Code: static void tm_reclaim_thread(struct thread_struct *thr,
			      struct thread_info *ti, uint8_t cause)
{
	unsigned long msr_diff = 0;

	/*
	 * If FP/VSX registers have been already saved to the
	 * thread_struct, move them to the transact_fp array.
	 * We clear the TIF_RESTORE_TM bit since after the reclaim
	 * the thread will no longer be transactional.
	 */
	if (test_ti_thread_flag(ti, TIF_RESTORE_TM)) {
		msr_diff = thr->ckpt_regs.msr & ~thr->regs->msr;
		if (msr_diff & MSR_FP)
			memcpy(&thr->transact_fp, &thr->fp_state,
			       sizeof(struct thread_fp_state));
		if (msr_diff & MSR_VEC)
			memcpy(&thr->transact_vr, &thr->vr_state,
			       sizeof(struct thread_vr_state));
		clear_ti_thread_flag(ti, TIF_RESTORE_TM);
 		msr_diff &= MSR_FP | MSR_VEC | MSR_VSX | MSR_FE0 | MSR_FE1;
 	}
 
 	tm_reclaim(thr, thr->regs->msr, cause);
 
 	/* Having done the reclaim, we now have the checkpointed
	 * FP/VSX values in the registers.  These might be valid
	 * even if we have previously called enable_kernel_fp() or
	 * flush_fp_to_thread(), so update thr->regs->msr to
	 * indicate their current validity.
	 */
	thr->regs->msr |= msr_diff;
}

Vulnerability Type: DoS 
CWE ID: CWE-284
Summary: The tm_reclaim_thread function in arch/powerpc/kernel/process.c in the Linux kernel before 4.4.1 on powerpc platforms does not ensure that TM suspend mode exists before proceeding with a tm_reclaim call, which allows local users to cause a denial of service (TM Bad Thing exception and panic) via a crafted application.
Commit Message: powerpc/tm: Check for already reclaimed tasks

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")
Cc: stable@vger.kernel.org # v3.9+
Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>