[PATCH] alpha: fix crash if pthread_create races with signal delivery

[PATCH] alpha: fix crash if pthread_create races with signal delivery

[Mikulas Patocka]

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>
Cc: stable@vger.kernel.org

---
 arch/alpha/kernel/process.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

Index: linux-stable/arch/alpha/kernel/process.c
===================================================================
--- linux-stable.orig/arch/alpha/kernel/process.c	2017-12-31 17:42:12.000000000 +0100
+++ linux-stable/arch/alpha/kernel/process.c	2018-01-02 18:06:24.000000000 +0100
@@ -265,12 +265,13 @@ copy_thread(unsigned long clone_flags, u
 	   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;

Re: [PATCH] alpha: fix crash if pthread_create races with signal delivery

[Michael Cree]

On Tue, Jan 02, 2018 at 02:01:34PM -0500, Mikulas Patocka wrote:
> 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.

This bug was identified some three years ago; it triggers a failure
in the glibc nptl/tst-eintr3 test.  See:

https://marc.info/?l=linux-alpha&m=140610647213217&w=2

and a fix was proposed by RTH, namely:

https://marc.info/?l=linux-alpha&m=140675667715872&w=2

but was never included in the kernel because someone objected to
breaking the ability to run OSF/1 executables.  That patch also
deleted the line to set childregs->r20 to 1 which I mark below.

> 
> Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> Cc: stable@vger.kernel.org
> 
> ---
>  arch/alpha/kernel/process.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> Index: linux-stable/arch/alpha/kernel/process.c
> ===================================================================
> --- linux-stable.orig/arch/alpha/kernel/process.c	2017-12-31 17:42:12.000000000 +0100
> +++ linux-stable/arch/alpha/kernel/process.c	2018-01-02 18:06:24.000000000 +0100
> @@ -265,12 +265,13 @@ copy_thread(unsigned long clone_flags, u
>  	   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.  */

This line.  Is it not also problematic?

Cheers
Michael.

> -	regs->r20 = 0;
>  	stack = ((struct switch_stack *) regs) - 1;
>  	*childstack = *stack;
>  	childstack->r26 = (unsigned long) ret_from_fork;
> --
> To unsubscribe from this list: send the line "unsubscribe linux-alpha" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

Re: [PATCH] alpha: fix crash if pthread_create races with signal delivery

[Mikulas Patocka]

On Wed, 3 Jan 2018, Michael Cree wrote:

> On Tue, Jan 02, 2018 at 02:01:34PM -0500, Mikulas Patocka wrote:
> > 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.
> 
> This bug was identified some three years ago; it triggers a failure
> in the glibc nptl/tst-eintr3 test.  See:
> 
> https://marc.info/?l=linux-alpha&m=140610647213217&w=2
> 
> and a fix was proposed by RTH, namely:
> 
> https://marc.info/?l=linux-alpha&m=140675667715872&w=2
> 
> but was never included in the kernel because someone objected to
> breaking the ability to run OSF/1 executables.  That patch also
> deleted the line to set childregs->r20 to 1 which I mark below.
> 
> > 
> > Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
> > Cc: stable@vger.kernel.org
> > 
> > ---
> >  arch/alpha/kernel/process.c |    3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > Index: linux-stable/arch/alpha/kernel/process.c
> > ===================================================================
> > --- linux-stable.orig/arch/alpha/kernel/process.c	2017-12-31 17:42:12.000000000 +0100
> > +++ linux-stable/arch/alpha/kernel/process.c	2018-01-02 18:06:24.000000000 +0100
> > @@ -265,12 +265,13 @@ copy_thread(unsigned long clone_flags, u
> >  	   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.  */
> 
> This line.  Is it not also problematic?

If a signal is delivered to the parent process, the incomplete child 
process is deleted and it is recreated when the syscall is restarted.

So, setting "childregs->r20 = 1" shouldn't cause any problems.

Mikulas

> Cheers
> Michael.
> 
> > -	regs->r20 = 0;
> >  	stack = ((struct switch_stack *) regs) - 1;
> >  	*childstack = *stack;
> >  	childstack->r26 = (unsigned long) ret_from_fork;
> > --
> > To unsubscribe from this list: send the line "unsubscribe linux-alpha" in
> > the body of a message to majordomo@vger.kernel.org
> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
>