Kernel+LibC: Introduce a "dumpable" flag for processes

This new flag controls two things:
- Whether the kernel will generate core dumps for the process
- Whether the EUID:EGID should own the process's files in /proc

Processes are automatically made non-dumpable when their EUID or EGID is
changed, either via syscalls that specifically modify those ID's, or via
sys$execve(), when a set-uid or set-gid program is executed.

A process can change its own dumpable flag at any time by calling the
new sys$prctl(PR_SET_DUMPABLE) syscall.

Fixes #4504.

Kernel/Syscalls/execve.cpp

@@ -363,19 +363,25 @@ int Process::do_exec(NonnullRefPtr<FileDescription> main_program_description, Ve
     auto old_suid = m_suid;
     auto old_egid = m_egid;
     auto old_sgid = m_sgid;
+    auto was_dumpable = is_dumpable();
 
     ArmedScopeGuard cred_restore_guard = [&] {
         m_euid = old_euid;
         m_suid = old_suid;
         m_egid = old_egid;
         m_sgid = old_sgid;
+        set_dumpable(was_dumpable);
     };
 
     if (!(main_program_description->custody()->mount_flags() & MS_NOSUID)) {
-        if (main_program_metadata.is_setuid())
+        if (main_program_metadata.is_setuid()) {
+            set_dumpable(false);
             m_euid = m_suid = main_program_metadata.uid;
-        if (main_program_metadata.is_setgid())
+        }
+        if (main_program_metadata.is_setgid()) {
+            set_dumpable(false);
             m_egid = m_sgid = main_program_metadata.gid;
+        }
     }
 
     auto load_result_or_error = load(main_program_description, interpreter_description);

Kernel/Syscalls/prctl.cpp

@@ -0,0 +1,46 @@
+/*
+ * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <Kernel/Process.h>
+#include <LibC/sys/prctl_numbers.h>
+
+namespace Kernel {
+
+int Process::sys$prctl(int option, FlatPtr arg1, [[maybe_unused]] FlatPtr arg2)
+{
+    switch (option) {
+    case PR_GET_DUMPABLE:
+        return is_dumpable();
+    case PR_SET_DUMPABLE:
+        set_dumpable(arg1);
+        return 0;
+    default:
+        return -EINVAL;
+    }
+    return 0;
+}
+
+}

Kernel/Syscalls/setuid.cpp

@@ -35,6 +35,8 @@ int Process::sys$seteuid(uid_t euid)
     if (euid != m_uid && euid != m_suid && !is_superuser())
         return -EPERM;
 
+    if (m_euid != euid)
+        set_dumpable(false);
     m_euid = euid;
     return 0;
 }
@@ -46,6 +48,9 @@ int Process::sys$setegid(gid_t egid)
     if (egid != m_gid && egid != m_sgid && !is_superuser())
         return -EPERM;
 
+    if (m_egid != egid)
+        set_dumpable(false);
+
     m_egid = egid;
     return 0;
 }
@@ -57,6 +62,9 @@ int Process::sys$setuid(uid_t uid)
     if (uid != m_uid && uid != m_euid && !is_superuser())
         return -EPERM;
 
+    if (m_euid != uid)
+        set_dumpable(false);
+
     m_uid = uid;
     m_euid = uid;
     m_suid = uid;
@@ -70,6 +78,9 @@ int Process::sys$setgid(gid_t gid)
     if (gid != m_gid && gid != m_egid && !is_superuser())
         return -EPERM;
 
+    if (m_egid != gid)
+        set_dumpable(false);
+
     m_gid = gid;
     m_egid = gid;
     m_sgid = gid;
@@ -91,6 +102,9 @@ int Process::sys$setresuid(uid_t ruid, uid_t euid, uid_t suid)
     if ((!ok(ruid) || !ok(euid) || !ok(suid)) && !is_superuser())
         return -EPERM;
 
+    if (m_euid != euid)
+        set_dumpable(false);
+
     m_uid = ruid;
     m_euid = euid;
     m_suid = suid;
@@ -112,6 +126,9 @@ int Process::sys$setresgid(gid_t rgid, gid_t egid, gid_t sgid)
     if ((!ok(rgid) || !ok(egid) || !ok(sgid)) && !is_superuser())
         return -EPERM;
 
+    if (m_egid != egid)
+        set_dumpable(false);
+
     m_gid = rgid;
     m_egid = egid;
     m_sgid = sgid;