Subject: Re: [PATCH 4/5] powerpc/tm: Check for already reclaimed tasks

From: Michael Neuling <[email-masked]>
Date: Mon, 16 Nov 2015 20:23:19 +1100

On Mon, 2015-11-16 at 12:51 +0530, Anshuman Khandual wrote:
> On 11/13/2015 10:27 AM, Michael Neuling wrote:
> > Currently we can hit a scenario where we'll tm_reclaim() twice.=20
> >  This
> > results in a TM bad thing exception because the second reclaim
> > occurs
> > when not in suspend mode.
> >=20
> > 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().
> >=20
> > 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!
> >=20
> > 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.
>=20
> Can this situation be created using a test and verified that with
> this new change, the kernel can handle it successfully. I guess
> the self test in the series does not cover this scenario.

No it doesn't.  The syscall fuzzer I have does hit it but I don't have
permission to post that.

> >=20
> > Found using syscall fuzzer.
> >=20
> > Signed-off-by: Michael Neuling <[email-masked]>
> > Cc: [email-masked]
> > ---
> >  arch/powerpc/kernel/process.c | 19 +++++++++++++++++++
> >  1 file changed, 19 insertions(+)
> >=20
> > diff --git a/arch/powerpc/kernel/process.c
> > b/arch/powerpc/kernel/process.c
> > index 5fbe5d8..a1b41d1 100644
> > --- a/arch/powerpc/kernel/process.c
> > +++ b/arch/powerpc/kernel/process.c
> > @@ -551,6 +551,25 @@ static void tm_reclaim_thread(struct
> > thread_struct *thr,
> >  		msr_diff &=3D MSR_FP | MSR_VEC | MSR_VSX | MSR_FE0 |
> > MSR_FE1;
> >  	}
> > =20
> > +	/*
> > +	 * 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 bit thread_struct bit, as it has the
>=20
> There is one extra "bit" here ^^^^^.

Thanks!

Mikey
>=20