[powerpc*] Use emergency stack for kernel TM Bad Thing program (CVE-2017-1000255)

debian/changelog

@@ -6,6 +6,8 @@ linux (4.13.4-2) UNRELEASED; urgency=medium
   [ Salvatore Bonaccorso ]
   * brcmfmac: add length check in brcmf_cfg80211_escan_handler()
     (CVE-2017-0786)
+  * [powerpc*] Use emergency stack for kernel TM Bad Thing program
+    (CVE-2017-1000255)
 
  -- Ben Hutchings <ben@decadent.org.uk>  Wed, 04 Oct 2017 23:14:54 +0100
 

debian/patches/bugfix/all/powerpc-64s-Use-emergency-stack-for-kernel-TM-Bad-Th.patch

@@ -0,0 +1,79 @@
+From: Cyril Bur <cyrilbur@gmail.com>
+Date: Thu, 17 Aug 2017 20:42:26 +1000
+Subject: powerpc/64s: Use emergency stack for kernel TM Bad Thing program
+ checks
+Origin: https://git.kernel.org/linus/265e60a170d0a0ecfc2d20490134ed2c48dd45ab
+Bug-Debian-Security: https://security-tracker.debian.org/tracker/CVE-2017-1000255
+
+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 | 24 +++++++++++++++++++++++-
+ 1 file changed, 23 insertions(+), 1 deletion(-)
+
+diff --git a/arch/powerpc/kernel/exceptions-64s.S b/arch/powerpc/kernel/exceptions-64s.S
+index 48da0f5d2f7f..b82586c53560 100644
+--- a/arch/powerpc/kernel/exceptions-64s.S
++++ b/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
+-- 
+2.11.0
+

debian/patches/series

@@ -120,6 +120,7 @@ bugfix/all/scsi-fix-the-issue-that-iscsi_if_rx-doesn-t-parse-nlmsg-properly.patc
 bugfix/x86/kvm-vmx-do-not-bug-on-out-of-bounds-guest-irq.patch
 bugfix/all/fix-infoleak-in-waitid-2.patch
 bugfix/all/brcmfmac-add-length-check-in-brcmf_cfg80211_escan_ha.patch
+bugfix/all/powerpc-64s-Use-emergency-stack-for-kernel-TM-Bad-Th.patch
 
 # Fix exported symbol versions
 bugfix/alpha/alpha-restore-symbol-versions-for-symbols-exported-f.patch