Re: [PATCH] powerpc/tm: Avoid machine crash on rt_sigreturn

[Michael Neuling <mikey@neuling.org>]

On Wed, 2019-01-16 at 14:47 -0200, Breno Leitao wrote:
> There is a kernel crash that happens if rt_sigreturn is called inside a
> transactional block.
> 
> This crash happens if the kernel hits an in-kernel page fault when
> accessing userspace memory, usually through copy_ckvsx_to_user(). A major
> page fault calls might_sleep() function, which can cause a task reschedule.
> A task reschedule (switch_to()) reclaim and recheckpoint the TM states,
> but, in the signal return path, the checkpointed memory was already
> reclaimed, thus the exception stack has MSR that points to MSR[TS]=0.
> 
> When the code returns from might_sleep() and a task reschedule happened,
> then this task is returned with the memory recheckpointed, and
> CPU MSR[TS] = suspended.
> 
> This means that there is a side effect at might_sleep() if it is called
> with CPU MSR[TS] = 0 and the task has regs->msr[TS] != 0.
> 
> This side effect can cause a TM bad thing, since at the exception entrance,
> the stack saves MSR[TS]=0, and this is what will be used at RFID, but,
> the processor has MSR[TS] = Suspended, and this transition will be invalid
> and a TM Bad thing will be raised, causing the following crash:
> 
> 	Unexpected TM Bad Thing exception at c00000000000e9ec (msr
> 0x8000000302a03031) tm_scratch=800000010280b033
> 	cpu 0xc: Vector: 700 (Program Check) at [c00000003ff1fd70]
> 	    pc: c00000000000e9ec: fast_exception_return+0x100/0x1bc
> 	    lr: c000000000032948: handle_rt_signal64+0xb8/0xaf0
> 	    sp: c0000004263ebc40
> 	   msr: 8000000302a03031
> 	  current = 0xc000000415050300
> 	  paca    = 0xc00000003ffc4080	 irqmask: 0x03	 irq_happened: 0x01
> 	    pid   = 25006, comm = sigfuz
> 	Linux version 5.0.0-rc1-00001-g3bd6e94bec12 (breno@debian) (gcc version
> 8.2.0 (Debian 8.2.0-3)) #899 SMP Mon Jan 7 11:30:07 EST 2019
> 	WARNING: exception is not recoverable, can't continue
> 	enter ? for help
> 	[c0000004263ebc40] c000000000032948 handle_rt_signal64+0xb8/0xaf0
> (unreliable)
> 	[c0000004263ebd30] c000000000022780 do_notify_resume+0x2f0/0x430
> 	[c0000004263ebe20] c00000000000e844 ret_from_except_lite+0x70/0x74
> 	--- Exception: c00 (System Call) at 00007fffbaac400c
> 	SP (7fffeca90f40) is in userspace
> 
> The solution for this problem is running the sigreturn code with
> regs->msr[TS] disabled, thus, avoiding hitting the side effect above. This
> does not seem to be a problem since regs->msr will be replaced by the
> ucontext value, so, it is being flushed already. In this case, it is
> flushed earlier.
> 
> Signed-off-by: Breno Leitao <leitao@debian.org>

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

This still applies on powerpc/next so just acking rather than reposting

> ---
>  arch/powerpc/kernel/signal_64.c | 27 ++++++++++++++++++++++++++-
>  1 file changed, 26 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/kernel/signal_64.c b/arch/powerpc/kernel/signal_64.c
> index 6794466f6420..06c299ef6132 100644
> --- a/arch/powerpc/kernel/signal_64.c
> +++ b/arch/powerpc/kernel/signal_64.c
> @@ -565,7 +565,7 @@ static long restore_tm_sigcontexts(struct task_struct
> *tsk,
>  	preempt_disable();
>  
>  	/* pull in MSR TS bits from user context */
> -	regs->msr = (regs->msr & ~MSR_TS_MASK) | (msr & MSR_TS_MASK);
> +	regs->msr |= msr & MSR_TS_MASK;
>  
>  	/*
>  	 * Ensure that TM is enabled in regs->msr before we leave the signal
> @@ -745,6 +745,31 @@ SYSCALL_DEFINE0(rt_sigreturn)
>  	if (MSR_TM_SUSPENDED(mfmsr()))
>  		tm_reclaim_current(0);
>  
> +	/*
> +	 * Disable MSR[TS] bit also, so, if there is an exception in the
> +	 * code below (as a page fault in copy_ckvsx_to_user()), it does
> +	 * not recheckpoint this task if there was a context switch inside
> +	 * the exception.
> +	 *
> +	 * A major page fault can indirectly call schedule(). A reschedule
> +	 * process in the middle of an exception can have a side effect
> +	 * (Changing the CPU MSR[TS] state), since schedule() is called
> +	 * with the CPU MSR[TS] disable and returns with MSR[TS]=Suspended
> +	 * (switch_to() calls tm_recheckpoint() for the 'new' process). In
> +	 * this case, the process continues to be the same in the CPU, but
> +	 * the CPU state just changed.
> +	 *
> +	 * This can cause a TM Bad Thing, since the MSR in the stack will
> +	 * have the MSR[TS]=0, and this is what will be used to RFID.
> +	 *
> +	 * Clearing MSR[TS] state here will avoid a recheckpoint if there
> +	 * is any process reschedule in kernel space. The MSR[TS] state
> +	 * does not need to be saved also, since it will be replaced with
> +	 * the MSR[TS] that came from user context later, at
> +	 * restore_tm_sigcontexts.
> +	 */
> +	regs->msr &= ~MSR_TS_MASK;
> +
>  	if (__get_user(msr, &uc->uc_mcontext.gp_regs[PT_MSR]))
>  		goto badframe;
>  	if (MSR_TM_ACTIVE(msr)) {