !set_iovec_string_field(iovec, n_iovec, "COREDUMP_EXE=", context[CONTEXT_EXE]))
                 return log_oom();
 
         if (sd_pid_get_session(pid, &t) >= 0)
@@ -1222,7 +1213,7 @@ static int gather_pid_metadata(
                 iovec[(*n_iovec)++] = IOVEC_MAKE_STRING(t);
 
         if (safe_atoi(context[CONTEXT_SIGNAL], &signo) >= 0 && SIGNAL_VALID(signo))
-                set_iovec_field(iovec, n_iovec, "COREDUMP_SIGNAL_NAME=SIG", signal_to_string(signo));
+                set_iovec_string_field(iovec, n_iovec, "COREDUMP_SIGNAL_NAME=SIG", signal_to_string(signo));
 
         return 0; /* we successfully acquired all metadata */
 }
diff --git a/src/journal/journald-server.c b/src/journal/journald-server.c
index f09672531..2a960ebb3 100644
--- a/src/journal/journald-server.c
+++ b/src/journal/journald-server.c
@@ -905,6 +905,7 @@ static void dispatch_message_real(
                 pid_t object_pid) {
 
         char source_time[sizeof("_SOURCE_REALTIME_TIMESTAMP=") + DECIMAL_STR_MAX(usec_t)];
+        _cleanup_free_ char *cmdline1 = NULL, *cmdline2 = NULL;
         uid_t journal_uid;
         ClientContext *o;
 
@@ -921,20 +922,23 @@ static void dispatch_message_real(
                 IOVEC_ADD_NUMERIC_FIELD(iovec, n, c->uid, uid_t, uid_is_valid, UID_FMT, "_UID");
                 IOVEC_ADD_NUMERIC_FIELD(iovec, n, c->gid, gid_t, gid_is_valid, GID_FMT, "_GID");
 
-                IOVEC_ADD_STRING_FIELD(iovec, n, c->comm, "_COMM");
-                IOVEC_ADD_STRING_FIELD(iovec, n, c->exe, "_EXE");
-                IOVEC_ADD_STRING_FIELD(iovec, n, c->cmdline, "_CMDLINE");
-                IOVEC_ADD_STRING_FIELD(iovec, n, c->capeff, "_CAP_EFFECTIVE");
+                IOVEC_ADD_STRING_FIELD(iovec, n, c->comm, "_COMM"); /* At most TASK_COMM_LENGTH (16 bytes) */
+                IOVEC_ADD_STRING_FIELD(iovec, n, c->exe, "_EXE"); /* A path, so at most PATH_MAX (4096 bytes) */
 
-                IOVEC_ADD_SIZED_FIELD(iovec, n, c->label, c->label_size, "_SELINUX_CONTEXT");
+                if (c->cmdline)
+                        /* At most _SC_ARG_MAX (2MB usually), which is too much to put on stack.
+                         * Let's use a heap allocation for this one. */
+                        cmdline1 = set_iovec_string_field(iovec, &n, "_CMDLINE=", c->cmdline);
 
+                IOVEC_ADD_STRING_FIELD(iovec, n, c->capeff, "_CAP_EFFECTIVE"); /* Read from /proc/.../status */
+                IOVEC_ADD_SIZED_FIELD(iovec, n, c->label, c->label_size, "_SELINUX_CONTEXT");
                 IOVEC_ADD_NUMERIC_FIELD(iovec, n, c->auditid, uint32_t, audit_session_is_valid, "%" PRIu32, "_AUDIT_SESSION");
                 IOVEC_ADD_NUMERIC_FIELD(iovec, n, c->loginuid, uid_t, uid_is_valid, UID_FMT, "_AUDIT_LOGINUID");
 
-                IOVEC_ADD_STRING_FIELD(iovec, n, c->cgroup, "_SYSTEMD_CGROUP");
+                IOVEC_ADD_STRING_FIELD(iovec, n, c->cgroup, "_SYSTEMD_CGROUP"); /* A path */
                 IOVEC_ADD_STRING_FIELD(iovec, n, c->session, "_SYSTEMD_SESSION");
                 IOVEC_ADD_NUMERIC_FIELD(iovec, n, c->owner_uid, uid_t, uid_is_valid, UID_FMT, "_SYSTEMD_OWNER_UID");
-                IOVEC_ADD_STRING_FIELD(iovec, n, c->unit, "_SYSTEMD_UNIT");
+                IOVEC_ADD_STRING_FIELD(iovec, n, c->unit, "_SYSTEMD_UNIT"); /* Unit names are bounded by UNIT_NAME_MAX */
                 IOVEC_ADD_STRING_FIELD(iovec, n, c->user_unit, "_SYSTEMD_USER_UNIT");
                 IOVEC_ADD_STRING_FIELD(iovec, n, c->slice, "_SYSTEMD_SLICE");
                 IOVEC_ADD_STRING_FIELD(iovec, n, c->user_slice, "_SYSTEMD_USER_SLICE");
@@ -955,13 +959,14 @@ static void dispatch_message_real(
                 IOVEC_ADD_NUMERIC_FIELD(iovec, n, o->uid, uid_t, uid_is_valid, UID_FMT, "OBJECT_UID");
                 IOVEC_ADD_NUMERIC_FIELD(iovec, n, o->gid, gid_t, gid_is_valid, GID_FMT, "OBJECT_GID");
 
+                /* See above for size limits, only ->cmdline may be large, so use a heap allocation for it. */
                 IOVEC_ADD_STRING_FIELD(iovec, n, o->comm, "OBJECT_COMM");
                 IOVEC_ADD_STRING_FIELD(iovec, n, o->exe, "OBJECT_EXE");
-                IOVEC_ADD_STRING_FIELD(iovec, n, o->cmdline, "OBJECT_CMDLINE");
-                IOVEC_ADD_STRING_FIELD(iovec, n, o->capeff, "OBJECT_CAP_EFFECTIVE");
+                if (o->cmdline)
+                        cmdline2 = set_iovec_string_field(iovec, &n, "OBJECT_CMDLINE=", o->cmdline);
 
+                IOVEC_ADD_STRING_FIELD(iovec, n, o->capeff, "OBJECT_CAP_EFFECTIVE");
                 IOVEC_ADD_SIZED_FIELD(iovec, n, o->label, o->label_size, "OBJECT_SELINUX_CONTEXT");
-
                 IOVEC_ADD_NUMERIC_FIELD(iovec, n, o->auditid, uint32_t, audit_session_is_valid, "%" PRIu32, "OBJECT_AUDIT_SESSION");
                 IOVEC_ADD_NUMERIC_FIELD(iovec, n, o->loginuid, uid_t, uid_is_valid, UID_FMT, "OBJECT_AUDIT_LOGINUID");