From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from ozlabs.org ([103.22.144.67]:37268 "EHLO ozlabs.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755690AbcECLEU convert rfc822-to-8bit (ORCPT ); Tue, 3 May 2016 07:04:20 -0400 Message-ID: <1462273457.4115.29.camel@neuling.org> Subject: Re: [PATCH 4.4 60/67] powerpc/tm: Check for already reclaimed tasks From: Michael Neuling To: Jiri Slaby , Greg Kroah-Hartman , linux-kernel@vger.kernel.org Cc: stable@vger.kernel.org, Michael Ellerman Date: Tue, 03 May 2016 21:04:17 +1000 In-Reply-To: <57284604.5070408@suse.cz> References: <20160127180907.419868641@linuxfoundation.org> <20160127180913.484826415@linuxfoundation.org> <57284604.5070408@suse.cz> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT Mime-Version: 1.0 Sender: stable-owner@vger.kernel.org List-ID: 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 > > > > 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 > > Signed-off-by: Michael Ellerman > > Signed-off-by: Greg Kroah-Hartman > > > > --- > >  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? > It is harmless though, it seems? Yes, that should be harmless, other than a small performance penalty. Mikey