Re: [PATCH 4.4 60/67] powerpc/tm: Check for already reclaimed tasks

[Greg Kroah-Hartman]

On Tue, May 03, 2016 at 09:04:17PM +1000, Michael Neuling wrote:
> On Tue, 2016-05-03 at 08:32 +0200, Jiri Slaby wrote:
> > On 01/27/2016, 07:12 PM, Greg Kroah-Hartman wrote:
> > > 
> > > 4.4-stable review patch.  If anyone has any objections, please let me
> > > know.
> > > 
> > > ------------------
> > > 
> > > From: Michael Neuling <mikey@xxxxxxxxxxx>
> > > 
> > > commit 7f821fc9c77a9b01fe7b1d6e72717b33d8d64142 upstream.
> > > 
> > > Currently we can hit a scenario where we'll tm_reclaim() twice.  This
> > > results in a TM bad thing exception because the second reclaim occurs
> > > when not in suspend mode.
> > > 
> > > The scenario in which this can happen is the following.  We attempt to
> > > deliver a signal to userspace.  To do this we need obtain the stack
> > > pointer to write the signal context.  To get this stack pointer we
> > > must tm_reclaim() in case we need to use the checkpointed stack
> > > pointer (see get_tm_stackpointer()).  Normally we'd then return
> > > directly to userspace to deliver the signal
> > > without going through
> > > __switch_to().
> > > 
> > > Unfortunatley, if at this point we get an error (such as a bad
> > > userspace stack pointer), we need to exit the process.  The exit will
> > > result in a __switch_to().  __switch_to() will attempt to save the
> > > process state which results in another tm_reclaim().  This
> > > tm_reclaim() now causes a TM Bad Thing exception as this state has
> > > already been saved and the processor is no longer in TM suspend mode.
> > > Whee!
> > > 
> > > This patch checks the state of the MSR to ensure we are TM suspended
> > > before we attempt the tm_reclaim().  If we've already saved the state
> > > away, we should no longer be in TM suspend mode.  This has the
> > > additional advantage of checking for a potential TM Bad Thing
> > > exception.
> > > 
> > > Found using syscall fuzzer.
> > > 
> > > Fixes: fb09692e71f1 ("powerpc: Add reclaim and recheckpoint functions
> > > for context switching transactional memory processes")
> > > Signed-off-by: Michael Neuling <mikey@xxxxxxxxxxx>
> > > Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> > > Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > > 
> > > ---
> > >  arch/powerpc/kernel/process.c |   18 ++++++++++++++++++
> > >  1 file changed, 18 insertions(+)
> > > 
> > > --- a/arch/powerpc/kernel/process.c
> > > +++ b/arch/powerpc/kernel/process.c
> > > @@ -569,6 +569,24 @@ static void tm_reclaim_thread(struct thr
> > >  	if (!MSR_TM_SUSPENDED(mfmsr()))
> > >  		return;
> > >  
> > > +	/*
> > > +	 * Use the current MSR TM suspended bit to track if we have
> > > +	 * checkpointed state outstanding.
> > > +	 * On signal delivery, we'd normally reclaim the checkpointed
> > > +	 * state to obtain stack pointer (see:get_tm_stackpointer()).
> > > +	 * This will then directly return to userspace without going
> > > +	 * through __switch_to(). However, if the stack frame is bad,
> > > +	 * we need to exit this thread which calls __switch_to() which
> > > +	 * will again attempt to reclaim the already saved tm state.
> > > +	 * Hence we need to check that we've not already reclaimed
> > > +	 * this state.
> > > +	 * We do this using the current MSR, rather tracking it in
> > > +	 * some specific thread_struct bit, as it has the additional
> > > +	 * benifit of checking for a potential TM bad thing exception.
> > > +	 */
> > > +	if (!MSR_TM_SUSPENDED(mfmsr()))
> > > +		return;
> > 
> > This one should have not been applied to 4.4. The patch is in mainline
> > since 4.4-rc6. Hence the check is duplicated as can be seen above.
> 
> Greg, surely your scripts could check for that?

My "scripts" are usually me looking at them manually, I messed up here,
sorry.  I'll revert it in the next round of stable kernel releases after
this one.

thanks,

greg k-h