[PATCH 2/2] powerpc/tm: Avoid SPR flush if TM is disabled

[Michael Neuling]

On Mon, 2018-10-01 at 16:47 -0300, Breno Leitao wrote:
> There is a bug in the flush_tmregs_to_thread() function, where it forces
> TM SPRs to be saved to the thread even if the TM facility is disabled.
> 
> This bug could be reproduced using a simple test case:
> 
>   mtspr(SPRN_TEXASR, XX);
>   sleep until load_tm == 0
>   cause a coredump
>   read SPRN_TEXASR in the coredump
> 
> In this case, the coredump may contain an invalid SPR, because the
> current code is flushing live SPRs (Used by the last thread with TM
> active) into the current thread, overwriting the latest SPRs (which were
> valid).
> 
> This patch checks if TM is enabled for current task before
> saving the SPRs, otherwise, the TM is lazily disabled and the thread
> value is already up-to-date and could be used directly, and saving is
> not required.

Acked-by: Michael Neuling <mikey at neuling.org>

Breno, can you send your selftest upstream that also?

Mikey

> 
> Fixes: cd63f3cf1d5 ("powerpc/tm: Fix saving of TM SPRs in core dump")
> Signed-off-by: Breno Leitao <leitao at debian.org>
> ---
>  arch/powerpc/kernel/ptrace.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/kernel/ptrace.c b/arch/powerpc/kernel/ptrace.c
> index 9667666eb18e..e0a2ee865032 100644
> --- a/arch/powerpc/kernel/ptrace.c
> +++ b/arch/powerpc/kernel/ptrace.c
> @@ -138,7 +138,12 @@ static void flush_tmregs_to_thread(struct task_struct
> *tsk)
>  
>  	if (MSR_TM_SUSPENDED(mfmsr())) {
>  		tm_reclaim_current(TM_CAUSE_SIGNAL);
> -	} else {
> +	} else if (tm_enabled(tsk)) {
> +		/*
> +		 * Only flush TM SPRs to the thread if TM was enabled,
> +		 * otherwise (TM lazily disabled), the thread already
> +		 * contains the latest SPR value
> +		 */
>  		tm_enable();
>  		tm_save_sprs(&(tsk->thread));
>  	}