Kernel: Disallow executing SUID binaries if process is jailed

Check if the process we are currently running is in a jail, and if that
is the case, fail early with the EPERM error code.

Also, as Brian noted, we should also disallow attaching to a jail in
case of already running within a setid executable, as this leaves the
user with false thinking of being secure (because you can't exec new
setid binaries), but the current program is still marked setid, which
means that at the very least we gained permissions while we didn't
expect it, so let's block it.

Base/usr/share/man/man7/Mitigations.md

@@ -105,6 +105,7 @@ Special restrictions on filesystem also apply:
 - Write access is forbidden to kernel variables (which are located in `/sys/kernel/variables`).
 - Open access is forbidden to all device nodes except for `/dev/full`, `/dev/null`, `/dev/zero`, `/dev/random` and various
     other TTY/PTY devices (not including Kernel virtual consoles).
+- Executing SUID binaries is forbidden.
 
 It was first added in the following [commit](https://github.com/SerenityOS/serenity/commit/5e062414c11df31ed595c363990005eef00fa263),
 for kernel support, and the following commits added basic userspace utilities:

Kernel/Process.h

@@ -118,6 +118,7 @@ class Process final
         // FIXME: This should be a NonnullRefPtr
         RefPtr<Credentials> credentials;
         bool dumpable { false };
+        bool executable_is_setid { false };
         Atomic<bool> has_promises { false };
         Atomic<u32> promises { 0 };
         Atomic<bool> has_execpromises { false };

Kernel/Syscalls/execve.cpp

@@ -472,6 +472,15 @@ ErrorOr<void> Process::do_exec(NonnullLockRefPtr<OpenFileDescription> main_progr
 {
     VERIFY(is_user_process());
     VERIFY(!Processor::in_critical());
+    auto main_program_metadata = main_program_description->metadata();
+    // NOTE: Don't allow running SUID binaries at all if we are in a jail.
+    TRY(Process::current().jail().with([&](auto& my_jail) -> ErrorOr<void> {
+        if (my_jail && (main_program_metadata.is_setuid() || main_program_metadata.is_setgid())) {
+            return Error::from_errno(EPERM);
+        }
+        return {};
+    }));
+
     // Although we *could* handle a pseudo_path here, trying to execute something that doesn't have
     // a custody (e.g. BlockDevice or RandomDevice) is pretty suspicious anyway.
     auto path = TRY(main_program_description->original_absolute_path());
@@ -507,8 +516,6 @@ ErrorOr<void> Process::do_exec(NonnullLockRefPtr<OpenFileDescription> main_progr
     bool executable_is_setid = false;
 
     if (!(main_program_description->custody()->mount_flags() & MS_NOSUID)) {
-        auto main_program_metadata = main_program_description->metadata();
-
         auto new_euid = old_credentials->euid();
         auto new_egid = old_credentials->egid();
         auto new_suid = old_credentials->suid();
@@ -551,6 +558,7 @@ ErrorOr<void> Process::do_exec(NonnullLockRefPtr<OpenFileDescription> main_progr
     with_mutable_protected_data([&](auto& protected_data) {
         protected_data.credentials = move(new_credentials);
         protected_data.dumpable = !executable_is_setid;
+        protected_data.executable_is_setid = executable_is_setid;
     });
 
     // We make sure to enter the new address space before destroying the old one.

Kernel/Syscalls/jail.cpp

@@ -45,6 +45,17 @@ ErrorOr<FlatPtr> Process::sys$jail_attach(Userspace<Syscall::SC_jail_attach_para
     VERIFY_NO_PROCESS_BIG_LOCK(this);
     TRY(require_promise(Pledge::jail));
 
+    // NOTE: Because the user might run a binary that is using this syscall and
+    // that binary was marked as SUID, then the user might be unaware of the
+    // fact that while no new setuid binaries might be executed, he is already
+    // running within such binary so for the sake of completeness and preventing
+    // naive sense of being secure, we should block that.
+    TRY(with_protected_data([&](auto& protected_data) -> ErrorOr<void> {
+        if (protected_data.executable_is_setid)
+            return EPERM;
+        return {};
+    }));
+
     auto params = TRY(copy_typed_from_user(user_params));
     return m_attached_jail.with([&](auto& my_jail) -> ErrorOr<FlatPtr> {
         // Note: If we are already in a jail, don't let the process escape it even if