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

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Message-ID: <57284604.5070408@suse.cz>
Date:	Tue, 3 May 2016 08:32:36 +0200
From:	Jiri Slaby <jslaby@...e.cz>
To:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org
Cc:	stable@...r.kernel.org, Michael Neuling <mikey@...ling.org>,
	Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH 4.4 60/67] powerpc/tm: Check for already reclaimed tasks

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@...ling.org>
> 
> 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@...ling.org>
> Signed-off-by: Michael Ellerman <mpe@...erman.id.au>
> Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> 
> ---
>  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.

It is harmless though, it seems?

thanks,
-- 
js
suse labs