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

[Breno Leitao]

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 at 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 at debian.org>
---
 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)) {
-- 
2.19.0