[RFC PATCH 06/10] hurd: Make sure to not use tcb->self

[Sergey Bugaev]

Unlike sigstate->thread, tcb->self did not hold a Mach port reference on
the thread port it names. This means that the port can be deallocated,
and the name reused for something else, without anyone noticing. Using
tcb->self will then lead to port use-after-free.

Fortunately nothing was accessing tcb->self, other than it being
intially set to then-valid thread port name upon TCB initialization. To
assert that this keeps being the case without altering TCB layout,
rename self -> self_do_not_use, and stop initializing it.

Also, do not (re-)allocate a whole separate and unused stack for the
main thread, and just exit __pthread_setup early in this case.

Found upon attempting to use tcb->self and getting unexpected crashes.

Signed-off-by: Sergey Bugaev <bugaevc@gmail.com>
---
 sysdeps/mach/hurd/i386/tls.h         |  3 +--
 sysdeps/mach/hurd/x86/htl/pt-setup.c | 34 ++++++++++------------------
 sysdeps/mach/hurd/x86_64/tls.h       |  3 +--
 3 files changed, 14 insertions(+), 26 deletions(-)

diff --git a/sysdeps/mach/hurd/i386/tls.h b/sysdeps/mach/hurd/i386/tls.h
index e124fb10..ba283008 100644
--- a/sysdeps/mach/hurd/i386/tls.h
+++ b/sysdeps/mach/hurd/i386/tls.h
@@ -32,7 +32,7 @@ typedef struct
 {
   void *tcb;                   /* Points to this structure.  */
   dtv_t *dtv;                  /* Vector of pointers to TLS data.  */
-  thread_t self;               /* This thread's control port.  */
+  thread_t self_do_not_use;    /* This thread's control port.  */
   int multiple_threads;
   uintptr_t sysinfo;
   uintptr_t stack_guard;
@@ -419,7 +419,6 @@ _hurd_tls_new (thread_t child, tcbhead_t *tcb)
   HURD_TLS_DESC_DECL (desc, tcb);
 
   tcb->tcb = tcb;
-  tcb->self = child;
 
   if (HURD_SEL_LDT (sel))
     err = __i386_set_ldt (child, sel, &desc, 1);
diff --git a/sysdeps/mach/hurd/x86/htl/pt-setup.c 
b/sysdeps/mach/hurd/x86/htl/pt-setup.c
index 3abd92b2..686124d7 100644
--- a/sysdeps/mach/hurd/x86/htl/pt-setup.c
+++ b/sysdeps/mach/hurd/x86/htl/pt-setup.c
@@ -19,6 +19,7 @@
 #include <stdint.h>
 #include <assert.h>
 #include <mach.h>
+#include <hurd.h>
 
 #include <pt-internal.h>
 
@@ -76,35 +77,24 @@ __pthread_setup (struct __pthread *thread,
                                      void *), void *(*start_routine) (void *),
                 void *arg)
 {
-  tcbhead_t *tcb;
   error_t err;
-  mach_port_t ktid;
 
-  thread->mcontext.pc = entry_point;
-  thread->mcontext.sp = stack_setup (thread, start_routine, arg);
-
-  ktid = __mach_thread_self ();
-  if (thread->kernel_thread == ktid)
+  if (thread->kernel_thread == hurd_thread_self ())
     /* Fix up the TCB for the main thread.  The C library has already
        installed a TCB, which we want to keep using.  This TCB must not
        be freed so don't register it in the thread structure.  On the
        other hand, it's not yet possible to reliably release a TCB.
-       Leave the unused one registered so that it doesn't leak.  The
-       only thing left to do is to correctly set the `self' member in
-       the already existing TCB.  */
-    tcb = THREAD_SELF;
-  else
-    {
-      err = __thread_set_pcsptp (thread->kernel_thread,
-                                1, thread->mcontext.pc,
-                                1, thread->mcontext.sp,
-                                1, thread->tcb);
-      assert_perror (err);
-      tcb = thread->tcb;
-    }
-  __mach_port_deallocate (__mach_task_self (), ktid);
+       Leave the unused one registered so that it doesn't leak.  */
+    return 0;
+
+  thread->mcontext.pc = entry_point;
+  thread->mcontext.sp = stack_setup (thread, start_routine, arg);
 
-  tcb->self = thread->kernel_thread;
+  err = __thread_set_pcsptp (thread->kernel_thread,
+                            1, thread->mcontext.pc,
+                            1, thread->mcontext.sp,
+                            1, thread->tcb);
+  assert_perror (err);
 
   return 0;
 }
diff --git a/sysdeps/mach/hurd/x86_64/tls.h b/sysdeps/mach/hurd/x86_64/tls.h
index 1274723a..35dcef44 100644
--- a/sysdeps/mach/hurd/x86_64/tls.h
+++ b/sysdeps/mach/hurd/x86_64/tls.h
@@ -35,7 +35,7 @@ typedef struct
 {
   void *tcb;                   /* Points to this structure.  */
   dtv_t *dtv;                  /* Vector of pointers to TLS data.  */
-  thread_t self;               /* This thread's control port.  */
+  thread_t self_do_no_use;     /* This thread's control port.  */
   int __glibc_padding1;
   int multiple_threads;
   int gscope_flag;
@@ -158,7 +158,6 @@ _hurd_tls_new (thread_t child, tcbhead_t *tcb)
   struct i386_fsgs_base_state state;
 
   tcb->tcb = tcb;
-  tcb->self = child;
 
   /* Install the TCB address into FS base.  */
   state.fs_base = (uintptr_t) tcb;
-- 
2.40.1