powerpc/64s: Use emergency stack for kernel TM Bad Thing program checks

When using transactional memory (TM), the CPU can be in one of six
states as far as TM is concerned, encoded in the Machine State
Register (MSR). Certain state transitions are illegal and if attempted
trigger a "TM Bad Thing" type program check exception.

If we ever hit one of these exceptions it's treated as a bug, ie. we
oops, and kill the process and/or panic, depending on configuration.

One case where we can trigger a TM Bad Thing, is when returning to
userspace after a system call or interrupt, using RFID. When this
happens the CPU first restores the user register state, in particular
r1 (the stack pointer) and then attempts to update the MSR. However
the MSR update is not allowed and so we take the program check with
the user register state, but the kernel MSR.

This tricks the exception entry code into thinking we have a bad
kernel stack pointer, because the MSR says we're coming from the
kernel, but r1 is pointing to userspace.

To avoid this we instead always switch to the emergency stack if we
take a TM Bad Thing from the kernel. That way none of the user
register values are used, other than for printing in the oops message.

This is the fix for CVE-2017-1000255.

Fixes: 5d176f751ee3 ("powerpc: tm: Enable transactional memory (TM) lazily for userspace")
Cc: stable@vger.kernel.org # v4.9+
Signed-off-by: Cyril Bur <cyrilbur@gmail.com>
[mpe: Rewrite change log & comments, tweak asm slightly]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/exceptions-64s.S

@@ -734,7 +734,29 @@ EXC_REAL(program_check, 0x700, 0x100)
 EXC_VIRT(program_check, 0x4700, 0x100, 0x700)
 TRAMP_KVM(PACA_EXGEN, 0x700)
 EXC_COMMON_BEGIN(program_check_common)
-	EXCEPTION_PROLOG_COMMON(0x700, PACA_EXGEN)
+	/*
+	 * It's possible to receive a TM Bad Thing type program check with
+	 * userspace register values (in particular r1), but with SRR1 reporting
+	 * that we came from the kernel. Normally that would confuse the bad
+	 * stack logic, and we would report a bad kernel stack pointer. Instead
+	 * we switch to the emergency stack if we're taking a TM Bad Thing from
+	 * the kernel.
+	 */
+	li	r10,MSR_PR		/* Build a mask of MSR_PR ..	*/
+	oris	r10,r10,0x200000@h	/* .. and SRR1_PROGTM		*/
+	and	r10,r10,r12		/* Mask SRR1 with that.		*/
+	srdi	r10,r10,8		/* Shift it so we can compare	*/
+	cmpldi	r10,(0x200000 >> 8)	/* .. with an immediate.	*/
+	bne 1f				/* If != go to normal path.	*/
+
+	/* SRR1 had PR=0 and SRR1_PROGTM=1, so use the emergency stack	*/
+	andi.	r10,r12,MSR_PR;		/* Set CR0 correctly for label	*/
+					/* 3 in EXCEPTION_PROLOG_COMMON	*/
+	mr	r10,r1			/* Save r1			*/
+	ld	r1,PACAEMERGSP(r13)	/* Use emergency stack		*/
+	subi	r1,r1,INT_FRAME_SIZE	/* alloc stack frame		*/
+	b 3f				/* Jump into the macro !!	*/
+1:	EXCEPTION_PROLOG_COMMON(0x700, PACA_EXGEN)
 	bl	save_nvgprs
 	RECONCILE_IRQ_STATE(r10, r11)
 	addi	r3,r1,STACK_FRAME_OVERHEAD