alpha: fix crash if pthread_create races with signal delivery

commit 21ffceda1c8b3807615c40d440d7815e0c85d366 upstream.

On alpha, a process will crash if it attempts to start a thread and a
signal is delivered at the same time. The crash can be reproduced with
this program: https://cygwin.com/ml/cygwin/2014-11/msg00473.html

The reason for the crash is this:
* we call the clone syscall
* we go to the function copy_process
* copy process calls copy_thread_tls, it is a wrapper around copy_thread
* copy_thread sets the tls pointer: childti->pcb.unique = regs->r20
* copy_thread sets regs->r20 to zero
* we go back to copy_process
* copy process checks "if (signal_pending(current))" and returns
  -ERESTARTNOINTR
* the clone syscall is restarted, but this time, regs->r20 is zero, so
  the new thread is created with zero tls pointer
* the new thread crashes in start_thread when attempting to access tls

The comment in the code says that setting the register r20 is some
compatibility with OSF/1. But OSF/1 doesn't use the CLONE_SETTLS flag, so
we don't have to zero r20 if CLONE_SETTLS is set. This patch fixes the bug
by zeroing regs->r20 only if CLONE_SETTLS is not set.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/alpha/kernel/process.c b/arch/alpha/kernel/process.c
index 74bfb1f2d68e32b11e0d4da70da1c1fa19d58059..3a885253f486438e86aa161a0ba8845320f40b54 100644 (file)
--- a/arch/alpha/kernel/process.c
+++ b/arch/alpha/kernel/process.c
@@ -269,12 +269,13 @@ copy_thread(unsigned long clone_flags, unsigned long usp,
           application calling fork.  */
        if (clone_flags & CLONE_SETTLS)
                childti->pcb.unique = regs->r20;
+       else
+               regs->r20 = 0;  /* OSF/1 has some strange fork() semantics.  */
        childti->pcb.usp = usp ?: rdusp();
        *childregs = *regs;
        childregs->r0 = 0;
        childregs->r19 = 0;
        childregs->r20 = 1;     /* OSF/1 has some strange fork() semantics.  */
-       regs->r20 = 0;
        stack = ((struct switch_stack *) regs) - 1;
        *childstack = *stack;
        childstack->r26 = (unsigned long) ret_from_fork;